Hi Bowen, > I've been reading the source code and the Java documentation. > > In the code > <https://github.com/apache/cassandra/blob/8d91b469afd3fcafef7ef85c10c8acc11703ba2d/src/java/org/apache/cassandra/utils/concurrent/WaitQueue.java#L244>: > > public void signalAll() > { > // ... > Iterator<RegisteredSignal> iter = queue.iterator(); > while (iter.hasNext()) > { > RegisteredSignal signal = iter.next(); > // ... > iter.remove(); // <--- here > } > } > > and in the documentation > <https://docs.oracle.com/javase/8/docs/api///java/util/Iterator.html#remove-->: > > > The behavior of an iterator is unspecified if the underlying collection is > > modified while the iteration is in progress in any way other than by > > calling this method. The documentation for the Iterator interface is misleading because most implementations of this interface are not thread-safe, but the implementation provided by the ConcurrentLinkedQueue is different.
The documentation for the ConcurrentLinkedQueue.iterator() method says: public Iterator <dfile:///Users/termi/Library/Application%20Support/Dash/Versioned%20DocSets/Java%20-%20DHDocsetDownloader/SE11/Java.docset/Contents/Resources/Documents/java.base/java/util/Iterator.html><E <dfile:///Users/termi/Library/Application%20Support/Dash/Versioned%20DocSets/Java%20-%20DHDocsetDownloader/SE11/Java.docset/Contents/Resources/Documents/java.base/java/util/concurrent/ConcurrentLinkedQueue.html>> iterator() Returns an iterator over the elements in this queue in proper sequence. The elements will be returned in order from first (head) to last (tail). The returned iterator is weakly consistent <dfile:///Users/termi/Library/Application%20Support/Dash/Versioned%20DocSets/Java%20-%20DHDocsetDownloader/SE11/Java.docset/Contents/Resources/Documents/java.base/java/util/concurrent/package-summary.html#Weakly>. And when following the link for “weakly consistent”, we find the following definition: Most concurrent Collection implementations (including most Queues) also differ from the usual java.util conventions in that their Iterators <dfile:///Users/termi/Library/Application%20Support/Dash/Versioned%20DocSets/Java%20-%20DHDocsetDownloader/SE11/Java.docset/Contents/Resources/Documents/java.base/java/util/Iterator.html> and Spliterators <dfile:///Users/termi/Library/Application%20Support/Dash/Versioned%20DocSets/Java%20-%20DHDocsetDownloader/SE11/Java.docset/Contents/Resources/Documents/java.base/java/util/Spliterator.html> provide weakly consistent rather than fast-fail traversal: they may proceed concurrently with other operations they will never throw ConcurrentModificationException <dfile:///Users/termi/Library/Application%20Support/Dash/Versioned%20DocSets/Java%20-%20DHDocsetDownloader/SE11/Java.docset/Contents/Resources/Documents/java.base/java/util/ConcurrentModificationException.html> they are guaranteed to traverse elements as they existed upon construction exactly once, and may (but are not guaranteed to) reflect any modifications subsequent to construction. So, concurrent modifications to a ConcurrentLinkedQueue should not result in undefined behavior of an iterator returned by such a queue. -Sebastian
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