1 /*
   2  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
   3  *
   4  * This code is free software; you can redistribute it and/or modify it
   5  * under the terms of the GNU General Public License version 2 only, as
   6  * published by the Free Software Foundation.  Oracle designates this
   7  * particular file as subject to the "Classpath" exception as provided
   8  * by Oracle in the LICENSE file that accompanied this code.
   9  *
  10  * This code is distributed in the hope that it will be useful, but WITHOUT
  11  * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
  12  * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
  13  * version 2 for more details (a copy is included in the LICENSE file that
  14  * accompanied this code).
  15  *
  16  * You should have received a copy of the GNU General Public License version
  17  * 2 along with this work; if not, write to the Free Software Foundation,
  18  * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
  19  *
  20  * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
  21  * or visit www.oracle.com if you need additional information or have any
  22  * questions.
  23  */
  24 
  25 /*
  26  * This file is available under and governed by the GNU General Public
  27  * License version 2 only, as published by the Free Software Foundation.
  28  * However, the following notice accompanied the original version of this
  29  * file:
  30  *
  31  * Written by Doug Lea with assistance from members of JCP JSR-166
  32  * Expert Group and released to the public domain, as explained at
  33  * http://creativecommons.org/publicdomain/zero/1.0/
  34  */
  35 
  36 package java.util.concurrent;
  37 
  38 import java.lang.invoke.MethodHandles;
  39 import java.lang.invoke.VarHandle;
  40 import java.util.AbstractQueue;
  41 import java.util.Arrays;
  42 import java.util.Collection;
  43 import java.util.Iterator;
  44 import java.util.NoSuchElementException;
  45 import java.util.Objects;
  46 import java.util.Queue;
  47 import java.util.Spliterator;
  48 import java.util.Spliterators;
  49 import java.util.concurrent.locks.LockSupport;
  50 import java.util.function.Consumer;
  51 import java.util.function.Predicate;
  52 
  53 /**
  54  * An unbounded {@link TransferQueue} based on linked nodes.
  55  * This queue orders elements FIFO (first-in-first-out) with respect
  56  * to any given producer.  The <em>head</em> of the queue is that
  57  * element that has been on the queue the longest time for some
  58  * producer.  The <em>tail</em> of the queue is that element that has
  59  * been on the queue the shortest time for some producer.
  60  *
  61  * <p>Beware that, unlike in most collections, the {@code size} method
  62  * is <em>NOT</em> a constant-time operation. Because of the
  63  * asynchronous nature of these queues, determining the current number
  64  * of elements requires a traversal of the elements, and so may report
  65  * inaccurate results if this collection is modified during traversal.
  66  *
  67  * <p>Bulk operations that add, remove, or examine multiple elements,
  68  * such as {@link #addAll}, {@link #removeIf} or {@link #forEach},
  69  * are <em>not</em> guaranteed to be performed atomically.
  70  * For example, a {@code forEach} traversal concurrent with an {@code
  71  * addAll} operation might observe only some of the added elements.
  72  *
  73  * <p>This class and its iterator implement all of the <em>optional</em>
  74  * methods of the {@link Collection} and {@link Iterator} interfaces.
  75  *
  76  * <p>Memory consistency effects: As with other concurrent
  77  * collections, actions in a thread prior to placing an object into a
  78  * {@code LinkedTransferQueue}
  79  * <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
  80  * actions subsequent to the access or removal of that element from
  81  * the {@code LinkedTransferQueue} in another thread.
  82  *
  83  * <p>This class is a member of the
  84  * <a href="{@docRoot}/java.base/java/util/package-summary.html#CollectionsFramework">
  85  * Java Collections Framework</a>.
  86  *
  87  * @since 1.7
  88  * @author Doug Lea
  89  * @param <E> the type of elements held in this queue
  90  */
  91 public class LinkedTransferQueue<E> extends AbstractQueue<E>
  92     implements TransferQueue<E>, java.io.Serializable {
  93     private static final long serialVersionUID = -3223113410248163686L;
  94 
  95     /*
  96      * *** Overview of Dual Queues with Slack ***
  97      *
  98      * Dual Queues, introduced by Scherer and Scott
  99      * (http://www.cs.rochester.edu/~scott/papers/2004_DISC_dual_DS.pdf)
 100      * are (linked) queues in which nodes may represent either data or
 101      * requests.  When a thread tries to enqueue a data node, but
 102      * encounters a request node, it instead "matches" and removes it;
 103      * and vice versa for enqueuing requests. Blocking Dual Queues
 104      * arrange that threads enqueuing unmatched requests block until
 105      * other threads provide the match. Dual Synchronous Queues (see
 106      * Scherer, Lea, & Scott
 107      * http://www.cs.rochester.edu/u/scott/papers/2009_Scherer_CACM_SSQ.pdf)
 108      * additionally arrange that threads enqueuing unmatched data also
 109      * block.  Dual Transfer Queues support all of these modes, as
 110      * dictated by callers.
 111      *
 112      * A FIFO dual queue may be implemented using a variation of the
 113      * Michael & Scott (M&S) lock-free queue algorithm
 114      * (http://www.cs.rochester.edu/~scott/papers/1996_PODC_queues.pdf).
 115      * It maintains two pointer fields, "head", pointing to a
 116      * (matched) node that in turn points to the first actual
 117      * (unmatched) queue node (or null if empty); and "tail" that
 118      * points to the last node on the queue (or again null if
 119      * empty). For example, here is a possible queue with four data
 120      * elements:
 121      *
 122      *  head                tail
 123      *    |                   |
 124      *    v                   v
 125      *    M -> U -> U -> U -> U
 126      *
 127      * The M&S queue algorithm is known to be prone to scalability and
 128      * overhead limitations when maintaining (via CAS) these head and
 129      * tail pointers. This has led to the development of
 130      * contention-reducing variants such as elimination arrays (see
 131      * Moir et al http://portal.acm.org/citation.cfm?id=1074013) and
 132      * optimistic back pointers (see Ladan-Mozes & Shavit
 133      * http://people.csail.mit.edu/edya/publications/OptimisticFIFOQueue-journal.pdf).
 134      * However, the nature of dual queues enables a simpler tactic for
 135      * improving M&S-style implementations when dual-ness is needed.
 136      *
 137      * In a dual queue, each node must atomically maintain its match
 138      * status. While there are other possible variants, we implement
 139      * this here as: for a data-mode node, matching entails CASing an
 140      * "item" field from a non-null data value to null upon match, and
 141      * vice-versa for request nodes, CASing from null to a data
 142      * value. (Note that the linearization properties of this style of
 143      * queue are easy to verify -- elements are made available by
 144      * linking, and unavailable by matching.) Compared to plain M&S
 145      * queues, this property of dual queues requires one additional
 146      * successful atomic operation per enq/deq pair. But it also
 147      * enables lower cost variants of queue maintenance mechanics. (A
 148      * variation of this idea applies even for non-dual queues that
 149      * support deletion of interior elements, such as
 150      * j.u.c.ConcurrentLinkedQueue.)
 151      *
 152      * Once a node is matched, its match status can never again
 153      * change.  We may thus arrange that the linked list of them
 154      * contain a prefix of zero or more matched nodes, followed by a
 155      * suffix of zero or more unmatched nodes. (Note that we allow
 156      * both the prefix and suffix to be zero length, which in turn
 157      * means that we do not use a dummy header.)  If we were not
 158      * concerned with either time or space efficiency, we could
 159      * correctly perform enqueue and dequeue operations by traversing
 160      * from a pointer to the initial node; CASing the item of the
 161      * first unmatched node on match and CASing the next field of the
 162      * trailing node on appends.  While this would be a terrible idea
 163      * in itself, it does have the benefit of not requiring ANY atomic
 164      * updates on head/tail fields.
 165      *
 166      * We introduce here an approach that lies between the extremes of
 167      * never versus always updating queue (head and tail) pointers.
