More logging for IPC failures in parallel checking

mypy/build.py

@@ -242,22 +242,26 @@ def __init__(self, status_file: str, options_data: str, env: Mapping[str, str])
 
     def connect(self) -> None:
         end_time = time.time() + WORKER_START_TIMEOUT
+        last_exception: Exception | None = None
         while time.time() < end_time:
             try:
                 data = read_status(self.status_file)
-            except BadStatus:
+            except BadStatus as exc:
+                last_exception = exc
                 time.sleep(WORKER_START_INTERVAL)
                 continue
             try:
                 pid, connection_name = data["pid"], data["connection_name"]
-                assert isinstance(pid, int) and isinstance(connection_name, str)
+                assert isinstance(pid, int), f"Bad PID: {pid}"
+                assert isinstance(connection_name, str), f"Bad connection name: {connection_name}"
                 # Double-check this status file is created by us.
-                assert pid == self.proc.pid
+                assert pid == self.proc.pid, f"PID mismatch: {pid} vs {self.proc.pid}"
                 self.conn = IPCClient(connection_name, WORKER_CONNECTION_TIMEOUT)
                 return
-            except Exception:
+            except Exception as exc:
+                last_exception = exc
                 break
-        print("Failed to establish connection with worker")
+        print("Failed to establish connection with worker:", last_exception)
         sys.exit(2)
 
     def close(self) -> None:

mypy/build_worker/worker.py

@@ -92,8 +92,9 @@ def main(argv: list[str]) -> None:
     try:
         with server:
             serve(server, ctx)
-    except OSError:
-        pass
+    except OSError as exc:
+        if options.verbosity >= 1:
+            print("Error communicating with coordinator:", exc)
     except Exception as exc:
         report_internal_error(exc, errors.file, 0, errors, options)
     finally:

mypy/ipc.py

@@ -350,7 +350,7 @@ def read_status(status_file: str) -> dict[str, object]:
         except Exception as e:
             raise BadStatus("Malformed status file (not JSON)") from e
     if not isinstance(data, dict):
-        raise BadStatus("Invalid status file (not a dict)")
+        raise BadStatus(f"Invalid status file (not a dict): {data}")
     return data