[PD-Disagg] Fix bootstrap server race condition when prefill workers not yet registered (#19288)

Co-authored-by: Cursor <cursoragent@cursor.com>

python/sglang/srt/disaggregation/common/conn.py

@@ -157,13 +157,26 @@ class CommonKVManager(BaseKVManager):
         with self.failure_lock:
             self.failure_records[bootstrap_room] = failure_reason
 
-    def ensure_parallel_info(self, bootstrap_addr: str) -> bool:
+    def ensure_parallel_info(
+        self, bootstrap_addr: str, max_retries: int = 20, retry_interval: float = 1.0
+    ) -> bool:
         """Fetch and cache prefill parallel info if not yet available.
         Returns True if info is available (cached or freshly fetched).
+        Retries with backoff if the prefill server hasn't registered yet.
         """
         if bootstrap_addr in self.prefill_info_table:
             return True
-        info = self._fetch_prefill_server_info(bootstrap_addr)
+        info = None
+        for attempt in range(max_retries):
+            info = self._fetch_prefill_server_info(bootstrap_addr)
+            if info is not None:
+                break
+            if attempt < max_retries - 1:
+                logger.info(
+                    f"Prefill server info not available from {bootstrap_addr}, "
+                    f"retrying ({attempt + 1}/{max_retries})..."
+                )
+                time.sleep(retry_interval)
         if info is None:
             return False
 
@@ -595,6 +608,7 @@ class CommonKVBootstrapServer(BaseKVBootstrapServer):
             int, Dict[int, Dict[int, Dict[str, Union[str, int]]]]
         ] = {}
         self.room_to_dp_rank: Dict[int, Dict[str, Union[int, float]]] = {}
+        self._registered_count = 0
         self.entry_cleanup_interval = (
             envs.SGLANG_DISAGGREGATION_BOOTSTRAP_ENTRY_CLEANUP_INTERVAL.get()
         )
@@ -606,6 +620,14 @@ class CommonKVBootstrapServer(BaseKVBootstrapServer):
     def run(self):
         self.thread.start()
 
+    def _is_ready(self) -> bool:
+        if self.attn_tp_size is None or self.pp_size is None:
+            return False
+        # TODO: verify this expected count is correct for all parallelism
+        # combinations (CP / DP attention / system DP / TP / PP).
+        expected = self.dp_size * self.attn_tp_size * self.pp_size
+        return self._registered_count >= expected
+
     def _setup_routes(self):
         self.app.router.add_route("*", "/route", self._handle_route)
         self.app.router.add_post("/register_dp_rank", self._handle_register_dp_rank)
@@ -676,8 +698,11 @@ class CommonKVBootstrapServer(BaseKVBootstrapServer):
                 "rank_ip": rank_ip,
                 "rank_port": rank_port,
             }
+            self._registered_count += 1
+            expected = self.dp_size * self.attn_tp_size * self.pp_size
             logger.debug(
                 f"Register prefill bootstrap: DP{dp_group} TP{attn_tp_rank} PP{pp_rank} with rank_ip: {rank_ip} and rank_port: {rank_port}"
+                f" ({self._registered_count}/{expected} registered)"
             )
 
         return web.Response(text="OK", status=200)
@@ -694,6 +719,12 @@ class CommonKVBootstrapServer(BaseKVBootstrapServer):
             and int(prefill_dp_rank) == -1
             and int(target_pp_rank) == -1
         ):
+            if not self._is_ready():
+                return web.Response(
+                    text=f"Prefill server not fully registered yet"
+                    f" ({self._registered_count} workers registered).",
+                    status=503,
+                )
             info = PrefillServerInfo(
                 attn_tp_size=self.attn_tp_size,
                 dp_size=self.dp_size,
@@ -707,16 +738,27 @@ class CommonKVBootstrapServer(BaseKVBootstrapServer):
             )
             return web.json_response(dataclasses.asdict(info), status=200)
 
-        # Find corresponding prefill info
-        async with self.lock:
-            bootstrap_info = self.prefill_port_table[int(prefill_dp_rank)][
-                int(engine_rank)
-            ][int(target_pp_rank)]
+        if not self._is_ready():
+            return web.Response(
+                text=f"Prefill server not fully registered yet"
+                f" ({self._registered_count} workers registered).",
+                status=503,
+            )
 
-        if bootstrap_info is not None:
-            return web.json_response(bootstrap_info, status=200)
-        else:
-            return web.Response(text="Bootstrap info not Found", status=404)
+        # Find corresponding prefill info
+        try:
+            async with self.lock:
+                bootstrap_info = self.prefill_port_table[int(prefill_dp_rank)][
+                    int(engine_rank)
+                ][int(target_pp_rank)]
+        except KeyError:
+            return web.Response(
+                text=f"Bootstrap info not found for dp_rank={prefill_dp_rank} "
+                f"engine_rank={engine_rank} pp_rank={target_pp_rank}",
+                status=404,
+            )
+
+        return web.json_response(bootstrap_info, status=200)
 
     async def _handle_register_dp_rank(self, request: web.Request):
         data = await request.json()