 168      * This offers a tradeoff between sometimes requiring extra
 169      * traversal steps to locate the first and/or last unmatched
 170      * nodes, versus the reduced overhead and contention of fewer
 171      * updates to queue pointers. For example, a possible snapshot of
 172      * a queue is:
 173      *
 174      *  head           tail
 175      *    |              |
 176      *    v              v
 177      *    M -> M -> U -> U -> U -> U
 178      *
 179      * The best value for this "slack" (the targeted maximum distance
 180      * between the value of "head" and the first unmatched node, and
 181      * similarly for "tail") is an empirical matter. We have found
 182      * that using very small constants in the range of 1-3 work best
 183      * over a range of platforms. Larger values introduce increasing
 184      * costs of cache misses and risks of long traversal chains, while
 185      * smaller values increase CAS contention and overhead.
 186      *
 187      * Dual queues with slack differ from plain M&S dual queues by
 188      * virtue of only sometimes updating head or tail pointers when
 189      * matching, appending, or even traversing nodes; in order to
 190      * maintain a targeted slack.  The idea of "sometimes" may be
 191      * operationalized in several ways. The simplest is to use a
 192      * per-operation counter incremented on each traversal step, and
 193      * to try (via CAS) to update the associated queue pointer
 194      * whenever the count exceeds a threshold. Another, that requires
 195      * more overhead, is to use random number generators to update
 196      * with a given probability per traversal step.
 197      *
 198      * In any strategy along these lines, because CASes updating
 199      * fields may fail, the actual slack may exceed targeted slack.
 200      * However, they may be retried at any time to maintain targets.
 201      * Even when using very small slack values, this approach works
 202      * well for dual queues because it allows all operations up to the
 203      * point of matching or appending an item (hence potentially
 204      * allowing progress by another thread) to be read-only, thus not
 205      * introducing any further contention.  As described below, we
 206      * implement this by performing slack maintenance retries only
 207      * after these points.
 208      *
 209      * As an accompaniment to such techniques, traversal overhead can
 210      * be further reduced without increasing contention of head
 211      * pointer updates: Threads may sometimes shortcut the "next" link
 212      * path from the current "head" node to be closer to the currently
 213      * known first unmatched node, and similarly for tail. Again, this
 214      * may be triggered with using thresholds or randomization.
 215      *
 216      * These ideas must be further extended to avoid unbounded amounts
 217      * of costly-to-reclaim garbage caused by the sequential "next"
 218      * links of nodes starting at old forgotten head nodes: As first
 219      * described in detail by Boehm
 220      * (http://portal.acm.org/citation.cfm?doid=503272.503282), if a GC
 221      * delays noticing that any arbitrarily old node has become
 222      * garbage, all newer dead nodes will also be unreclaimed.
 223      * (Similar issues arise in non-GC environments.)  To cope with
 224      * this in our implementation, upon CASing to advance the head
 225      * pointer, we set the "next" link of the previous head to point
 226      * only to itself; thus limiting the length of chains of dead nodes.
 227      * (We also take similar care to wipe out possibly garbage
 228      * retaining values held in other Node fields.)  However, doing so
 229      * adds some further complexity to traversal: If any "next"
 230      * pointer links to itself, it indicates that the current thread
 231      * has lagged behind a head-update, and so the traversal must
 232      * continue from the "head".  Traversals trying to find the
 233      * current tail starting from "tail" may also encounter
 234      * self-links, in which case they also continue at "head".
 235      *
 236      * It is tempting in slack-based scheme to not even use CAS for
 237      * updates (similarly to Ladan-Mozes & Shavit). However, this
 238      * cannot be done for head updates under the above link-forgetting
 239      * mechanics because an update may leave head at a detached node.
 240      * And while direct writes are possible for tail updates, they
 241      * increase the risk of long retraversals, and hence long garbage
 242      * chains, which can be much more costly than is worthwhile
 243      * considering that the cost difference of performing a CAS vs
 244      * write is smaller when they are not triggered on each operation
 245      * (especially considering that writes and CASes equally require
 246      * additional GC bookkeeping ("write barriers") that are sometimes
 247      * more costly than the writes themselves because of contention).
 248      *
 249      * *** Overview of implementation ***
 250      *
 251      * We use a threshold-based approach to updates, with a slack
 252      * threshold of two -- that is, we update head/tail when the
 253      * current pointer appears to be two or more steps away from the
 254      * first/last node. The slack value is hard-wired: a path greater
 255      * than one is naturally implemented by checking equality of
 256      * traversal pointers except when the list has only one element,
 257      * in which case we keep slack threshold at one. Avoiding tracking
 258      * explicit counts across method calls slightly simplifies an
 259      * already-messy implementation. Using randomization would
 260      * probably work better if there were a low-quality dirt-cheap
 261      * per-thread one available, but even ThreadLocalRandom is too
 262      * heavy for these purposes.
 263      *
 264      * With such a small slack threshold value, it is not worthwhile
 265      * to augment this with path short-circuiting (i.e., unsplicing
 266      * interior nodes) except in the case of cancellation/removal (see
 267      * below).
 268      *
 269      * All enqueue/dequeue operations are handled by the single method
 270      * "xfer" with parameters indicating whether to act as some form
 271      * of offer, put, poll, take, or transfer (each possibly with
 272      * timeout). The relative complexity of using one monolithic
 273      * method outweighs the code bulk and maintenance problems of
 274      * using separate methods for each case.
 275      *
 276      * Operation consists of up to two phases. The first is implemented
 277      * in method xfer, the second in method awaitMatch.
 278      *
 279      * 1. Traverse until matching or appending (method xfer)
 280      *
 281      *    Conceptually, we simply traverse all nodes starting from head.
 282      *    If we encounter an unmatched node of opposite mode, we match
 283      *    it and return, also updating head (by at least 2 hops) to
 284      *    one past the matched node (or the node itself if it's the
 285      *    pinned trailing node).  Traversals also check for the
 286      *    possibility of falling off-list, in which case they restart.
 287      *
 288      *    If the trailing node of the list is reached, a match is not
 289      *    possible.  If this call was untimed poll or tryTransfer
 290      *    (argument "how" is NOW), return empty-handed immediately.
 291      *    Else a new node is CAS-appended.  On successful append, if
 292      *    this call was ASYNC (e.g. offer), an element was
 293      *    successfully added to the end of the queue and we return.
 294      *
 295      *    Of course, this naive traversal is O(n) when no match is
 296      *    possible.  We optimize the traversal by maintaining a tail
 297      *    pointer, which is expected to be "near" the end of the list.
 298      *    It is only safe to fast-forward to tail (in the presence of
 299      *    arbitrary concurrent changes) if it is pointing to a node of
 300      *    the same mode, even if it is dead (in this case no preceding
 301      *    node could still be matchable by this traversal).  If we
 302      *    need to restart due to falling off-list, we can again
 303      *    fast-forward to tail, but only if it has changed since the
 304      *    last traversal (else we might loop forever).  If tail cannot
 305      *    be used, traversal starts at head (but in this case we
 306      *    expect to be able to match near head).  As with head, we
 307      *    CAS-advance the tail pointer by at least two hops.
 308      *
 309      * 2. Await match or cancellation (method awaitMatch)
 310      *
 311      *    Wait for another thread to match node; instead cancelling if
 312      *    the current thread was interrupted or the wait timed out. To
 313      *    improve performance in common single-source / single-sink
 314      *    usages when there are more tasks that cores, an initial
 315      *    Thread.yield is tried when there is apparently only one
 316      *    waiter.  In other cases, waiters may help with some
 317      *    bookkeeping, then park/unpark.
