Fix issue #15698 Distinguish between timeout and task rejection in DefaultFuture

dubbo-remoting/dubbo-remoting-api/src/main/java/org/apache/dubbo/remoting/exchange/support/DefaultFuture.java

@@ -328,7 +328,7 @@ public void run(Timeout timeout) {
                 try {
                     executor.execute(() -> notifyTimeout(future));
                 } catch (RejectedExecutionException e) {
-                    notifyTimeout(future);
+                    notifyExecutionError(future, e);
                     throw e;
                 }
             } else {
@@ -345,5 +345,17 @@ private void notifyTimeout(DefaultFuture future) {
             // handle response.
             DefaultFuture.received(future.getChannel(), timeoutResponse, true);
         }
+
+        private void notifyExecutionError(DefaultFuture future, Throwable e) {
+            // create exception response.
+            Response errorResponse = new Response(future.getId());
+            // set error status
+            errorResponse.setStatus(Response.SERVER_THREADPOOL_EXHAUSTED_ERROR);
+            // set detailed error message
+            errorResponse.setErrorMessage("Executor rejected the task for handling timeout notification: "
+                    + e.getClass().getSimpleName() + " - " + e.getMessage());
+            // handle response.
+            DefaultFuture.received(future.getChannel(), errorResponse, true);
+        }
     }
 }