Add http3 module: HTTP/3 in the Java HTTP Client (JEP 517), opt-in, discovery modes, fallback timings, proxy downgrade and a packet-loss comparison

Co-Authored-By: Claude Sonnet 5.5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01KqJyCidz3ZgRyHABv2GVJh

http3/server/connect-proxy.py

@@ -0,0 +1,25 @@
+"""A 25-line HTTP CONNECT proxy, just enough to show that the JDK will not send HTTP/3 through one."""
+import asyncio, sys
+
+async def pipe(r, w):
+    try:
+        while data := await r.read(65536):
+            w.write(data); await w.drain()
+    finally:
+        w.close()
+
+async def handle(cr, cw):
+    line = (await cr.readline()).decode()
+    while (await cr.readline()) not in (b"\r\n", b""):
+        pass
+    method, target, _ = line.split()
+    print("proxy saw:", method, target, flush=True)
+    host, port = target.rsplit(":", 1)
+    ur, uw = await asyncio.open_connection(host, int(port))
+    cw.write(b"HTTP/1.1 200 Connection established\r\n\r\n"); await cw.drain()
+    await asyncio.gather(pipe(cr, uw), pipe(ur, cw))
+
+async def main():
+    srv = await asyncio.start_server(handle, "127.0.0.1", int(sys.argv[1]))
+    async with srv: await srv.serve_forever()
+asyncio.run(main())