 318      *
 319      * ** Unlinking removed interior nodes **
 320      *
 321      * In addition to minimizing garbage retention via self-linking
 322      * described above, we also unlink removed interior nodes. These
 323      * may arise due to timed out or interrupted waits, or calls to
 324      * remove(x) or Iterator.remove.  Normally, given a node that was
 325      * at one time known to be the predecessor of some node s that is
 326      * to be removed, we can unsplice s by CASing the next field of
 327      * its predecessor if it still points to s (otherwise s must
 328      * already have been removed or is now offlist). But there are two
 329      * situations in which we cannot guarantee to make node s
 330      * unreachable in this way: (1) If s is the trailing node of list
 331      * (i.e., with null next), then it is pinned as the target node
 332      * for appends, so can only be removed later after other nodes are
 333      * appended. (2) We cannot necessarily unlink s given a
 334      * predecessor node that is matched (including the case of being
 335      * cancelled): the predecessor may already be unspliced, in which
 336      * case some previous reachable node may still point to s.
 337      * (For further explanation see Herlihy & Shavit "The Art of
 338      * Multiprocessor Programming" chapter 9).  Although, in both
 339      * cases, we can rule out the need for further action if either s
 340      * or its predecessor are (or can be made to be) at, or fall off
 341      * from, the head of list.
 342      *
 343      * Without taking these into account, it would be possible for an
 344      * unbounded number of supposedly removed nodes to remain reachable.
 345      * Situations leading to such buildup are uncommon but can occur
 346      * in practice; for example when a series of short timed calls to
 347      * poll repeatedly time out at the trailing node but otherwise
 348      * never fall off the list because of an untimed call to take() at
 349      * the front of the queue.
 350      *
 351      * When these cases arise, rather than always retraversing the
 352      * entire list to find an actual predecessor to unlink (which
 353      * won't help for case (1) anyway), we record the need to sweep the
 354      * next time any thread would otherwise block in awaitMatch. Also,
 355      * because traversal operations on the linked list of nodes are a
 356      * natural opportunity to sweep dead nodes, we generally do so,
 357      * including all the operations that might remove elements as they
 358      * traverse, such as removeIf and Iterator.remove.  This largely
 359      * eliminates long chains of dead interior nodes, except from
 360      * cancelled or timed out blocking operations.
 361      *
 362      * Note that we cannot self-link unlinked interior nodes during
 363      * sweeps. However, the associated garbage chains terminate when
 364      * some successor ultimately falls off the head of the list and is
 365      * self-linked.
 366      */
 367 
 368     /**
 369      * The number of nanoseconds for which it is faster to spin
 370      * rather than to use timed park. A rough estimate suffices.
 371      * Using a power of two minus one simplifies some comparisons.
 372      */
 373     static final long SPIN_FOR_TIMEOUT_THRESHOLD = 1023L;
 374 
 375     /**
 376      * The maximum number of estimated removal failures (sweepVotes)
 377      * to tolerate before sweeping through the queue unlinking
 378      * cancelled nodes that were not unlinked upon initial
 379      * removal. See above for explanation. The value must be at least
 380      * two to avoid useless sweeps when removing trailing nodes.
 381      */
 382     static final int SWEEP_THRESHOLD = 32;
 383 
 384     /**
 385      * Queue nodes. Uses Object, not E, for items to allow forgetting
 386      * them after use.  Writes that are intrinsically ordered wrt
 387      * other accesses or CASes use simple relaxed forms.
 388      */
 389     static final class Node implements ForkJoinPool.ManagedBlocker {
 390         final boolean isData;   // false if this is a request node
 391         volatile Object item;   // initially non-null if isData; CASed to match
 392         volatile Node next;
 393         volatile Thread waiter; // null when not waiting for a match
 394 
 395         /**
 396          * Constructs a data node holding item if item is non-null,
 397          * else a request node.  Uses relaxed write because item can
 398          * only be seen after piggy-backing publication via CAS.
 399          */
 400         Node(Object item) {
 401             ITEM.set(this, item);
 402             isData = (item != null);
 403         }
 404 
 405         /** Constructs a (matched data) dummy node. */
 406         Node() {
 407             isData = true;
 408         }
 409 
 410         final boolean casNext(Node cmp, Node val) {
 411             // assert val != null;
 412             return NEXT.compareAndSet(this, cmp, val);
 413         }
 414 
 415         final boolean casItem(Object cmp, Object val) {
 416             // assert isData == (cmp != null);
 417             // assert isData == (val == null);
 418             // assert !(cmp instanceof Node);
 419             return ITEM.compareAndSet(this, cmp, val);
 420         }
 421 
 422         /**
 423          * Links node to itself to avoid garbage retention.  Called
 424          * only after CASing head field, so uses relaxed write.
 425          */
 426         final void selfLink() {
 427             // assert isMatched();
 428             NEXT.setRelease(this, this);
 429         }
 430 
 431         final void appendRelaxed(Node next) {
 432             // assert next != null;
 433             // assert this.next == null;
 434             NEXT.setOpaque(this, next);
 435         }
 436 
 437         /**
 438          * Returns true if this node has been matched, including the
 439          * case of artificial matches due to cancellation.
 440          */
 441         final boolean isMatched() {
 442             return isData == (item == null);
 443         }
 444 
 445         /** Tries to CAS-match this node; if successful, wakes waiter. */
 446         final boolean tryMatch(Object cmp, Object val) {
 447             if (casItem(cmp, val)) {
 448                 LockSupport.unpark(waiter);
 449                 return true;
 450             }
 451             return false;
 452         }
 453 
 454         /**
 455          * Returns true if a node with the given mode cannot be
 456          * appended to this node because this node is unmatched and
 457          * has opposite data mode.
 458          */
 459         final boolean cannotPrecede(boolean haveData) {
 460             boolean d = isData;
 461             return d != haveData && d != (item == null);
 462         }
 463 
 464         public final boolean isReleasable() {
 465             return (isData == (item == null)) ||
 466                 Thread.currentThread().isInterrupted();
 467         }
 468 
 469         public final boolean block() {
 470             while (!isReleasable()) LockSupport.park();
 471             return true;
 472         }
 473 
 474         private static final long serialVersionUID = -3375979862319811754L;
 475     }
 476 
 477     /**
 478      * A node from which the first live (non-matched) node (if any)
 479      * can be reached in O(1) time.
 480      * Invariants:
 481      * - all live nodes are reachable from head via .next
 482      * - head != null
 483      * - (tmp = head).next != tmp || tmp != head
 484      * Non-invariants:
 485      * - head may or may not be live
 486      * - it is permitted for tail to lag behind head, that is, for tail
 487      *   to not be reachable from head!
 488      */
 489     transient volatile Node head;
 490 
 491     /**
 492      * A node from which the last node on list (that is, the unique
 493      * node with node.next == null) can be reached in O(1) time.
 494      * Invariants:
 495      * - the last node is always reachable from tail via .next
 496      * - tail != null
 497      * Non-invariants:
 498      * - tail may or may not be live
 499      * - it is permitted for tail to lag behind head, that is, for tail
 500      *   to not be reachable from head!
 501      * - tail.next may or may not be self-linked.
 502      */
 503     private transient volatile Node tail;
 504 
 505     /** The number of apparent failures to unsplice cancelled nodes */
 506     private transient volatile boolean needSweep;
 507 
 508     private boolean casTail(Node cmp, Node val) {
 509         // assert cmp != null;
 510         // assert val != null;
 511         return TAIL.compareAndSet(this, cmp, val);
 512     }
 513 
 514     private boolean casHead(Node cmp, Node val) {
 515         return HEAD.compareAndSet(this, cmp, val);
 516     }
 517 
 518     /**
 519      * Tries to CAS pred.next (or head, if pred is null) from c to p.
 520      * Caller must ensure that we're not unlinking the trailing node.
 521      */
 522     private boolean tryCasSuccessor(Node pred, Node c, Node p) {
 523         // assert p != null;
 524         // assert c.isData != (c.item != null);
 525         // assert c != p;
 526         if (pred != null)
 527             return pred.casNext(c, p);
 528         if (casHead(c, p)) {
 529             c.selfLink();
 530             return true;
 531         }
 532         return false;
 533     }
 534 
 535     /**
 536      * Collapses dead (matched) nodes between pred and q.
