[GitHub] [mina-sshd] lgoldstein commented on a change in pull request #166: [SSHD-1070] Limit the amount of data that is kept in memory for forwa…

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lgoldstein commented on a change in pull request #166:
URL: https://github.com/apache/mina-sshd/pull/166#discussion_r489990336



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File path: sshd-core/src/main/java/org/apache/sshd/common/io/nio2/Nio2Session.java
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@@ -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(....)`
   
   Result: reading is not resumed.
   
   I think the suspend/resume should have some mutual exclusion mechanism to prevent this.




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