[Bugfix] fix race condition that leads to wrong order of token returned

During the startup of the api server the setup function is called
multiple times (every 5s). So the longer the longer the startup time
(generally for larger models) the more consumers are contending for the
output. This can then lead to race condition where the order of the
answer token is wrong.

Introduce here: #9973

References:
#10376
#10589
#10782

Signed-off-by: Jannis Schönleber <[email protected]>

vllm/engine/multiprocessing/client.py

@@ -255,7 +255,12 @@ async def setup(self):
         """Setup the client before it starts sending server requests."""
 
         # Start output_loop
-        self.output_loop = asyncio.create_task(self.run_output_handler_loop())
+        if self.output_loop is None:
+            # only generate once to avoid multiple concurrent output_loops
+            # this will lead to race conditions and wrong orders of tokens returned
+            # by the engine
+            # setup will be called multiple times during the startup of the engine
+            self.output_loop = asyncio.create_task(self.run_output_handler_loop())
 
         with self.get_data_socket() as socket:
             # Wait until server is ready.
@@ -264,8 +269,9 @@ async def setup(self):
             self.tracing_flag = response.tracing_enabled
 
             # Start health_loop.
-            self.health_loop = asyncio.create_task(
-                self.run_heartbeat_loop(timeout=VLLM_RPC_TIMEOUT))
+            if self.health_loop is None:
+                self.health_loop = asyncio.create_task(
+                    self.run_heartbeat_loop(timeout=VLLM_RPC_TIMEOUT))
 
     def close(self):
         """Destroy the ZeroMQ Context."""