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/*
* Copyright 2008-present MongoDB, Inc.
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
/*
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package com.mongodb.internal.connection;
import java.util.AbstractCollection;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Deque;
import java.util.Iterator;
import java.util.NoSuchElementException;
import java.util.concurrent.atomic.AtomicReference;
/**
* A concurrent linked-list implementation of a {@link Deque} (double-ended queue).
*
* This class should not be considered a part of the public API.
* Concurrent insertion, removal, and access
* operations execute safely across multiple threads. Iterators are
* weakly consistent, returning elements reflecting the state
* of the deque at some point at or since the creation of the
* iterator. They do not throw {@link
* java.util.ConcurrentModificationException}, and may proceed concurrently with
* other operations.
*
*
This class and its iterators implement all of the
* optional methods of the {@link java.util.Collection} and {@link
* java.util.Iterator} interfaces. Like most other concurrent collection
* implementations, this class does not permit the use of
* {@code null} elements. because some null arguments and return
* values cannot be reliably distinguished from the absence of
* elements. Arbitrarily, the {@link java.util.Collection#remove} method is
* mapped to {@code removeFirstOccurrence}, and {@link
* java.util.Collection#add} is mapped to {@code addLast}.
*
*
Beware that, unlike in most collections, the {@code size}
* method is NOT a constant-time operation. Because of the
* asynchronous nature of these deques, determining the current number
* of elements requires a traversal of the elements.
*
*
This class is {@code Serializable}, but relies on default
* serialization mechanisms. Usually, it is a better idea for any
* serializable class using a {@code ConcurrentLinkedDeque} to instead
* serialize a snapshot of the elements obtained by method
* {@code toArray}.
*
* @author Doug Lea
* @param the type of elements held in this collection
*/
public class ConcurrentLinkedDeque
extends AbstractCollection
implements Deque, java.io.Serializable {
// CHECKSTYLE:OFF
/*
* This is an adaptation of an algorithm described in Paul
* Martin's "A Practical Lock-Free Doubly-Linked List". Sun Labs
* Tech report. The basic idea is to primarily rely on
* next-pointers to ensure consistency. Prev-pointers are in part
* optimistic, reconstructed using forward pointers as needed.
* The main forward list uses a variant of HM-list algorithm
* similar to the one used in ConcurrentSkipListMap class, but a
* little simpler. It is also basically similar to the approach
* in Edya Ladan-Mozes and Nir Shavit "An Optimistic Approach to
* Lock-Free FIFO Queues" in DISC04.
*
* Quoting a summary in Paul Martin's tech report:
*
* All cleanups work to maintain these invariants:
* (1) forward pointers are the ground truth.
* (2) forward pointers to dead nodes can be improved by swinging them
* further forward around the dead node.
* (2.1) forward pointers are still correct when pointing to dead
* nodes, and forward pointers from dead nodes are left
* as they were when the node was deleted.
* (2.2) multiple dead nodes may point forward to the same node.
* (3) backward pointers were correct when they were installed
* (3.1) backward pointers are correct when pointing to any
* node which points forward to them, but since more than
* one forward pointer may point to them, the live one is best.
* (4) backward pointers that are out of date due to deletion
* point to a deleted node, and need to point further back until
* they point to the live node that points to their source.
* (5) backward pointers that are out of date due to insertion
* point too far backwards, so shortening their scope (by searching
* forward) fixes them.
* (6) backward pointers from a dead node cannot be "improved" since
* there may be no live node pointing forward to their origin.
* (However, it does no harm to try to improve them while
* racing with a deletion.)
*
*
* Notation guide for local variables
* n, b, f : a node, its predecessor, and successor
* s : some other successor
*/
/**
* Linked Nodes. As a minor efficiency hack, this class
* opportunistically inherits from AtomicReference, with the
* atomic ref used as the "next" link.
*
* Nodes are in doubly-linked lists. There are three
* kinds of special nodes, distinguished by:
* * The list header has a null prev link.
* * The list trailer has a null next link.
* * A deletion marker has a prev link pointing to itself.
* All three kinds of special nodes have null element fields.
*
* Regular nodes have non-null element, next, and prev fields. To
* avoid visible inconsistencies when deletions overlap element
* replacement, replacements are done by replacing the node, not
* just setting the element.