 537      * @param pred the last known live node, or null if none
 538      * @param c the first dead node
 539      * @param p the last dead node
 540      * @param q p.next: the next live node, or null if at end
 541      * @return pred if pred still alive and CAS succeeded; else p
 542      */
 543     private Node skipDeadNodes(Node pred, Node c, Node p, Node q) {
 544         // assert pred != c;
 545         // assert p != q;
 546         // assert c.isMatched();
 547         // assert p.isMatched();
 548         if (q == null) {
 549             // Never unlink trailing node.
 550             if (c == p) return pred;
 551             q = p;
 552         }
 553         return (tryCasSuccessor(pred, c, q)
 554                 && (pred == null || !pred.isMatched()))
 555             ? pred : p;
 556     }
 557 
 558     /**
 559      * Collapses dead (matched) nodes from h (which was once head) to p.
 560      * Caller ensures all nodes from h up to and including p are dead.
 561      */
 562     private void skipDeadNodesNearHead(Node h, Node p) {
 563         // assert h != null;
 564         // assert h != p;
 565         // assert p.isMatched();
 566         for (;;) {
 567             final Node q;
 568             if ((q = p.next) == null) break;
 569             else if (!q.isMatched()) { p = q; break; }
 570             else if (p == (p = q)) return;
 571         }
 572         if (casHead(h, p))
 573             h.selfLink();
 574     }
 575 
 576     /* Possible values for "how" argument in xfer method. */
 577 
 578     private static final int NOW   = 0; // for untimed poll, tryTransfer
 579     private static final int ASYNC = 1; // for offer, put, add
 580     private static final int SYNC  = 2; // for transfer, take
 581     private static final int TIMED = 3; // for timed poll, tryTransfer
 582 
 583     /**
 584      * Implements all queuing methods. See above for explanation.
 585      *
 586      * @param e the item or null for take
 587      * @param haveData true if this is a put, else a take
 588      * @param how NOW, ASYNC, SYNC, or TIMED
 589      * @param nanos timeout in nanosecs, used only if mode is TIMED
 590      * @return an item if matched, else e
 591      * @throws NullPointerException if haveData mode but e is null
 592      */
 593     @SuppressWarnings("unchecked")
 594     private E xfer(E e, boolean haveData, int how, long nanos) {
 595         if (haveData && (e == null))
 596             throw new NullPointerException();
 597 
 598         restart: for (Node s = null, t = null, h = null;;) {
 599             for (Node p = (t != (t = tail) && t.isData == haveData) ? t
 600                      : (h = head);; ) {
 601                 final Node q; final Object item;
 602                 if (p.isData != haveData
 603                     && haveData == ((item = p.item) == null)) {
 604                     if (h == null) h = head;
 605                     if (p.tryMatch(item, e)) {
 606                         if (h != p) skipDeadNodesNearHead(h, p);
 607                         return (E) item;
 608                     }
 609                 }
 610                 if ((q = p.next) == null) {
 611                     if (how == NOW) return e;
 612                     if (s == null) s = new Node(e);
 613                     if (!p.casNext(null, s)) continue;
 614                     if (p != t) casTail(t, s);
 615                     if (how == ASYNC) return e;
 616                     return awaitMatch(s, p, e, (how == TIMED), nanos);
 617                 }
 618                 if (p == (p = q)) continue restart;
 619             }
 620         }
 621     }
 622 
 623     /**
 624      * Possibly blocks until node s is matched or caller gives up.
 625      *
 626      * @param s the waiting node
 627      * @param pred the predecessor of s, or null if unknown (the null
 628      * case does not occur in any current calls but may in possible
 629      * future extensions)
 630      * @param e the comparison value for checking match
 631      * @param timed if true, wait only until timeout elapses
 632      * @param nanos timeout in nanosecs, used only if timed is true
 633      * @return matched item, or e if unmatched on interrupt or timeout
 634      */
 635     @SuppressWarnings("unchecked")
 636     private E awaitMatch(Node s, Node pred, E e, boolean timed, long nanos) {
 637         final boolean isData = s.isData;
 638         final long deadline = timed ? System.nanoTime() + nanos : 0L;
 639         final Thread w = Thread.currentThread();
 640         int stat = -1;                   // -1: may yield, +1: park, else 0
 641         Object item;
 642         while ((item = s.item) == e) {
 643             if (needSweep)               // help clean
 644                 sweep();
 645             else if ((timed && nanos <= 0L) || w.isInterrupted()) {
 646                 if (s.casItem(e, (e == null) ? s : null)) {
 647                     unsplice(pred, s);   // cancelled
 648                     return e;
 649                 }
 650             }
 651             else if (stat <= 0) {
 652                 if (pred != null && pred.next == s) {
 653                     if (stat < 0 &&
 654                         (pred.isData != isData || pred.isMatched())) {
 655                         stat = 0;        // yield once if first
 656                         Thread.yield();
 657                     }
 658                     else {
 659                         stat = 1;
 660                         s.waiter = w;    // enable unpark
 661                     }
 662                 }                        // else signal in progress
 663             }
 664             else if ((item = s.item) != e)
 665                 break;                   // recheck
 666             else if (!timed) {
 667                 LockSupport.setCurrentBlocker(this);
 668                 try {
 669                     ForkJoinPool.managedBlock(s);
 670                 } catch (InterruptedException cannotHappen) { }
 671                 LockSupport.setCurrentBlocker(null);
 672             }
 673             else {
 674                 nanos = deadline - System.nanoTime();
 675                 if (nanos > SPIN_FOR_TIMEOUT_THRESHOLD)
 676                     LockSupport.parkNanos(this, nanos);
 677             }
 678         }
 679         if (stat == 1)
 680             WAITER.set(s, null);
 681         if (!isData)
 682             ITEM.set(s, s);              // self-link to avoid garbage
 683         return (E) item;
 684     }
 685 
 686     /* -------------- Traversal methods -------------- */
 687 
 688     /**
 689      * Returns the first unmatched data node, or null if none.
 690      * Callers must recheck if the returned node is unmatched
 691      * before using.
 692      */
 693     final Node firstDataNode() {
 694         Node first = null;
 695         restartFromHead: for (;;) {
 696             Node h = head, p = h;
 697             while (p != null) {
 698                 if (p.item != null) {
 699                     if (p.isData) {
 700                         first = p;
 701                         break;
 702                     }
 703                 }
 704                 else if (!p.isData)
 705                     break;
 706                 final Node q;
 707                 if ((q = p.next) == null)
 708                     break;
 709                 if (p == (p = q))
 710                     continue restartFromHead;
 711             }
 712             if (p != h && casHead(h, p))
 713                 h.selfLink();
 714             return first;
 715         }
 716     }
 717 
 718     /**
 719      * Traverses and counts unmatched nodes of the given mode.
 720      * Used by methods size and getWaitingConsumerCount.
