proxy: split session_affinity per mode + vLLM patch self-check (M4, S2)

- Replace the global session_affinity dict with two namespace-isolated
  ones (combined / prefill) so a session_id never indexes the wrong
  instance list across mode switches. Keep `session_affinity` as a
  read-only alias to the combined dict for any existing tooling.
- Add a startup _verify_vllm_patch() that scans
  vllm.v1.core.sched.scheduler.Scheduler for the original
  `assert req_id in self.requests` line. If the patch was not
  re-applied after a vLLM upgrade we now print a loud warning at
  lifespan startup instead of dying mid-experiment on a KV-transfer
  abort race.

scripts/cache_aware_proxy.py

@@ -169,7 +169,12 @@ global_args = None
 combined_instances: list[InstanceState] = []
 prefill_instances: list[InstanceState] = []
 decode_instances: list[InstanceState] = []
-session_affinity: dict[str, int] = {}
+# Session affinity is namespace-isolated: combined-mode and pd-sep mode index
+# different instance lists, so a shared dict could mis-route after a mode switch.
+session_affinity_combined: dict[str, int] = {}
+session_affinity_prefill: dict[str, int] = {}
+# Backwards-compat alias used by /stats etc.
+session_affinity = session_affinity_combined
 is_pd_sep = False
 _breakdown_log: list[dict] = []
 
@@ -224,11 +229,37 @@ async def _reconcile_loop():
                 inst.active_p_offloads = 0
 
 
+def _verify_vllm_patch():
+    """Startup self-check for patches/0001-fix-kv-transfer-abort-race.patch.
+
+    The patch turns an `assert req_id in self.requests` into a soft warn so
+    that engines do not crash on the KV-transfer abort race (see REPORT
+    §3.x). If somebody upgrades vLLM without re-applying the patch, the
+    assert returns and elastic mode dies under load. Print a loud warning
+    so we catch the regression before the first HEAVY request.
+    """
+    try:
+        import inspect
+        from vllm.v1.core.sched.scheduler import Scheduler
+        src = inspect.getsource(Scheduler)
+        if "assert req_id in self.requests" in src:
+            print("WARNING: vLLM scheduler still contains the unpatched "
+                  "`assert req_id in self.requests` line; expect engine "
+                  "death on KV-transfer abort race. Apply "
+                  "patches/0001-fix-kv-transfer-abort-race.patch.")
+        else:
+            print("vLLM patch self-check: kv-transfer-abort assert is patched.")
+    except Exception as exc:
+        print(f"vLLM patch self-check skipped: {exc!r}")
+
+
 @asynccontextmanager
 async def lifespan(app: FastAPI):
     global is_pd_sep
     app.state.ready = asyncio.Event()
 
+    _verify_vllm_patch()
+
     reconcile_task = asyncio.create_task(_reconcile_loop())
 
     if global_args.combined:
@@ -313,7 +344,7 @@ async def _handle_combined(api, req_data, token_ids, input_length, session_id, h
     policy = getattr(global_args, 'policy', 'linear') if global_args else 'linear'
     picker = pick_instance_lmetric if policy == 'lmetric' else pick_instance
     best_inst, best_idx = picker(combined_instances, token_ids, session_id,
-                                  input_length, session_affinity)
+                                  input_length, session_affinity_combined)
     cache_hit = best_inst.estimate_cache_hit(token_ids)
     estimated_new = max(0, input_length - cache_hit)
 
@@ -395,7 +426,7 @@ async def _handle_combined(api, req_data, token_ids, input_length, session_id, h
         breakdown["cache_hit_tokens"] = cache_hit
 
         if session_id:
-            session_affinity[session_id] = d_idx
+            session_affinity_combined[session_id] = d_idx
 
         return await _handle_direct_read_offload(
             api, req_data, headers, token_ids, input_length,
@@ -509,7 +540,7 @@ async def _handle_pd_sep(api, req_data, request_id, token_ids, input_length,
     }
 
     p_inst, _ = pick_instance(prefill_instances, token_ids, session_id,
-                               input_length, session_affinity)
+                               input_length, session_affinity_prefill)
     d_inst = min(decode_instances, key=lambda x: x.ongoing_tokens)
     breakdown["p_inst"] = p_inst.url
     breakdown["d_inst"] = d_inst.url