*
* Nodes can be traversed by read-only ConcurrentLinkedDeque class
* operations just by following raw next pointers, so long as they
* ignore any special nodes seen along the way. (This is automated
* in method forward.) However, traversal using prev pointers is
* not guaranteed to see all live nodes since a prev pointer of a
* deleted node can become unrecoverably stale.
*/
static final class Node extends AtomicReference> {
private volatile Node prev;
final E element;
private static final long serialVersionUID = 876323262645176354L;
/** Creates a node with given contents. */
Node(E element, Node next, Node prev) {
super(next);
this.prev = prev;
this.element = element;
}
/** Creates a marker node with given successor. */
Node(Node next) {
super(next);
this.prev = this;
this.element = null;
}
/**
* Gets next link (which is actually the value held
* as atomic reference).
*/
private Node getNext() {
return get();
}
/**
* Sets next link.
* @param n the next node
*/
void setNext(Node n) {
set(n);
}
/**
* compareAndSet next link
*/
private boolean casNext(Node cmp, Node val) {
return compareAndSet(cmp, val);
}
/**
* Gets prev link.
*/
private Node getPrev() {
return prev;
}
/**
* Sets prev link.
* @param b the previous node
*/
void setPrev(Node b) {
prev = b;
}
/**
* Returns true if this is a header, trailer, or marker node.
*/
boolean isSpecial() {
return element == null;
}
/**
* Returns true if this is a trailer node.
*/
boolean isTrailer() {
return getNext() == null;
}
/**
* Returns true if this is a header node.
*/
boolean isHeader() {
return getPrev() == null;
}
/**
* Returns true if this is a marker node.
*/
boolean isMarker() {
return getPrev() == this;
}
/**
* Returns true if this node is followed by a marker node,
* meaning that this node is deleted.
*
* @return true if this node is deleted
*/
boolean isDeleted() {
Node f = getNext();
return f != null && f.isMarker();
}
/**
* Returns next node, ignoring deletion marker.
*/
private Node nextNonmarker() {
Node f = getNext();
return (f == null || !f.isMarker()) ? f : f.getNext();
}
/**
* Returns the next non-deleted node, swinging next pointer
* around any encountered deleted nodes, and also patching up
* successor's prev link to point back to this. Returns
* null if this node is trailer so has no successor.
*
* @return successor, or null if no such
*/
Node successor() {
Node f = nextNonmarker();
for (;;) {
if (f == null)
return null;
if (!f.isDeleted()) {
if (f.getPrev() != this && !isDeleted())
f.setPrev(this); // relink f's prev
return f;
}
Node s = f.nextNonmarker();
if (f == getNext())
casNext(f, s); // unlink f
f = s;
}
}
/**
* Returns the apparent predecessor of target by searching
* forward for it, starting at this node, patching up pointers
* while traversing. Used by predecessor().
*
* @return target's predecessor, or null if not found
*/
private Node findPredecessorOf(Node target) {
Node n = this;
for (;;) {
Node f = n.successor();
if (f == target)
return n;
if (f == null)
return null;
n = f;
}
}
/**
* Returns the previous non-deleted node, patching up pointers
* as needed. Returns null if this node is header so has no
* successor. May also return null if this node is deleted,
* so doesn't have a distinct predecessor.
*
* @return predecessor, or null if not found
*/
Node predecessor() {
Node n = this;
for (;;) {
Node b = n.getPrev();
if (b == null)
return n.findPredecessorOf(this);
Node s = b.getNext();
if (s == this)
return b;
if (s == null || !s.isMarker()) {
Node p = b.findPredecessorOf(this);
if (p != null)
return p;
}
n = b;
}
}
/**
* Returns the next node containing a nondeleted user element.
* Use for forward list traversal.
*
* @return successor, or null if no such
*/
Node forward() {
Node f = successor();
return (f == null || f.isSpecial()) ? null : f;
}
/**
* Returns previous node containing a nondeleted user element, if
* possible. Use for backward list traversal, but beware that
* if this method is called from a deleted node, it might not
* be able to determine a usable predecessor.
*
* @return predecessor, or null if no such could be found
*/
Node back() {
Node f = predecessor();
return (f == null || f.isSpecial()) ? null : f;
}
/**
* Tries to insert a node holding element as successor, failing
* if this node is deleted.