 721      */
 722     private int countOfMode(boolean data) {
 723         restartFromHead: for (;;) {
 724             int count = 0;
 725             for (Node p = head; p != null;) {
 726                 if (!p.isMatched()) {
 727                     if (p.isData != data)
 728                         return 0;
 729                     if (++count == Integer.MAX_VALUE)
 730                         break;  // @see Collection.size()
 731                 }
 732                 if (p == (p = p.next))
 733                     continue restartFromHead;
 734             }
 735             return count;
 736         }
 737     }
 738 
 739     public String toString() {
 740         String[] a = null;
 741         restartFromHead: for (;;) {
 742             int charLength = 0;
 743             int size = 0;
 744             for (Node p = head; p != null;) {
 745                 Object item = p.item;
 746                 if (p.isData) {
 747                     if (item != null) {
 748                         if (a == null)
 749                             a = new String[4];
 750                         else if (size == a.length)
 751                             a = Arrays.copyOf(a, 2 * size);
 752                         String s = item.toString();
 753                         a[size++] = s;
 754                         charLength += s.length();
 755                     }
 756                 } else if (item == null)
 757                     break;
 758                 if (p == (p = p.next))
 759                     continue restartFromHead;
 760             }
 761 
 762             if (size == 0)
 763                 return "[]";
 764 
 765             return Helpers.toString(a, size, charLength);
 766         }
 767     }
 768 
 769     private Object[] toArrayInternal(Object[] a) {
 770         Object[] x = a;
 771         restartFromHead: for (;;) {
 772             int size = 0;
 773             for (Node p = head; p != null;) {
 774                 Object item = p.item;
 775                 if (p.isData) {
 776                     if (item != null) {
 777                         if (x == null)
 778                             x = new Object[4];
 779                         else if (size == x.length)
 780                             x = Arrays.copyOf(x, 2 * (size + 4));
 781                         x[size++] = item;
 782                     }
 783                 } else if (item == null)
 784                     break;
 785                 if (p == (p = p.next))
 786                     continue restartFromHead;
 787             }
 788             if (x == null)
 789                 return new Object[0];
 790             else if (a != null && size <= a.length) {
 791                 if (a != x)
 792                     System.arraycopy(x, 0, a, 0, size);
 793                 if (size < a.length)
 794                     a[size] = null;
 795                 return a;
 796             }
 797             return (size == x.length) ? x : Arrays.copyOf(x, size);
 798         }
 799     }
 800 
 801     /**
 802      * Returns an array containing all of the elements in this queue, in
 803      * proper sequence.
 804      *
 805      * <p>The returned array will be "safe" in that no references to it are
 806      * maintained by this queue.  (In other words, this method must allocate
 807      * a new array).  The caller is thus free to modify the returned array.
 808      *
 809      * <p>This method acts as bridge between array-based and collection-based
 810      * APIs.
 811      *
 812      * @return an array containing all of the elements in this queue
 813      */
 814     public Object[] toArray() {
 815         return toArrayInternal(null);
 816     }
 817 
 818     /**
 819      * Returns an array containing all of the elements in this queue, in
 820      * proper sequence; the runtime type of the returned array is that of
 821      * the specified array.  If the queue fits in the specified array, it
 822      * is returned therein.  Otherwise, a new array is allocated with the
 823      * runtime type of the specified array and the size of this queue.
 824      *
 825      * <p>If this queue fits in the specified array with room to spare
 826      * (i.e., the array has more elements than this queue), the element in
 827      * the array immediately following the end of the queue is set to
 828      * {@code null}.
 829      *
 830      * <p>Like the {@link #toArray()} method, this method acts as bridge between
 831      * array-based and collection-based APIs.  Further, this method allows
 832      * precise control over the runtime type of the output array, and may,
 833      * under certain circumstances, be used to save allocation costs.
 834      *
 835      * <p>Suppose {@code x} is a queue known to contain only strings.
 836      * The following code can be used to dump the queue into a newly
 837      * allocated array of {@code String}:
 838      *
 839      * <pre> {@code String[] y = x.toArray(new String[0]);}</pre>
 840      *
 841      * Note that {@code toArray(new Object[0])} is identical in function to
 842      * {@code toArray()}.
 843      *
 844      * @param a the array into which the elements of the queue are to
 845      *          be stored, if it is big enough; otherwise, a new array of the
 846      *          same runtime type is allocated for this purpose
 847      * @return an array containing all of the elements in this queue
 848      * @throws ArrayStoreException if the runtime type of the specified array
 849      *         is not a supertype of the runtime type of every element in
 850      *         this queue
 851      * @throws NullPointerException if the specified array is null
 852      */
 853     @SuppressWarnings("unchecked")
 854     public <T> T[] toArray(T[] a) {
 855         Objects.requireNonNull(a);
 856         return (T[]) toArrayInternal(a);
 857     }
 858 
 859     /**
 860      * Weakly-consistent iterator.
 861      *
 862      * Lazily updated ancestor is expected to be amortized O(1) remove(),
 863      * but O(n) in the worst case, when lastRet is concurrently deleted.
 864      */
 865     final class Itr implements Iterator<E> {
 866         private Node nextNode;   // next node to return item for
 867         private E nextItem;      // the corresponding item
 868         private Node lastRet;    // last returned node, to support remove
 869         private Node ancestor;   // Helps unlink lastRet on remove()
 870 
 871         /**
 872          * Moves to next node after pred, or first node if pred null.
 873          */
 874         @SuppressWarnings("unchecked")
 875         private void advance(Node pred) {
 876             for (Node p = (pred == null) ? head : pred.next, c = p;
 877                  p != null; ) {
 878                 final Object item;
 879                 if ((item = p.item) != null && p.isData) {
 880                     nextNode = p;
 881                     nextItem = (E) item;
 882                     if (c != p)
 883                         tryCasSuccessor(pred, c, p);
 884                     return;
 885                 }
 886                 else if (!p.isData && item == null)
 887                     break;
 888                 if (c != p && !tryCasSuccessor(pred, c, c = p)) {
 889                     pred = p;
 890                     c = p = p.next;
 891                 }
 892                 else if (p == (p = p.next)) {
 893                     pred = null;
 894                     c = p = head;
 895                 }
 896             }
 897             nextItem = null;
 898             nextNode = null;
 899         }
 900 
 901         Itr() {
 902             advance(null);
 903         }
 904 
 905         public final boolean hasNext() {
 906             return nextNode != null;
 907         }
 908 
 909         public final E next() {
 910             final Node p;
 911             if ((p = nextNode) == null) throw new NoSuchElementException();
 912             E e = nextItem;
 913             advance(lastRet = p);
 914             return e;
 915         }
 916 
 917         public void forEachRemaining(Consumer<? super E> action) {
 918             Objects.requireNonNull(action);
 919             Node q = null;
 920             for (Node p; (p = nextNode) != null; advance(q = p))
 921                 action.accept(nextItem);
 922             if (q != null)
 923                 lastRet = q;
 924         }
 925 
 926         public final void remove() {
 927             final Node lastRet = this.lastRet;
 928             if (lastRet == null)
 929                 throw new IllegalStateException();
 930             this.lastRet = null;
 931             if (lastRet.item == null)   // already deleted?
 932                 return;
 933             // Advance ancestor, collapsing intervening dead nodes
 934             Node pred = ancestor;
 935             for (Node p = (pred == null) ? head : pred.next, c = p, q;
 936                  p != null; ) {
 937                 if (p == lastRet) {
 938                     final Object item;
 939                     if ((item = p.item) != null)
 940                         p.tryMatch(item, null);
 941                     if ((q = p.next) == null) q = p;
 942                     if (c != q) tryCasSuccessor(pred, c, q);
 943                     ancestor = pred;
 944                     return;
 945                 }
 946                 final Object item; final boolean pAlive;
 947                 if (pAlive = ((item = p.item) != null && p.isData)) {
 948                     // exceptionally, nothing to do
 949                 }
 950                 else if (!p.isData && item == null)
 951                     break;
 952                 if ((c != p && !tryCasSuccessor(pred, c, c = p)) || pAlive) {
 953                     pred = p;
 954                     c = p = p.next;
 955                 }
 956                 else if (p == (p = p.next)) {
 957                     pred = null;
 958                     c = p = head;
 959                 }
 960             }
 961             // traversal failed to find lastRet; must have been deleted;
 962             // leave ancestor at original location to avoid overshoot;
 963             // better luck next time!
