[release-branch.go1.26] net: correctly handle non-nil empty cmsg buffer on windows

Updates #77875
Fixes #77885

Change-Id: I0ed91be7ed10c04ddea3af55548a8dbf0be5f3a8
Reviewed-on: https://go-review.googlesource.com/c/go/+/750420
Reviewed-by: Quim Muntal <quimmuntal@gmail.com>
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-by: David Chase <drchase@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
(cherry picked from commit 9e0b5684e1f7166d6427635673001c80218ac9f4)
Reviewed-on: https://go-review.googlesource.com/c/go/+/753060
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

2 files changed, 47 insertions, 3 deletions

diff --git a/src/internal/poll/fd_windows.go b/src/internal/poll/fd_windows.go
index edad656350..2ba967f990 100644
--- a/src/internal/poll/fd_windows.go
+++ b/src/internal/poll/fd_windows.go
@@ -159,9 +159,11 @@ func newWSAMsg(p []byte, oob []byte, flags int, unconnected bool) *windows.WSAMs
 	msg := wsaMsgPool.Get().(*windows.WSAMsg)
 	msg.Buffers.Len = uint32(len(p))
 	msg.Buffers.Buf = unsafe.SliceData(p)
-	msg.Control = syscall.WSABuf{
-		Len: uint32(len(oob)),
-		Buf: unsafe.SliceData(oob),
+	if len(oob) > 0 {
+		msg.Control = syscall.WSABuf{
+			Len: uint32(len(oob)),
+			Buf: unsafe.SliceData(oob),
+		}
 	}
 	msg.Flags = uint32(flags)
 	if unconnected {
diff --git a/src/net/udpsock_test.go b/src/net/udpsock_test.go
index a79e9f83c1..1700ebaad1 100644
--- a/src/net/udpsock_test.go
+++ b/src/net/udpsock_test.go
@@ -750,3 +750,45 @@ func TestIPv4WriteMsgUDPAddrPortTargetAddrIPVersion(t *testing.T) {
 		t.Errorf("conn.WriteMsgUDPAddrPort(buf, nil, daddr6) should have failed, but got no error")
 	}
 }
+
+// TestReadWriteMsgUDPAddrPortEmptyCmsg verifies that ReadMsgUDPAddrPort and
+// WriteMsgUDPAddrPort won't fail when the cmsg buffer (the oob parameter) is
+// non-nil but empty.
+//
+// This is a regression test for https://golang.org/issue/77875.
+func TestReadWriteMsgUDPAddrPortEmptyCmsg(t *testing.T) {
+	switch runtime.GOOS {
+	case "plan9":
+		t.Skipf("not supported on %s", runtime.GOOS)
+	}
+
+	if !testableNetwork("udp4") {
+		t.Skipf("skipping: udp4 not available")
+	}
+
+	conn, err := ListenUDP("udp4", &UDPAddr{IP: IPv4(127, 0, 0, 1)})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+
+	buf := make([]byte, 8)
+	cmsgBuf := make([]byte, 8)[:0]
+	daddr := conn.LocalAddr().(*UDPAddr).AddrPort()
+
+	_, cmsgn, err := conn.WriteMsgUDPAddrPort(buf, cmsgBuf, daddr)
+	if err != nil {
+		t.Fatalf("WriteMsgUDPAddrPort failed: %v", err)
+	}
+	if cmsgn != 0 {
+		t.Errorf("WriteMsgUDPAddrPort wrote %d cmsg bytes; want 0", cmsgn)
+	}
+
+	_, cmsgn, _, _, err = conn.ReadMsgUDPAddrPort(buf, cmsgBuf)
+	if err != nil {
+		t.Fatalf("ReadMsgUDPAddrPort failed: %v", err)
+	}
+	if cmsgn != 0 {
+		t.Errorf("ReadMsgUDPAddrPort read %d cmsg bytes; want 0", cmsgn)
+	}
+}