Complete implementation of stdio stub

This class replaced stdout and stderr to capture output, but caused
issues because it didn't implement the full API of sys.stdout and
sys.stderr.

By fully stubbing the TextIO API we prevent issues with other code
which uses more of the API than we had previously accounted for.

Fixes #16678

mypy/dmypy_server.py

@@ -221,8 +221,8 @@ def serve(self) -> None:
             while True:
                 with server:
                     data = receive(server)
-                    sys.stdout = WriteToConn(server, "stdout")  # type: ignore[assignment]
-                    sys.stderr = WriteToConn(server, "stderr")  # type: ignore[assignment]
+                    sys.stdout = WriteToConn(server, "stdout", sys.stdout.isatty())
+                    sys.stderr = WriteToConn(server, "stderr", sys.stderr.isatty())
                     resp: dict[str, Any] = {}
                     if "command" not in data:
                         resp = {"error": "No command found in request"}

mypy/dmypy_util.py

@@ -5,8 +5,10 @@
 
 from __future__ import annotations
 
+import io
 import json
-from typing import Any, Final, Iterable
+from types import TracebackType
+from typing import Any, Final, Iterable, Iterator, TextIO
 
 from mypy.ipc import IPCBase
 
@@ -40,19 +42,76 @@ def send(connection: IPCBase, data: Any) -> None:
     connection.write(json.dumps(data))
 
 
-class WriteToConn:
+class WriteToConn(TextIO):
     """Helper class to write to a connection instead of standard output."""
 
-    def __init__(self, server: IPCBase, output_key: str) -> None:
+    def __init__(self, server: IPCBase, output_key: str, isatty: bool) -> None:
         self.server = server
         self.output_key = output_key
+        self._isatty = isatty
+
+    def __enter__(self) -> TextIO:
+        return self
+
+    def __exit__(
+        self,
+        t: type[BaseException] | None,
+        value: BaseException | None,
+        traceback: TracebackType | None,
+    ) -> None:
+        pass
+
+    def __iter__(self) -> Iterator[str]:
+        raise io.UnsupportedOperation
+
+    def __next__(self) -> str:
+        raise io.UnsupportedOperation
+
+    def close(self) -> None:
+        pass
+
+    def fileno(self) -> int:
+        raise OSError
+
+    def flush(self) -> None:
+        pass
+
+    def isatty(self) -> bool:
+        return self._isatty
+
+    def read(self, n: int = 0) -> str:
+        raise io.UnsupportedOperation
+
+    def readable(self) -> bool:
+        return False
+
+    def readline(self, limit: int = 0) -> str:
+        raise io.UnsupportedOperation
+
+    def readlines(self, hint: int = 0) -> list[str]:
+        raise io.UnsupportedOperation
+
+    def seek(self, offset: int, whence: int = 0) -> int:
+        raise io.UnsupportedOperation
+
+    def seekable(self) -> bool:
+        return False
+
+    def tell(self) -> int:
+        raise io.UnsupportedOperation
+
+    def truncate(self, size: int | None = 0) -> int:
+        raise io.UnsupportedOperation
 
     def write(self, output: str) -> int:
         resp: dict[str, Any] = {}
         resp[self.output_key] = output
         send(self.server, resp)
         return len(output)
 
+    def writable(self) -> bool:
+        return True
+
     def writelines(self, lines: Iterable[str]) -> None:
         for s in lines:
             self.write(s)