 964 
 965             // assert lastRet.isMatched();
 966         }
 967     }
 968 
 969     /** A customized variant of Spliterators.IteratorSpliterator */
 970     final class LTQSpliterator implements Spliterator<E> {
 971         static final int MAX_BATCH = 1 << 25;  // max batch array size;
 972         Node current;       // current node; null until initialized
 973         int batch;          // batch size for splits
 974         boolean exhausted;  // true when no more nodes
 975         LTQSpliterator() {}
 976 
 977         public Spliterator<E> trySplit() {
 978             Node p, q;
 979             if ((p = current()) == null || (q = p.next) == null)
 980                 return null;
 981             int i = 0, n = batch = Math.min(batch + 1, MAX_BATCH);
 982             Object[] a = null;
 983             do {
 984                 final Object item = p.item;
 985                 if (p.isData) {
 986                     if (item != null) {
 987                         if (a == null)
 988                             a = new Object[n];
 989                         a[i++] = item;
 990                     }
 991                 } else if (item == null) {
 992                     p = null;
 993                     break;
 994                 }
 995                 if (p == (p = q))
 996                     p = firstDataNode();
 997             } while (p != null && (q = p.next) != null && i < n);
 998             setCurrent(p);
 999             return (i == 0) ? null :
1000                 Spliterators.spliterator(a, 0, i, (Spliterator.ORDERED |
1001                                                    Spliterator.NONNULL |
1002                                                    Spliterator.CONCURRENT));
1003         }
1004 
1005         public void forEachRemaining(Consumer<? super E> action) {
1006             Objects.requireNonNull(action);
1007             final Node p;
1008             if ((p = current()) != null) {
1009                 current = null;
1010                 exhausted = true;
1011                 forEachFrom(action, p);
1012             }
1013         }
1014 
1015         @SuppressWarnings("unchecked")
1016         public boolean tryAdvance(Consumer<? super E> action) {
1017             Objects.requireNonNull(action);
1018             Node p;
1019             if ((p = current()) != null) {
1020                 E e = null;
1021                 do {
1022                     final Object item = p.item;
1023                     final boolean isData = p.isData;
1024                     if (p == (p = p.next))
1025                         p = head;
1026                     if (isData) {
1027                         if (item != null) {
1028                             e = (E) item;
1029                             break;
1030                         }
1031                     }
1032                     else if (item == null)
1033                         p = null;
1034                 } while (p != null);
1035                 setCurrent(p);
1036                 if (e != null) {
1037                     action.accept(e);
1038                     return true;
1039                 }
1040             }
1041             return false;
1042         }
1043 
1044         private void setCurrent(Node p) {
1045             if ((current = p) == null)
1046                 exhausted = true;
1047         }
1048 
1049         private Node current() {
1050             Node p;
1051             if ((p = current) == null && !exhausted)
1052                 setCurrent(p = firstDataNode());
1053             return p;
1054         }
1055 
1056         public long estimateSize() { return Long.MAX_VALUE; }
1057 
1058         public int characteristics() {
1059             return (Spliterator.ORDERED |
1060                     Spliterator.NONNULL |
1061                     Spliterator.CONCURRENT);
1062         }
1063     }
1064 
1065     /**
1066      * Returns a {@link Spliterator} over the elements in this queue.
1067      *
1068      * <p>The returned spliterator is
1069      * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
1070      *
1071      * <p>The {@code Spliterator} reports {@link Spliterator#CONCURRENT},
1072      * {@link Spliterator#ORDERED}, and {@link Spliterator#NONNULL}.
1073      *
1074      * @implNote
1075      * The {@code Spliterator} implements {@code trySplit} to permit limited
1076      * parallelism.
1077      *
1078      * @return a {@code Spliterator} over the elements in this queue
1079      * @since 1.8
1080      */
1081     public Spliterator<E> spliterator() {
1082         return new LTQSpliterator();
1083     }
1084 
1085     /* -------------- Removal methods -------------- */
1086 
1087     /**
1088      * Unsplices (now or later) the given deleted/cancelled node with
1089      * the given predecessor.
1090      *
1091      * @param pred a node that was at one time known to be the
1092      * predecessor of s
1093      * @param s the node to be unspliced
1094      */
1095     final void unsplice(Node pred, Node s) {
1096         // assert pred != null;
1097         // assert pred != s;
1098         // assert s != null;
1099         // assert s.isMatched();
1100         // assert (SWEEP_THRESHOLD & (SWEEP_THRESHOLD - 1)) == 0;
1101         s.waiter = null; // disable signals
1102         /*
1103          * See above for rationale. Briefly: if pred still points to
1104          * s, try to unlink s.  If s cannot be unlinked, because it is
1105          * trailing node or pred might be unlinked, and neither pred
1106          * nor s are head or offlist, set needSweep;
1107          */
1108         if (pred != null && pred.next == s) {
1109             Node n = s.next;
1110             if (n == null ||
1111                 (n != s && pred.casNext(s, n) && pred.isMatched())) {
1112                 for (;;) {               // check if at, or could be, head
1113                     Node h = head;
1114                     if (h == pred || h == s)
1115                         return;          // at head or list empty
1116                     if (!h.isMatched())
1117                         break;
1118                     Node hn = h.next;
1119                     if (hn == null)
1120                         return;          // now empty
1121                     if (hn != h && casHead(h, hn))
1122                         h.selfLink();  // advance head
1123                 }
1124                 if (pred.next != pred && s.next != s)
1125                     needSweep = true;
1126             }
1127         }
1128     }
1129 
1130     /**
1131      * Unlinks matched (typically cancelled) nodes encountered in a
1132      * traversal from head.
1133      */
1134     private void sweep() {
1135         needSweep = false;
1136         for (Node p = head, s, n; p != null && (s = p.next) != null; ) {
1137             if (!s.isMatched())
1138                 // Unmatched nodes are never self-linked
1139                 p = s;
1140             else if ((n = s.next) == null) // trailing node is pinned
1141                 break;
1142             else if (s == n)    // stale
1143                 // No need to also check for p == s, since that implies s == n
1144                 p = head;
1145             else
1146                 p.casNext(s, n);
1147         }
1148     }
1149 
1150     /**
1151      * Creates an initially empty {@code LinkedTransferQueue}.
1152      */
1153     public LinkedTransferQueue() {
1154         head = tail = new Node();
1155     }
1156 
1157     /**
1158      * Creates a {@code LinkedTransferQueue}
1159      * initially containing the elements of the given collection,
1160      * added in traversal order of the collection's iterator.
1161      *
1162      * @param c the collection of elements to initially contain
1163      * @throws NullPointerException if the specified collection or any
1164      *         of its elements are null
1165      */
1166     public LinkedTransferQueue(Collection<? extends E> c) {
1167         Node h = null, t = null;
1168         for (E e : c) {
1169             Node newNode = new Node(Objects.requireNonNull(e));
1170             if (h == null)
1171                 h = t = newNode;
1172             else
1173                 t.appendRelaxed(t = newNode);
1174         }
1175         if (h == null)
1176             h = t = new Node();
1177         head = h;
1178         tail = t;
1179     }
1180 
1181     /**
1182      * Inserts the specified element at the tail of this queue.
1183      * As the queue is unbounded, this method will never block.
1184      *
1185      * @throws NullPointerException if the specified element is null
1186      */
1187     public void put(E e) {
1188         xfer(e, true, ASYNC, 0L);
1189     }
1190 
1191     /**
1192      * Inserts the specified element at the tail of this queue.
1193      * As the queue is unbounded, this method will never block or
1194      * return {@code false}.
1195      *
1196      * @return {@code true} (as specified by
1197      *  {@link BlockingQueue#offer(Object,long,TimeUnit) BlockingQueue.offer})
1198      * @throws NullPointerException if the specified element is null
1199      */
1200     public boolean offer(E e, long timeout, TimeUnit unit) {
1201         xfer(e, true, ASYNC, 0L);
1202         return true;
1203     }
1204 
1205     /**
1206      * Inserts the specified element at the tail of this queue.
1207      * As the queue is unbounded, this method will never return {@code false}.
1208      *
1209      * @return {@code true} (as specified by {@link Queue#offer})
1210      * @throws NullPointerException if the specified element is null
1211      */
1212     public boolean offer(E e) {
1213         xfer(e, true, ASYNC, 0L);
1214         return true;
1215     }
1216 
1217     /**
1218      * Inserts the specified element at the tail of this queue.
1219      * As the queue is unbounded, this method will never throw
1220      * {@link IllegalStateException} or return {@code false}.
1221      *
1222      * @return {@code true} (as specified by {@link Collection#add})
1223      * @throws NullPointerException if the specified element is null
1224      */
1225     public boolean add(E e) {
1226         xfer(e, true, ASYNC, 0L);
1227         return true;
1228     }
1229 
1230     /**
1231      * Transfers the element to a waiting consumer immediately, if possible.
1232      *
1233      * <p>More precisely, transfers the specified element immediately
1234      * if there exists a consumer already waiting to receive it (in
1235      * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1236      * otherwise returning {@code false} without enqueuing the element.
1237      *
1238      * @throws NullPointerException if the specified element is null
1239      */
1240     public boolean tryTransfer(E e) {
1241         return xfer(e, true, NOW, 0L) == null;
1242     }
1243 
1244     /**
1245      * Transfers the element to a consumer, waiting if necessary to do so.
1246      *
1247      * <p>More precisely, transfers the specified element immediately
1248      * if there exists a consumer already waiting to receive it (in
1249      * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1250      * else inserts the specified element at the tail of this queue
1251      * and waits until the element is received by a consumer.
1252      *
1253      * @throws NullPointerException if the specified element is null
1254      */
1255     public void transfer(E e) throws InterruptedException {
1256         if (xfer(e, true, SYNC, 0L) != null) {
1257             Thread.interrupted(); // failure possible only due to interrupt
1258             throw new InterruptedException();
1259         }
1260     }
1261 
1262     /**
1263      * Transfers the element to a consumer if it is possible to do so
1264      * before the timeout elapses.
1265      *
1266      * <p>More precisely, transfers the specified element immediately
1267      * if there exists a consumer already waiting to receive it (in
1268      * {@link #take} or timed {@link #poll(long,TimeUnit) poll}),
1269      * else inserts the specified element at the tail of this queue
1270      * and waits until the element is received by a consumer,
1271      * returning {@code false} if the specified wait time elapses
1272      * before the element can be transferred.
1273      *
1274      * @throws NullPointerException if the specified element is null
1275      */
1276     public boolean tryTransfer(E e, long timeout, TimeUnit unit)
1277         throws InterruptedException {
1278         if (xfer(e, true, TIMED, unit.toNanos(timeout)) == null)
1279             return true;
1280         if (!Thread.interrupted())
1281             return false;
1282         throw new InterruptedException();
1283     }
1284 
1285     public E take() throws InterruptedException {
1286         E e = xfer(null, false, SYNC, 0L);
1287         if (e != null)
1288             return e;
1289         Thread.interrupted();
1290         throw new InterruptedException();
1291     }
1292 
1293     public E poll(long timeout, TimeUnit unit) throws InterruptedException {
1294         E e = xfer(null, false, TIMED, unit.toNanos(timeout));
1295         if (e != null || !Thread.interrupted())
1296             return e;
1297         throw new InterruptedException();
1298     }
1299 
1300     public E poll() {
1301         return xfer(null, false, NOW, 0L);
1302     }
1303 
1304     /**
1305      * @throws NullPointerException     {@inheritDoc}
1306      * @throws IllegalArgumentException {@inheritDoc}
1307      */
1308     public int drainTo(Collection<? super E> c) {
1309         Objects.requireNonNull(c);
1310         if (c == this)
1311             throw new IllegalArgumentException();
1312         int n = 0;
1313         for (E e; (e = poll()) != null; n++)
1314             c.add(e);
1315         return n;
1316     }
1317 
1318     /**
1319      * @throws NullPointerException     {@inheritDoc}
1320      * @throws IllegalArgumentException {@inheritDoc}
1321      */
1322     public int drainTo(Collection<? super E> c, int maxElements) {
1323         Objects.requireNonNull(c);
1324         if (c == this)
1325             throw new IllegalArgumentException();
1326         int n = 0;
1327         for (E e; n < maxElements && (e = poll()) != null; n++)
1328             c.add(e);
1329         return n;
1330     }
1331 
1332     /**
1333      * Returns an iterator over the elements in this queue in proper sequence.
1334      * The elements will be returned in order from first (head) to last (tail).
1335      *
1336      * <p>The returned iterator is
1337      * <a href="package-summary.html#Weakly"><i>weakly consistent</i></a>.
1338      *
1339      * @return an iterator over the elements in this queue in proper sequence
1340      */
1341     public Iterator<E> iterator() {
1342         return new Itr();
1343     }
1344 
1345     public E peek() {
1346         restartFromHead: for (;;) {
1347             for (Node p = head; p != null;) {
1348                 Object item = p.item;
1349                 if (p.isData) {
1350                     if (item != null) {
1351                         @SuppressWarnings("unchecked") E e = (E) item;
1352                         return e;
1353                     }
1354                 }
1355                 else if (item == null)
1356                     break;
1357                 if (p == (p = p.next))
1358                     continue restartFromHead;
1359             }
1360             return null;
1361         }
1362     }
1363 
1364     /**
1365      * Returns {@code true} if this queue contains no elements.
1366      *
1367      * @return {@code true} if this queue contains no elements
1368      */
1369     public boolean isEmpty() {
1370         return firstDataNode() == null;
1371     }
1372 
1373     public boolean hasWaitingConsumer() {
1374         restartFromHead: for (;;) {
1375             for (Node p = head; p != null;) {
1376                 Object item = p.item;
1377                 if (p.isData) {
1378                     if (item != null)
1379                         break;
1380                 }
1381                 else if (item == null)
1382                     return true;
1383                 if (p == (p = p.next))
1384                     continue restartFromHead;
1385             }
1386             return false;
1387         }
1388     }
1389 
1390     /**
1391      * Returns the number of elements in this queue.  If this queue
1392      * contains more than {@code Integer.MAX_VALUE} elements, returns
1393      * {@code Integer.MAX_VALUE}.
1394      *
1395      * <p>Beware that, unlike in most collections, this method is
1396      * <em>NOT</em> a constant-time operation. Because of the
1397      * asynchronous nature of these queues, determining the current
1398      * number of elements requires an O(n) traversal.
1399      *
1400      * @return the number of elements in this queue
1401      */
1402     public int size() {
1403         return countOfMode(true);
1404     }
1405 
1406     public int getWaitingConsumerCount() {
1407         return countOfMode(false);
1408     }
1409 
1410     /**
1411      * Removes a single instance of the specified element from this queue,
1412      * if it is present.  More formally, removes an element {@code e} such
1413      * that {@code o.equals(e)}, if this queue contains one or more such
1414      * elements.
1415      * Returns {@code true} if this queue contained the specified element
1416      * (or equivalently, if this queue changed as a result of the call).
1417      *
1418      * @param o element to be removed from this queue, if present
1419      * @return {@code true} if this queue changed as a result of the call
1420      */
1421     public boolean remove(Object o) {
1422         if (o == null) return false;
1423         restartFromHead: for (;;) {
1424             for (Node p = head, pred = null; p != null; ) {
1425                 Node q = p.next;
1426                 final Object item;
1427                 if ((item = p.item) != null) {
1428                     if (p.isData) {
1429                         if (o.equals(item) && p.tryMatch(item, null)) {
1430                             skipDeadNodes(pred, p, p, q);
1431                             return true;
1432                         }
1433                         pred = p; p = q; continue;
1434                     }
1435                 }
1436                 else if (!p.isData)
1437                     break;
1438                 for (Node c = p;; q = p.next) {
1439                     if (q == null || !q.isMatched()) {
1440                         pred = skipDeadNodes(pred, c, p, q); p = q; break;
1441                     }
1442                     if (p == (p = q)) continue restartFromHead;
1443                 }
1444             }
1445             return false;
1446         }
1447     }
1448 
1449     /**
1450      * Returns {@code true} if this queue contains the specified element.
