Synchronize in WebAsyncManager onError/onTimeout

On connection loss, in a race between application thread and onError
callback trying to set the DeferredResult and dispatch, the onError
callback must not exit until dispatch completes. Currently, it may do
so because the DeferredResult has checks to bypasses locking or even
trying to dispatch if result is already set.

Closes gh-34192

Author: rstoyanchev

spring-web/src/main/java/org/springframework/web/context/request/async/WebAsyncManager.java

@@ -425,6 +425,11 @@ public void startDeferredResultProcessing(
 			}
 			try {
 				interceptorChain.triggerAfterTimeout(this.asyncWebRequest, deferredResult);
+				synchronized (WebAsyncManager.this) {
+					// If application thread set the DeferredResult first in a race,
+					// we must still not return until setConcurrentResultAndDispatch is done
+					return;
+				}
 			}
 			catch (Throwable ex) {
 				setConcurrentResultAndDispatch(ex);
@@ -437,6 +442,11 @@ public void startDeferredResultProcessing(
 			}
 			try {
 				interceptorChain.triggerAfterError(this.asyncWebRequest, deferredResult, ex);
+				synchronized (WebAsyncManager.this) {
+					// If application thread set the DeferredResult first in a race,
+					// we must still not return until setConcurrentResultAndDispatch is done
+					return;
+				}
 			}
 			catch (Throwable interceptorEx) {
 				setConcurrentResultAndDispatch(interceptorEx);