lgoldstein commented on a change in pull request #166:
URL: https://github.com/apache/mina-sshd/pull/166#discussion_r489990336
##########
File path:
sshd-core/src/main/java/org/apache/sshd/common/io/nio2/Nio2Session.java
##########
@@ -382,7 +385,32 @@ protected void handleReadCycleFailure(ByteBuffer buffer,
Readable bufReader, Thr
exceptionCaught(exc);
}
+ @Override
+ public void suspendRead() {
+ log.trace("suspendRead({})", this);
+ if (suspendRead.compareAndSet(false, true)) {
+ log.debug("suspendRead({}) requesting read suspension", this);
+ }
+ }
+
+ @Override
+ public void resumeRead() {
+ log.trace("resumeRead({})", this);
+ Runnable runnable = readRunnable.getAndSet(null);
+ suspendRead.set(false);
+ if (runnable != null) {
+ log.debug("resumeRead({}) resuming read", this);
+ runnable.run();
+ }
+ }
+
protected void doReadCycle(ByteBuffer buffer,
Nio2CompletionHandler<Integer, Object> completion) {
+ if (suspendRead.get()) {
+ log.debug("doReadCycle({}) suspending reading", this);
+ readRunnable.set(() -> doReadCycle(buffer, completion));
+ return;
+ }
+
Review comment:
Seems to me that there is a chance of a race condition as follows:
* Thread A calls `suspendRead` and marks `suspendedRead` as _true_
* Thread A is pre-empted by thread B that enters `doReadCycle`
* Thread B sees that `suspendRead` is _true_ and enters the `if` block, but
is preempted by thread A before calling`readRunnable.set(...)`
* Thread A calls `resumeRead` - calls `readRunnable.getAndSet(null);` and
gets _null_ - so thinks there is nothing to do
* Thread B resumes its execution and calls `.set(....)` but no one will call
the set _Runnable_
Result: reading is not resumed.
I think the suspend/resume should have some mutual exclusion mechanism to
prevent this.
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