[serve] Cherry-pick fix for router queue length metric

python/ray/serve/_private/router.py

@@ -929,20 +929,25 @@ async def assign_request(
             },
         )
 
-        query = Query(
-            args=list(request_args),
-            kwargs=request_kwargs,
-            metadata=request_meta,
-        )
-        await query.resolve_async_tasks()
-        result = await self._replica_scheduler.assign_replica(query)
-
-        self.num_queued_queries -= 1
-        self.num_queued_queries_gauge.set(
-            self.num_queued_queries,
-            tags={
-                "application": request_meta.app_name,
-            },
-        )
+        try:
+            query = Query(
+                args=list(request_args),
+                kwargs=request_kwargs,
+                metadata=request_meta,
+            )
+            await query.resolve_async_tasks()
+            result = await self._replica_scheduler.assign_replica(query)
 
-        return result
+            return result
+        finally:
+            # If the query is disconnected before assignment, this coroutine
+            # gets cancelled by the caller and an asyncio.CancelledError is
+            # raised. The finally block ensures that num_queued_queries
+            # is correctly decremented in this case.
+            self.num_queued_queries -= 1
+            self.num_queued_queries_gauge.set(
+                self.num_queued_queries,
+                tags={
+                    "application": request_meta.app_name,
+                },
+            )

python/ray/serve/tests/test_metrics.py

@@ -1,4 +1,6 @@
 import os
+from functools import partial
+from multiprocessing import Pool
 from typing import List, Dict, DefaultDict
 
 import requests
@@ -841,6 +843,63 @@ def verify_metrics():
     )
 
 
+def test_queued_queries_disconnected(serve_start_shutdown):
+    """Check that queued_queries decrements when queued requests disconnect."""
+
+    signal = SignalActor.remote()
+
+    @serve.deployment(
+        max_concurrent_queries=1,
+        graceful_shutdown_timeout_s=0.0001,
+    )
+    async def hang_on_first_request():
+        await signal.wait.remote()
+
+    serve.run(hang_on_first_request.bind())
+
+    print("Deployed hang_on_first_request deployment.")
+
+    def queue_size() -> float:
+        metrics = requests.get("http://127.0.0.1:9999").text
+        queue_size = -1
+        for line in metrics.split("\n"):
+            if "ray_serve_deployment_queued_queries" in line:
+                queue_size = line.split(" ")[-1]
+
+        return float(queue_size)
+
+    def first_request_executing(request_future) -> bool:
+        try:
+            request_future.get(timeout=0.1)
+        except Exception:
+            return ray.get(signal.cur_num_waiters.remote()) == 1
+
+    url = "http://localhost:8000/"
+
+    pool = Pool()
+
+    # Make a request to block the deployment from accepting other requests
+    fut = pool.apply_async(partial(requests.get, url))
+    wait_for_condition(lambda: first_request_executing(fut), timeout=5)
+    print("Executed first request.")
+
+    num_requests = 5
+    for _ in range(num_requests):
+        pool.apply_async(partial(requests.get, url))
+    print(f"Executed {num_requests} more requests.")
+
+    # First request should be processing. All others should be queued.
+    wait_for_condition(lambda: queue_size() == num_requests, timeout=15)
+    print("ray_serve_deployment_queued_queries updated successfully.")
+
+    # Disconnect all requests by terminating the process pool.
+    pool.terminate()
+    print("Terminated all requests.")
+
+    wait_for_condition(lambda: queue_size() == 0, timeout=15)
+    print("ray_serve_deployment_queued_queries updated successfully.")
+
+
 def test_actor_summary(serve_instance):
     @serve.deployment
     def f():
@@ -855,7 +914,7 @@ def f():
 
 
 def get_metric_dictionaries(name: str, timeout: float = 20) -> List[Dict]:
-    """Gets a list of metric's dictionaries from metrics' text output.
+    """Gets a list of metric's tags from metrics' text output.
 
     Return:
         Example: