Debug startup errors on Windows

Author: ilevkivskyi

.github/workflows/test.yml

@@ -116,7 +116,7 @@ jobs:
           python: '3.14'
           os: windows-latest
           toxenv: py
-          tox_extra_args: "-n 2 --mypy-num-workers=2 mypy/test/testcheck.py -k 'incremental or modules or classes'"
+          tox_extra_args: "-n 4 --mypy-num-workers=2 mypy/test/testcheck.py"
 
         - name: Type check our own code (py310-ubuntu)
           python: '3.10'

mypy/build.py

@@ -283,6 +283,7 @@ def __init__(self, status_file: str, options_data: str, env: Mapping[str, str])
         ]
         # Return early without waiting, caller must call connect() before using the client.
         self.proc = subprocess.Popen(command, env=env)
+        self.connected = False
 
     def connect(self) -> None:
         end_time = time.time() + WORKER_START_TIMEOUT
@@ -303,12 +304,12 @@ def connect(self) -> None:
                     # verify PIDs reliably.
                     assert pid == self.proc.pid, f"PID mismatch: {pid} vs {self.proc.pid}"
                 self.conn = IPCClient(connection_name, WORKER_CONNECTION_TIMEOUT)
+                self.connected = True
                 return
             except Exception as exc:
                 last_exception = exc
                 break
         print("Failed to establish connection with worker:", last_exception)
-        sys.exit(2)
 
     def close(self) -> None:
         self.conn.close()
@@ -394,6 +395,8 @@ def default_flush_errors(
         def connect(wc: WorkerClient, data: bytes) -> None:
             # Start loading sources in each worker as soon as it is up.
             wc.connect()
+            if not wc.connected:
+                return
             wc.conn.write_bytes(data)
 
         # We don't wait for workers to be ready until they are actually needed.
@@ -432,6 +435,8 @@ def connect(wc: WorkerClient, data: bytes) -> None:
         for thread in connect_threads:
             thread.join()
         for worker in workers:
+            if not worker.connected:
+                continue
             try:
                 send(worker.conn, SccRequestMessage(scc_id=None, import_errors={}, mod_data={}))
             except (OSError, IPCException):
@@ -3972,6 +3977,8 @@ def dispatch(
         # Wait for workers since they may be needed at this point.
         for thread in connect_threads:
             thread.join()
+        if not all(wc.connected for wc in manager.workers):
+            raise OSError("Build workers failed to start")
         process_graph(graph, manager)
         # Update plugins snapshot.
         write_plugins_snapshot(manager)

mypy/build_worker/worker.py

@@ -94,6 +94,9 @@ def main(argv: list[str]) -> None:
     status_file = args.status_file
     server = IPCServer(CONNECTION_NAME, WORKER_CONNECTION_TIMEOUT)
 
+    if sys.platform == "win32":
+        print("Writing status file:", status_file)
+
     try:
         with open(status_file, "w") as f:
             json.dump({"pid": os.getpid(), "connection_name": server.connection_name}, f)

mypy/ipc.py

@@ -361,6 +361,8 @@ def read_status(status_file: str) -> dict[str, object]:
     Raise BadStatus if the status file doesn't exist or contains
     invalid JSON or the JSON is not a dict.
     """
+    if sys.platform == "win32":
+        print("Reading status file:", status_file)
     if not os.path.isfile(status_file):
         raise BadStatus("No status file found")
     with open(status_file) as f: