[release/6.0-rc1] Don't cache CanReuse value (#35676)

* Don't cache CanReuse value

* Comment on when to use CanReuse

Co-authored-by: Sebastien Ros <sebastienros@gmail.com>

src/Servers/Kestrel/Core/src/Internal/Http2/Http2Stream.cs

@@ -41,7 +41,6 @@ public void Initialize(Http2StreamContext context)
         {
             base.Initialize(context);
 
-            CanReuse = false;
             _decrementCalled = false;
             _completionState = StreamCompletionFlags.None;
             InputRemaining = null;
@@ -107,7 +106,16 @@ public bool ReceivedEmptyRequestBody
             }
         }
 
-        public bool CanReuse { get; private set; }
+        // We only want to reuse a stream that was not aborted and has completely finished writing.
+        // This ensures Http2OutputProducer.ProcessDataWrites is in the correct state to be reused.
+
+        // CanReuse must be evaluated on the main frame-processing looping after the stream is removed
+        // from the connection's active streams collection. This is required because a RST_STREAM
+        // frame could arrive after the END_STREAM flag is received. Only once the stream is removed
+        // from the connection's active stream collection can no longer be reset, and is safe to
+        // evaluate for pooling.
+
+        public bool CanReuse => !_connectionAborted && HasResponseCompleted;
 
         protected override void OnReset()
         {
@@ -164,9 +172,6 @@ public void CompleteStream(bool errored)
                 // connection's flow-control window.
                 _inputFlowControl.Abort();
 
-                // We only want to reuse a stream that was not aborted and has completely finished writing.
-                // This ensures Http2OutputProducer.ProcessDataWrites is in the correct state to be reused.
-                CanReuse = !_connectionAborted && HasResponseCompleted;
             }
             finally
             {