Document who can call the loopback API and let the user pick the listen port.

host/firefox_agent_bridge_host.py

@@ -20,9 +20,12 @@ if sys.platform == "win32":
     msvcrt.setmode(sys.stdout.fileno(), os.O_BINARY)
 
 HOST = "127.0.0.1"
-PORT = int(os.environ.get("FAB_PORT", "17634"))
+DEFAULT_PORT = int(os.environ.get("FAB_PORT", "17634"))
+PORT = DEFAULT_PORT
 MAX_BODY = 256 * 1024
 ALLOWED_HOSTS = {f"127.0.0.1:{PORT}", f"localhost:{PORT}"}
+HTTP_SERVER: ThreadingHTTPServer | None = None
+SERVER_LOCK = threading.Lock()
 ALLOWED_METHODS = frozenset(
     {
         "meta.ping",
@@ -45,6 +48,45 @@ PENDING: dict[str, tuple[threading.Event, dict[str, Any]]] = {}
 PENDING_LOCK = threading.Lock()
 
 
+def set_port_globals(port: int) -> None:
+    global PORT, ALLOWED_HOSTS
+    PORT = port
+    ALLOWED_HOSTS = {f"127.0.0.1:{port}", f"localhost:{port}"}
+
+
+def apply_listen_port(requested: Any) -> dict[str, Any]:
+    global HTTP_SERVER
+    try:
+        port = int(requested)
+    except (TypeError, ValueError):
+        return {"type": "config-result", "ok": False, "error": "invalid port"}
+    if port < 1024 or port > 65535:
+        return {
+            "type": "config-result",
+            "ok": False,
+            "port": port,
+            "error": "port must be 1024–65535",
+        }
+    with SERVER_LOCK:
+        if HTTP_SERVER is not None and PORT == port:
+            return {"type": "config-result", "ok": True, "port": port}
+        old = HTTP_SERVER
+        HTTP_SERVER = None
+        if old is not None:
+            old.shutdown()
+            old.server_close()
+        try:
+            set_port_globals(port)
+            HTTP_SERVER = ThreadingHTTPServer((HOST, port), Handler)
+        except OSError as exc:
+            return {"type": "config-result", "ok": False, "port": port, "error": str(exc)}
+        threading.Thread(
+            target=HTTP_SERVER.serve_forever, name="fab-http", daemon=True
+        ).start()
+        log(f"listening on http://{HOST}:{port}")
+        return {"type": "config-result", "ok": True, "port": port}
+
+
 def log(msg: str) -> None:
     sys.stderr.write(msg + "\n")
     sys.stderr.flush()
@@ -252,6 +294,9 @@ def stdin_loop() -> None:
             break
         if message is None:
             break
+        if message.get("type") == "config":
+            send_to_extension(apply_listen_port(message.get("port")))
+            continue
         req_id = str(message.get("id") or "")
         with PENDING_LOCK:
             pending = PENDING.pop(req_id, None)
@@ -265,10 +310,9 @@ def stdin_loop() -> None:
 
 def main() -> int:
     threading.Thread(target=stdin_loop, name="fab-stdin", daemon=True).start()
-    server = ThreadingHTTPServer((HOST, PORT), Handler)
-    log(f"listening on http://{HOST}:{PORT}")
+    apply_listen_port(DEFAULT_PORT)
     try:
-        server.serve_forever()
+        threading.Event().wait()
     except KeyboardInterrupt:
         return 0
     return 0