1451      * More formally, returns {@code true} if and only if this queue contains
1452      * at least one element {@code e} such that {@code o.equals(e)}.
1453      *
1454      * @param o object to be checked for containment in this queue
1455      * @return {@code true} if this queue contains the specified element
1456      */
1457     public boolean contains(Object o) {
1458         if (o == null) return false;
1459         restartFromHead: for (;;) {
1460             for (Node p = head, pred = null; p != null; ) {
1461                 Node q = p.next;
1462                 final Object item;
1463                 if ((item = p.item) != null) {
1464                     if (p.isData) {
1465                         if (o.equals(item))
1466                             return true;
1467                         pred = p; p = q; continue;
1468                     }
1469                 }
1470                 else if (!p.isData)
1471                     break;
1472                 for (Node c = p;; q = p.next) {
1473                     if (q == null || !q.isMatched()) {
1474                         pred = skipDeadNodes(pred, c, p, q); p = q; break;
1475                     }
1476                     if (p == (p = q)) continue restartFromHead;
1477                 }
1478             }
1479             return false;
1480         }
1481     }
1482 
1483     /**
1484      * Always returns {@code Integer.MAX_VALUE} because a
1485      * {@code LinkedTransferQueue} is not capacity constrained.
1486      *
1487      * @return {@code Integer.MAX_VALUE} (as specified by
1488      *         {@link BlockingQueue#remainingCapacity()})
1489      */
1490     public int remainingCapacity() {
1491         return Integer.MAX_VALUE;
1492     }
1493 
1494     /**
1495      * Saves this queue to a stream (that is, serializes it).
1496      *
1497      * @param s the stream
1498      * @throws java.io.IOException if an I/O error occurs
1499      * @serialData All of the elements (each an {@code E}) in
1500      * the proper order, followed by a null
1501      */
1502     private void writeObject(java.io.ObjectOutputStream s)
1503         throws java.io.IOException {
1504         s.defaultWriteObject();
1505         for (E e : this)
1506             s.writeObject(e);
1507         // Use trailing null as sentinel
1508         s.writeObject(null);
1509     }
1510 
1511     /**
1512      * Reconstitutes this queue from a stream (that is, deserializes it).
1513      * @param s the stream
1514      * @throws ClassNotFoundException if the class of a serialized object
1515      *         could not be found
1516      * @throws java.io.IOException if an I/O error occurs
1517      */
1518     private void readObject(java.io.ObjectInputStream s)
1519         throws java.io.IOException, ClassNotFoundException {
1520 
1521         // Read in elements until trailing null sentinel found
1522         Node h = null, t = null;
1523         for (Object item; (item = s.readObject()) != null; ) {
1524             Node newNode = new Node(item);
1525             if (h == null)
1526                 h = t = newNode;
1527             else
1528                 t.appendRelaxed(t = newNode);
1529         }
1530         if (h == null)
1531             h = t = new Node();
1532         head = h;
1533         tail = t;
1534     }
1535 
1536     /**
1537      * @throws NullPointerException {@inheritDoc}
1538      */
1539     public boolean removeIf(Predicate<? super E> filter) {
1540         Objects.requireNonNull(filter);
1541         return bulkRemove(filter);
1542     }
1543 
1544     /**
1545      * @throws NullPointerException {@inheritDoc}
1546      */
1547     public boolean removeAll(Collection<?> c) {
1548         Objects.requireNonNull(c);
1549         return bulkRemove(e -> c.contains(e));
1550     }
1551 
1552     /**
1553      * @throws NullPointerException {@inheritDoc}
1554      */
1555     public boolean retainAll(Collection<?> c) {
1556         Objects.requireNonNull(c);
1557         return bulkRemove(e -> !c.contains(e));
1558     }
1559 
1560     public void clear() {
1561         bulkRemove(e -> true);
1562     }
1563 
1564     /**
1565      * Tolerate this many consecutive dead nodes before CAS-collapsing.
1566      * Amortized cost of clear() is (1 + 1/MAX_HOPS) CASes per element.
1567      */
1568     private static final int MAX_HOPS = 8;
1569 
1570     /** Implementation of bulk remove methods. */
1571     @SuppressWarnings("unchecked")
1572     private boolean bulkRemove(Predicate<? super E> filter) {
1573         boolean removed = false;
1574         restartFromHead: for (;;) {
1575             int hops = MAX_HOPS;
1576             // c will be CASed to collapse intervening dead nodes between
1577             // pred (or head if null) and p.
1578             for (Node p = head, c = p, pred = null, q; p != null; p = q) {
1579                 q = p.next;
1580                 final Object item; boolean pAlive;
1581                 if (pAlive = ((item = p.item) != null && p.isData)) {
1582                     if (filter.test((E) item)) {
1583                         if (p.tryMatch(item, null))
1584                             removed = true;
1585                         pAlive = false;
1586                     }
1587                 }
1588                 else if (!p.isData && item == null)
1589                     break;
1590                 if (pAlive || q == null || --hops == 0) {
1591                     // p might already be self-linked here, but if so:
1592                     // - CASing head will surely fail
1593                     // - CASing pred's next will be useless but harmless.
1594                     if ((c != p && !tryCasSuccessor(pred, c, c = p))
1595                         || pAlive) {
1596                         // if CAS failed or alive, abandon old pred
1597                         hops = MAX_HOPS;
1598                         pred = p;
1599                         c = q;
1600                     }
1601                 } else if (p == q)
1602                     continue restartFromHead;
1603             }
1604             return removed;
1605         }
1606     }
1607 
1608     /**
1609      * Runs action on each element found during a traversal starting at p.
1610      * If p is null, the action is not run.
1611      */
1612     @SuppressWarnings("unchecked")
1613     void forEachFrom(Consumer<? super E> action, Node p) {
1614         for (Node pred = null; p != null; ) {
1615             Node q = p.next;
1616             final Object item;
1617             if ((item = p.item) != null) {
1618                 if (p.isData) {
1619                     action.accept((E) item);
1620                     pred = p; p = q; continue;
1621                 }
1622             }
1623             else if (!p.isData)
1624                 break;
1625             for (Node c = p;; q = p.next) {
1626                 if (q == null || !q.isMatched()) {
1627                     pred = skipDeadNodes(pred, c, p, q); p = q; break;
1628                 }
1629                 if (p == (p = q)) { pred = null; p = head; break; }
1630             }
1631         }
1632     }
1633 
1634     /**
1635      * @throws NullPointerException {@inheritDoc}
1636      */
1637     public void forEach(Consumer<? super E> action) {
1638         Objects.requireNonNull(action);
1639         forEachFrom(action, head);
1640     }
1641 
1642     // VarHandle mechanics
1643     private static final VarHandle HEAD;
1644     private static final VarHandle TAIL;
1645     static final VarHandle ITEM;
1646     static final VarHandle NEXT;
1647     static final VarHandle WAITER;
1648     static {
1649         try {
1650             MethodHandles.Lookup l = MethodHandles.lookup();
1651             HEAD = l.findVarHandle(LinkedTransferQueue.class, "head",
1652                                    Node.class);
1653             TAIL = l.findVarHandle(LinkedTransferQueue.class, "tail",
1654                                    Node.class);
1655             ITEM = l.findVarHandle(Node.class, "item", Object.class);
1656             NEXT = l.findVarHandle(Node.class, "next", Node.class);
1657             WAITER = l.findVarHandle(Node.class, "waiter", Thread.class);
1658         } catch (ReflectiveOperationException e) {
1659             throw new ExceptionInInitializerError(e);
1660         }
1661 
1662         // Reduce the risk of rare disastrous classloading in first call to
1663         // LockSupport.park: https://bugs.openjdk.java.net/browse/JDK-8074773
1664         Class<?> ensureLoaded = LockSupport.class;
1665     }
1666 }