Show logging written to stderr in mypy daemon (#14775)

Previously stdout from mypy daemon was propagated to the client, but
some verbose logging is written to stderr and it wasn't propagated. Now
propagate stderr as well. Tested manually.

This builds on top of #13769.

mypy/dmypy/client.py

@@ -665,10 +665,15 @@ def request(
         return {"error": str(err)}
     # TODO: Other errors, e.g. ValueError, UnicodeError
     else:
-        # Display debugging output written to stdout in the server process for convenience.
+        # Display debugging output written to stdout/stderr in the server process for convenience.
         stdout = response.get("stdout")
         if stdout:
             sys.stdout.write(stdout)
+        stderr = response.get("stderr")
+        if stderr:
+            print("-" * 79)
+            print("stderr:")
+            sys.stdout.write(stderr)
         return response
 
 

mypy/dmypy_server.py

@@ -215,7 +215,9 @@ def serve(self) -> None:
                 with server:
                     data = receive(server)
                     debug_stdout = io.StringIO()
+                    debug_stderr = io.StringIO()
                     sys.stdout = debug_stdout
+                    sys.stderr = debug_stderr
                     resp: dict[str, Any] = {}
                     if "command" not in data:
                         resp = {"error": "No command found in request"}
@@ -233,9 +235,11 @@ def serve(self) -> None:
                                 resp = {"error": "Daemon crashed!\n" + "".join(tb)}
                                 resp.update(self._response_metadata())
                                 resp["stdout"] = debug_stdout.getvalue()
+                                resp["stderr"] = debug_stderr.getvalue()
                                 server.write(json.dumps(resp).encode("utf8"))
                                 raise
                     resp["stdout"] = debug_stdout.getvalue()
+                    resp["stderr"] = debug_stderr.getvalue()
                     try:
                         resp.update(self._response_metadata())
                         server.write(json.dumps(resp).encode("utf8"))