*
* @param element the element
* @return the new node, or null on failure
*/
Node append(E element) {
for (;;) {
Node f = getNext();
if (f == null || f.isMarker())
return null;
Node x = new Node(element, f, this);
if (casNext(f, x)) {
f.setPrev(x); // optimistically link
return x;
}
}
}
/**
* Tries to insert a node holding element as predecessor, failing
* if no live predecessor can be found to link to.
*
* @param element the element
* @return the new node, or null on failure
*/
Node prepend(E element) {
for (;;) {
Node b = predecessor();
if (b == null)
return null;
Node x = new Node(element, this, b);
if (b.casNext(this, x)) {
setPrev(x); // optimistically link
return x;
}
}
}
/**
* Tries to mark this node as deleted, failing if already
* deleted or if this node is header or trailer.
*
* @return true if successful
*/
boolean delete() {
Node b = getPrev();
Node f = getNext();
if (b != null && f != null && !f.isMarker() &&
casNext(f, new Node(f))) {
if (b.casNext(this, f))
f.setPrev(b);
return true;
}
return false;
}
/**
* Tries to insert a node holding element to replace this node.
* failing if already deleted. A currently unused proof of
* concept that demonstrates atomic node content replacement.
*
* Although this implementation ensures that exactly one
* version of this Node is alive at a given time, it fails to
* maintain atomicity in the sense that iterators may
* encounter both the old and new versions of the element.
*
* @param newElement the new element
* @return the new node, or null on failure
*/
Node replace(E newElement) {
for (;;) {
Node b = getPrev();
Node f = getNext();
if (b == null || f == null || f.isMarker())
return null;
Node x = new Node(newElement, f, b);
if (casNext(f, new Node(x))) {
b.successor(); // to relink b
x.successor(); // to relink f
return x;
}
}
}
}
// Minor convenience utilities
/**
* Returns true if given reference is non-null and isn't a header,
* trailer, or marker.
*
* @param n (possibly null) node
* @return true if n exists as a user node
*/
private static boolean usable(Node n) {
return n != null && !n.isSpecial();
}
/**
* Throws NullPointerException if argument is null.
*
* @param v the element
*/
private static void checkNotNull(Object v) {
if (v == null)
throw new NullPointerException();
}
/**
* Returns element unless it is null, in which case throws
* NoSuchElementException.
*
* @param v the element
* @return the element
*/
private E screenNullResult(E v) {
if (v == null)
throw new NoSuchElementException();
return v;
}
/**
* Creates an array list and fills it with elements of this list.
* Used by toArray.
*
* @return the array list
*/
private ArrayList toArrayList() {
ArrayList c = new ArrayList();
for (Node n = header.forward(); n != null; n = n.forward())
c.add(n.element);
return c;
}
// Fields and constructors
private static final long serialVersionUID = 876323262645176354L;
/**
* List header. First usable node is at header.forward().
*/
private final Node header;
/**
* List trailer. Last usable node is at trailer.back().
*/
private final Node trailer;
/**
* Constructs an empty deque.
*/
public ConcurrentLinkedDeque() {
Node h = new Node(null, null, null);
Node t = new Node(null, null, h);
h.setNext(t);
header = h;
trailer = t;
}
/**
* Constructs a deque initially containing the elements of
* the given collection, added in traversal order of the
* collection's iterator.
*
* @param c the collection of elements to initially contain
* @throws NullPointerException if the specified collection or any
* of its elements are null
*/
public ConcurrentLinkedDeque(Collection c) {
this();
addAll(c);
}
/**
* Inserts the specified element at the front of this deque.
*
* @throws NullPointerException {@inheritDoc}
*/
public void addFirst(E e) {
checkNotNull(e);
while (header.append(e) == null)
;
}
/**
* Inserts the specified element at the end of this deque.
* This is identical in function to the {@code add} method.
*
* @throws NullPointerException {@inheritDoc}
*/
public void addLast(E e) {
checkNotNull(e);
while (trailer.prepend(e) == null)
;
}
/**
* Inserts the specified element at the front of this deque.
*
* @return {@code true} always
* @throws NullPointerException {@inheritDoc}
*/
public boolean offerFirst(E e) {
addFirst(e);
return true;
}
/**
* Inserts the specified element at the end of this deque.
*
* This method is equivalent to {@link #add}.
*
* @return {@code true} always
* @throws NullPointerException {@inheritDoc}
*/
public boolean offerLast(E e) {
addLast(e);
return true;
}
public E peekFirst() {
Node n = header.successor();
return (n == null) ? null : n.element;
}
public E peekLast() {
Node n = trailer.predecessor();
return (n == null) ? null : n.element;
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E getFirst() {
return screenNullResult(peekFirst());
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E getLast() {
return screenNullResult(peekLast());
}
public E pollFirst() {
for (;;) {
Node n = header.successor();
if (!usable(n))
return null;
if (n.delete())
return n.element;
}
}
public E pollLast() {
for (;;) {
Node n = trailer.predecessor();
if (!usable(n))
return null;
if (n.delete())
return n.element;
}
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E removeFirst() {
return screenNullResult(pollFirst());
}
/**
* @throws NoSuchElementException {@inheritDoc}
*/
public E removeLast() {
return screenNullResult(pollLast());
}
// *** Queue and stack methods ***
/**
* Inserts the specified element at the tail of this queue.
*
* @return {@code true}
* (as specified by {@link java.util.Queue#offer Queue.offer})
* @throws NullPointerException if the specified element is null
*/
public boolean offer(E e) {
return offerLast(e);
}
/**
* Inserts the specified element at the tail of this deque.
*
* @return {@code true} (as specified by {@link Collection#add})
* @throws NullPointerException if the specified element is null
*/
public boolean add(E e) {
return offerLast(e);
}
public E poll() { return pollFirst(); }
public E remove() { return removeFirst(); }
public E peek() { return peekFirst(); }
public E element() { return getFirst(); }
public void push(E e) { addFirst(e); }
public E pop() { return removeFirst(); }
/**
* Removes the first element {@code e} such that
* {@code o.equals(e)}, if such an element exists in this deque.
* If the deque does not contain the element, it is unchanged.
*
* @param o element to be removed from this deque, if present
* @return {@code true} if the deque contained the specified element
* @throws NullPointerException if the specified element is {@code null}
*/
public boolean removeFirstOccurrence(Object o) {
checkNotNull(o);
for (;;) {
Node n = header.forward();
for (;;) {
if (n == null)
return false;
if (o.equals(n.element)) {
if (n.delete())
return true;
else
break; // restart if interference
}
n = n.forward();
}
}
}
/**
* Removes the last element {@code e} such that
* {@code o.equals(e)}, if such an element exists in this deque.
* If the deque does not contain the element, it is unchanged.
*
* @param o element to be removed from this deque, if present
* @return {@code true} if the deque contained the specified element
* @throws NullPointerException if the specified element is {@code null}
*/
public boolean removeLastOccurrence(Object o) {
checkNotNull(o);
for (;;) {
Node s = trailer;
for (;;) {
Node n = s.back();
if (s.isDeleted() || (n != null && n.successor() != s))
break; // restart if pred link is suspect.
if (n == null)
return false;
if (o.equals(n.element)) {
if (n.delete())
return true;
else
break; // restart if interference
}
s = n;
}
}
}
/**
* Returns {@code true} if this deque contains at least one
* element {@code e} such that {@code o.equals(e)}.
*
* @param o element whose presence in this deque is to be tested
* @return {@code true} if this deque contains the specified element
*/
public boolean contains(Object o) {
if (o == null) return false;
for (Node n = header.forward(); n != null; n = n.forward())
if (o.equals(n.element))
return true;
return false;
}
/**
* Returns {@code true} if this collection contains no elements.
*
* @return {@code true} if this collection contains no elements
*/
public boolean isEmpty() {
return !usable(header.successor());
}
/**
* Returns the number of elements in this deque. If this deque
* contains more than {@code Integer.MAX_VALUE} elements, it
* returns {@code Integer.MAX_VALUE}.
*
* Beware that, unlike in most collections, this method is
* NOT a constant-time operation. Because of the
* asynchronous nature of these deques, determining the current
* number of elements requires traversing them all to count them.
* Additionally, it is possible for the size to change during
* execution of this method, in which case the returned result
* will be inaccurate. Thus, this method is typically not very
* useful in concurrent applications.
*
* @return the number of elements in this deque
*/
public int size() {
long count = 0;
for (Node n = header.forward(); n != null; n = n.forward())
++count;
return (count >= Integer.MAX_VALUE) ? Integer.MAX_VALUE : (int) count;
}
/**
* Removes the first element {@code e} such that
* {@code o.equals(e)}, if such an element exists in this deque.
* If the deque does not contain the element, it is unchanged.
