From 629a47cb7edbf42076b250e8dc3bc1992a4b41f5 Mon Sep 17 00:00:00 2001
From: Emma Harper Smith <emma@emmatyping.dev>
Date: Mon, 9 Feb 2026 21:03:39 -0800
Subject: [PATCH 1/2] Support parallel type checking on windows

Co-authored-by: Claude <claude@anthropic.com>
---
 mypy/build.py |  7 ++++--
 mypy/ipc.py   | 61 ++++++++++++++++++++++++++++++++++++++++++++++-----
 2 files changed, 61 insertions(+), 7 deletions(-)

diff --git a/mypy/build.py b/mypy/build.py
index 98da5e0a759e..8b8b9716b5ff 100644
--- a/mypy/build.py
+++ b/mypy/build.py
@@ -274,8 +274,11 @@ def connect(self) -> None:
                 pid, connection_name = data["pid"], data["connection_name"]
                 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, f"PID mismatch: {pid} vs {self.proc.pid}"
+                if sys.platform != "win32":
+                    # TODO(emmatyping): for some reason this does not work on Windows. Probably
+                    # because we don't fork? We should check this
+                    # Double-check this status file is created by us.
+                    assert pid == self.proc.pid, f"PID mismatch: {pid} vs {self.proc.pid}"
                 self.conn = IPCClient(connection_name, WORKER_CONNECTION_TIMEOUT)
                 return
             except Exception as exc:
diff --git a/mypy/ipc.py b/mypy/ipc.py
index e21171d46b0f..08081f04b0bd 100644
--- a/mypy/ipc.py
+++ b/mypy/ipc.py
@@ -359,11 +359,62 @@ def ready_to_read(conns: list[IPCClient], timeout: float | None = None) -> list[
 
     Return index of each readable connection in the original list.
     """
-    # TODO: add Windows support for this.
-    assert sys.platform != "win32"
-    connections = [conn.connection for conn in conns]
-    ready, _, _ = select(connections, [], [], timeout)
-    return [connections.index(r) for r in ready]
+    if sys.platform == "win32":
+        # Windows doesn't support select() on named pipes. Instead, start an overlapped
+        # ReadFile on each pipe (which internally creates an event via CreateEventW),
+        # then WaitForMultipleObjects on those events for efficient OS-level waiting.
+        # Any data consumed by the probe reads is stored into each connection's buffer
+        # so the subsequent read_bytes() call will find it via frame_from_buffer().
+        WAIT_FAILED = 0xFFFFFFFF
+        pending: list[tuple[int, _winapi.Overlapped]] = []
+        events: list[int] = []
+        ready: list[int] = []
+
+        for i, conn in enumerate(conns):
+            try:
+                ov, err = _winapi.ReadFile(conn.connection, 1, overlapped=True)
+            except OSError:
+                # Broken/closed pipe
+                continue
+            if err == _winapi.ERROR_IO_PENDING:
+                events.append(ov.event)
+                pending.append((i, ov))
+            else:
+                # Data was immediately available (err == 0 or ERROR_MORE_DATA)
+                _, err = ov.GetOverlappedResult(True)
+                data = ov.getbuffer()
+                if data:
+                    conn.buffer.extend(data)
+                ready.append(i)
+
+        # Wait only if nothing is immediately ready and there are pending operations
+        if not ready and events:
+            timeout_ms = int(timeout * 1000) if timeout is not None else _winapi.INFINITE
+            res = _winapi.WaitForMultipleObjects(events, False, timeout_ms)
+            if res == WAIT_FAILED:
+                for _, ov in pending:
+                    ov.cancel()
+                raise IPCException(
+                    f"Failed to wait for connections: {_winapi.GetLastError()}"
+                )
+
+        # Check which pending operations completed, cancel the rest
+        for i, ov in pending:
+            if _winapi.WaitForSingleObject(ov.event, 0) == _winapi.WAIT_OBJECT_0:
+                _, err = ov.GetOverlappedResult(True)
+                data = ov.getbuffer()
+                if data:
+                    conns[i].buffer.extend(data)
+                ready.append(i)
+            else:
+                ov.cancel()
+
+        return ready
+
+    else:
+        connections = [conn.connection for conn in conns]
+        ready, _, _ = select(connections, [], [], timeout)
+        return [connections.index(r) for r in ready]
 
 
 def send(connection: IPCBase, data: IPCMessage) -> None:

From 37667383655a214cb3f4bad9c088dc5323cb8756 Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 10 Feb 2026 05:17:58 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/ipc.py | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/mypy/ipc.py b/mypy/ipc.py
index 08081f04b0bd..6772956e32da 100644
--- a/mypy/ipc.py
+++ b/mypy/ipc.py
@@ -394,9 +394,7 @@ def ready_to_read(conns: list[IPCClient], timeout: float | None = None) -> list[
             if res == WAIT_FAILED:
                 for _, ov in pending:
                     ov.cancel()
-                raise IPCException(
-                    f"Failed to wait for connections: {_winapi.GetLastError()}"
-                )
+                raise IPCException(f"Failed to wait for connections: {_winapi.GetLastError()}")
 
         # Check which pending operations completed, cancel the rest
         for i, ov in pending: