[router] Speak cleartext h2c on both edges: serve it inbound, forward it outbound (#39006)

Co-authored-by: Kangyan Zhou <kangyan.zhou@radixark.ai>
Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

experimental/sgl-router/tests/e2e/k8s_integration/fake_worker.py

@@ -2,9 +2,21 @@
 
 Responds to:
   GET  /health                   -> {"status": "ok"}
-  GET  /server_info              -> {"served_model_name": MODEL_ID}
+  GET  /server_info              -> {"served_model_name": MODEL_ID, ...}
   GET  /v1/models                -> list with a single MODEL_ID model entry
-  POST /v1/chat/completions      -> echoes the last user message back
+  POST /v1/chat/completions      -> echoes the last user message back, plus
+                                    the HTTP version the request arrived on
+
+Set `FAKE_WORKER_HTTP2=1` to imitate an engine launched with
+`--enable-http2`: `/server_info` advertises the flag and the app is served by
+Granian in `HTTPModes.auto`, which is what the real engine runs, so one port
+serves cleartext h2c alongside HTTP/1.1. The default is uvicorn, which speaks
+HTTP/1.1 only.
+
+`x_http_version` on the chat response is the load-bearing part for
+`test_h2c_forwarding.py`: a chat completion returns 200 over either protocol,
+so without the worker reporting what it actually received, an h2c test passes
+whether or not h2c was used.
 """
 
 from __future__ import annotations
@@ -17,6 +29,16 @@ from fastapi import FastAPI, Request
 app = FastAPI()
 
 MODEL_ID = os.environ.get("MODEL_ID", "tiny")
+# Normalised before comparing: a manifest that writes the Python-idiomatic
+# "False" must not silently turn this worker into a Granian/h2c one, which
+# would fail test_h2c_forwarding with a message about a router bug.
+ENABLE_HTTP2 = os.environ.get("FAKE_WORKER_HTTP2", "").strip().lower() not in (
+    "",
+    "0",
+    "false",
+    "no",
+    "off",
+)
 
 
 @app.get("/health")
@@ -27,8 +49,12 @@ async def health():
 @app.get("/server_info")
 async def server_info():
     # The sgl-router worker manager fetches this on every Added event and
-    # uses `served_model_name` to populate the registry's model index.
-    return {"served_model_name": MODEL_ID}
+    # uses `served_model_name` to populate the registry's model index, and
+    # `enable_http2` to resolve the worker's forwarding protocol.
+    info = {"served_model_name": MODEL_ID}
+    if ENABLE_HTTP2:
+        info["enable_http2"] = True
+    return info
 
 
 @app.get("/v1/models")
@@ -55,6 +81,11 @@ async def chat_completions(request: Request):
         "id": "chatcmpl-mock",
         "object": "chat.completion",
         "model": payload.get("model", MODEL_ID),
+        # Non-standard, and deliberately so: the router returns the upstream
+        # body verbatim (`proxy::forward_*` hands back `resp.bytes()`), so this
+        # is how a test on the other side of the router learns which protocol
+        # the forward leg actually used. "1.1" or "2".
+        "x_http_version": request.scope.get("http_version"),
         "choices": [
             {
                 "index": 0,
@@ -70,4 +101,20 @@ async def chat_completions(request: Request):
 
 
 if __name__ == "__main__":
-    uvicorn.run(app, host="0.0.0.0", port=30000)
+    if ENABLE_HTTP2:
+        # Mirrors the engine's own server (`_run_granian_server` in
+        # sglang/srt/entrypoints/http_server.py): HTTPModes.auto dispatches per
+        # connection on the first bytes, so h2c prior-knowledge and HTTP/1.1
+        # share one cleartext port.
+        from granian import Granian
+        from granian.constants import HTTPModes, Interfaces
+
+        Granian(
+            target="fake_worker:app",
+            address="0.0.0.0",
+            port=30000,
+            interface=Interfaces.ASGI,
+            http=HTTPModes.auto,
+        ).serve()
+    else:
+        uvicorn.run(app, host="0.0.0.0", port=30000)