*
* @param o element to be removed from this deque, if present
* @return {@code true} if the deque contained the specified element
* @throws NullPointerException if the specified element is {@code null}
*/
public boolean remove(Object o) {
return removeFirstOccurrence(o);
}
/**
* Appends all of the elements in the specified collection to the end of
* this deque, in the order that they are returned by the specified
* collection's iterator. The behavior of this operation is undefined if
* the specified collection is modified while the operation is in
* progress. (This implies that the behavior of this call is undefined if
* the specified Collection is this deque, and this deque is nonempty.)
*
* @param c the elements to be inserted into this deque
* @return {@code true} if this deque changed as a result of the call
* @throws NullPointerException if {@code c} or any element within it
* is {@code null}
*/
public boolean addAll(Collection c) {
Iterator it = c.iterator();
if (!it.hasNext())
return false;
do {
addLast(it.next());
} while (it.hasNext());
return true;
}
/**
* Removes all of the elements from this deque.
*/
public void clear() {
while (pollFirst() != null)
;
}
/**
* Returns an array containing all of the elements in this deque, in
* proper sequence (from first to last element).
*
* The returned array will be "safe" in that no references to it are
* maintained by this deque. (In other words, this method must allocate
* a new array). The caller is thus free to modify the returned array.
*
*
This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this deque
*/
public Object[] toArray() {
return toArrayList().toArray();
}
/**
* Returns an array containing all of the elements in this deque,
* in proper sequence (from first to last element); the runtime
* type of the returned array is that of the specified array. If
* the deque fits in the specified array, it is returned therein.
* Otherwise, a new array is allocated with the runtime type of
* the specified array and the size of this deque.
*
*
If this deque fits in the specified array with room to spare
* (i.e., the array has more elements than this deque), the element in
* the array immediately following the end of the deque is set to
* {@code null}.
*
*
Like the {@link #toArray()} method, this method acts as bridge between
* array-based and collection-based APIs. Further, this method allows
* precise control over the runtime type of the output array, and may,
* under certain circumstances, be used to save allocation costs.
*
*
Suppose {@code x} is a deque known to contain only strings.
* The following code can be used to dump the deque into a newly
* allocated array of {@code String}:
*
*
* String[] y = x.toArray(new String[0]);
*
* Note that {@code toArray(new Object[0])} is identical in function to
* {@code toArray()}.
*
* @param a the array into which the elements of the deque are to
* be stored, if it is big enough; otherwise, a new array of the
* same runtime type is allocated for this purpose
* @return an array containing all of the elements in this deque
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in
* this deque
* @throws NullPointerException if the specified array is null
*/
public T[] toArray(T[] a) {
return toArrayList().toArray(a);
}
/**
* Returns an iterator over the elements in this deque in proper sequence.
* The elements will be returned in order from first (head) to last (tail).
* The returned {@code Iterator} is a "weakly consistent" iterator that
* will never throw {@link java.util.ConcurrentModificationException
* ConcurrentModificationException},
* and guarantees to traverse elements as they existed upon
* construction of the iterator, and may (but is not guaranteed to)
* reflect any modifications subsequent to construction.
*
* @return an iterator over the elements in this deque in proper sequence
*/
public RemovalReportingIterator iterator() {
return new CLDIterator();
}
final class CLDIterator implements RemovalReportingIterator {
Node last;
Node next = header.forward();
public boolean hasNext() {
return next != null;
}
public E next() {
Node l = last = next;
if (l == null)
throw new NoSuchElementException();
next = next.forward();
return l.element;
}
public void remove() {
reportingRemove();
}
@Override
public boolean reportingRemove() {
Node l = last;
if (l == null)
throw new IllegalStateException();
boolean successfullyRemoved = l.delete();
while (!successfullyRemoved && !l.isDeleted()) {
successfullyRemoved = l.delete();
}
return successfullyRemoved;
}
}
/**
* Not yet implemented.
*/
public Iterator descendingIterator() {
throw new UnsupportedOperationException();
}
public interface RemovalReportingIterator extends Iterator {
/**
* Removes from the underlying collection the last element returned by this iterator and reports whether the current element was
* removed by the call. This method can be called only once per call to {@link #next}.
*
* @return true if the element was successfully removed by this call, false if the element had already been removed by a concurrent
* removal
* @throws IllegalStateException if the {@code next} method has not
* yet been called, or the {@code remove} method has already
* been called after the last call to the {@code next}
* method
*/
boolean reportingRemove();
}
}