net: document caveats for (*syscall.RawConn).Write on Windows

Change-Id: I6e1fa67dc9d4d151c90eb19a6f736e4daa7d4fb3
Reviewed-on: https://go-review.googlesource.com/107615
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/poll/fd_windows.go

@@ -946,11 +946,13 @@ func (fd *FD) RawWrite(f func(uintptr) bool) error {
 		return err
 	}
 	defer fd.writeUnlock()
-	for {
-		if f(uintptr(fd.Sysfd)) {
-			return nil
-		}
+
+	if f(uintptr(fd.Sysfd)) {
+		return nil
 	}
+
+	// TODO(tmm1): find a way to detect socket writability
+	return syscall.EWINDOWS
 }
 
 func sockaddrToRaw(sa syscall.Sockaddr) (unsafe.Pointer, int32, error) {

src/net/rawconn.go

@@ -9,6 +9,12 @@ import (
 	"syscall"
 )
 
+// BUG(tmm1): On Windows, the Write method of syscall.RawConn
+// does not integrate with the runtime's network poller. It cannot
+// wait for the connection to become writeable, and does not respect
+// deadlines. If the user-provided callback returns false, the Write
+// method will fail immediately.
+
 // BUG(mikio): On NaCl and Plan 9, the Control, Read and Write methods
 // of syscall.RawConn are not implemented.