From 27af7477a99bdbee0c32697837f1ff261ee70d83 Mon Sep 17 00:00:00 2001
From: Spike Curtis <spike@coder.com>
Date: Mon, 14 Apr 2025 11:12:03 +0400
Subject: [PATCH] net: fix deadlock in lookupProtocol on Windows

If the context expires before acquireThread(ctx) succeeds, then the
goroutine can block like:

net.lookupProtocol.func1()
	src/net/lookup_windows.go:58 +0x105
created by net.lookupProtocol in goroutine 2834
	src/net/lookup_windows.go:56 +0xda

We saw this in our UTs with a leak detector, confirmed by inspection of the
source code.
---
 src/net/lookup_windows.go | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/src/net/lookup_windows.go b/src/net/lookup_windows.go
index 7a8fda0a30..38034dc601 100644
--- a/src/net/lookup_windows.go
+++ b/src/net/lookup_windows.go
@@ -52,7 +52,7 @@ func lookupProtocol(ctx context.Context, name string) (int, error) {
 		proto int
 		err   error
 	}
-	ch := make(chan result) // unbuffered
+	ch := make(chan result, 1) // buffer so that next goroutine never blocks
 	go func() {
 		if err := acquireThread(ctx); err != nil {
 			ch <- result{err: mapErr(err)}
@@ -62,10 +62,7 @@ func lookupProtocol(ctx context.Context, name string) (int, error) {
 		runtime.LockOSThread()
 		defer runtime.UnlockOSThread()
 		proto, err := getprotobyname(name)
-		select {
-		case ch <- result{proto: proto, err: err}:
-		case <-ctx.Done():
-		}
+		ch <- result{proto: proto, err: err}
 	}()
 	select {
 	case r := <-ch: