Server: a stop signal no longer crashes it (SIGABRT) while the app holds stdin

The --exit-on-eof watcher read stdin with a buffered read, which holds stdin's
lock. When SIGTERM stopped the server first, Python aborted at exit trying to
take that lock back, and the app showed "The server stopped unexpectedly
(SIGABRT)". It now uses os.read. The startup line is printed inside the try,
so a signal that arrives while it's printed still runs the cleanup.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

tests/test_server.py

@@ -279,6 +279,18 @@ class OneServer(unittest.TestCase):
         r = conn.getresponse()
         self.assertEqual(r.status, 403, r.read())
 
+    @unittest.skipIf(os.name == "nt", "no SIGTERM on Windows")
+    def test_sigterm_while_the_app_holds_stdin_exits_cleanly(self):
+        """The app keeps stdin open; a stop signal used to abort Python (SIGABRT) at exit."""
+        env = {**os.environ, "FRAME_CONTROL_DATA_DIR": tempfile.mkdtemp(prefix="frame-one-server-"),
+               "FRAME_ALIAS": "frame-control-test.invalid"}
+        proc = subprocess.Popen([sys.executable, str(ROOT / "ui" / "server.py"), "--port", "0", "--exit-on-eof"],
+                                env=env, stdin=subprocess.PIPE, stdout=subprocess.PIPE, stderr=subprocess.STDOUT, text=True)
+        self.addCleanup(lambda: (proc.stdin.close(), proc.stdout.close()))
+        self.assertIn("Frame Control on", proc.stdout.readline())
+        proc.terminate()
+        self.assertEqual(proc.wait(30), 0, proc.stdout.read())
+
 
 class ArtworkSettings(unittest.TestCase):
     """The settings panel's endpoints, with and without the page's X-Frame-UI key."""

ui/server.py

@@ -2656,12 +2656,16 @@ def main():
         signal.signal(signal.SIGTERM, lambda *_: (_ for _ in ()).throw(KeyboardInterrupt))
     if args.exit_on_eof:
         def watch_stdin():
-            sys.stdin.buffer.read()
+            # os.read, not sys.stdin.buffer.read: a buffered read holds stdin's lock,
+            # and if a signal stops the server first, Python aborts (SIGABRT) at exit
+            # when it can't take that lock back from this thread.
+            while os.read(0, 4096):
+                pass
             threading.Thread(target=httpd.shutdown, daemon=True).start()
         threading.Thread(target=watch_stdin, daemon=True).start()
-    # The real port, which --port 0 leaves to the system (the iPhone app reads it from here).
-    print(f"Frame Control on http://127.0.0.1:{httpd.server_address[1]}  (alias: {FRAME}; Ctrl-C to stop)", flush=True)
     try:
+        # The real port, which --port 0 leaves to the system (the iPhone app reads it from here).
+        print(f"Frame Control on http://127.0.0.1:{httpd.server_address[1]}  (alias: {FRAME}; Ctrl-C to stop)", flush=True)
         httpd.serve_forever()
     except KeyboardInterrupt:
         pass