internal/poll: do not call SetFileCompletionNotificationModes if it is broken

Current code assumes that SetFileCompletionNotificationModes
is safe to call even if we know that it is not safe to use
FILE_SKIP_COMPLETION_PORT_ON_SUCCESS flag. It appears (see issue #22149),
SetFileCompletionNotificationModes crashes when we call it without
FILE_SKIP_COMPLETION_PORT_ON_SUCCESS flag.

Do not call SetFileCompletionNotificationModes in that situation.
We are allowed to do that, because SetFileCompletionNotificationModes
is just an optimisation.

Fixes #22149

Change-Id: I0ad3aff4eabd8c27739417a62c286b1819ae166a
Reviewed-on: https://go-review.googlesource.com/69870Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/internal/poll/fd_windows.go

@@ -31,39 +31,49 @@ var (
 // package uses CancelIoEx API, if present, otherwise it fallback
 // to CancelIo.
 
-var (
-	canCancelIO                               bool // determines if CancelIoEx API is present
-	skipSyncNotif                             bool
-	hasLoadSetFileCompletionNotificationModes bool
-)
-
-func init() {
-	var d syscall.WSAData
-	e := syscall.WSAStartup(uint32(0x202), &d)
-	if e != nil {
-		initErr = e
+var canCancelIO bool // determines if CancelIoEx API is present
+
+// This package uses SetFileCompletionNotificationModes Windows API
+// to skip calling GetQueuedCompletionStatus if an IO operation completes
+// synchronously. Unfortuently SetFileCompletionNotificationModes is not
+// available on Windows XP. Also there is a known bug where
+// SetFileCompletionNotificationModes crashes on some systems
+// (see http://support.microsoft.com/kb/2568167 for details).
+
+var useSetFileCompletionNotificationModes bool // determines is SetFileCompletionNotificationModes is present and safe to use
+
+// checkSetFileCompletionNotificationModes verifies that
+// SetFileCompletionNotificationModes Windows API is present
+// on the system and is safe to use.
+// See http://support.microsoft.com/kb/2568167 for details.
+func checkSetFileCompletionNotificationModes() {
+	err := syscall.LoadSetFileCompletionNotificationModes()
+	if err != nil {
+		return
 	}
-	canCancelIO = syscall.LoadCancelIoEx() == nil
-	hasLoadSetFileCompletionNotificationModes = syscall.LoadSetFileCompletionNotificationModes() == nil
-	if hasLoadSetFileCompletionNotificationModes {
-		// It's not safe to use FILE_SKIP_COMPLETION_PORT_ON_SUCCESS if non IFS providers are installed:
-		// http://support.microsoft.com/kb/2568167
-		skipSyncNotif = true
 	protos := [2]int32{syscall.IPPROTO_TCP, 0}
 	var buf [32]syscall.WSAProtocolInfo
 	len := uint32(unsafe.Sizeof(buf))
 	n, err := syscall.WSAEnumProtocols(&protos[0], &buf[0], &len)
 	if err != nil {
-			skipSyncNotif = false
-		} else {
+		return
+	}
 	for i := int32(0); i < n; i++ {
 		if buf[i].ServiceFlags1&syscall.XP1_IFS_HANDLES == 0 {
-					skipSyncNotif = false
-					break
-				}
+			return
 		}
 	}
+	useSetFileCompletionNotificationModes = true
+}
+
+func init() {
+	var d syscall.WSAData
+	e := syscall.WSAStartup(uint32(0x202), &d)
+	if e != nil {
+		initErr = e
 	}
+	canCancelIO = syscall.LoadCancelIoEx() == nil
+	checkSetFileCompletionNotificationModes()
 }
 
 // operation contains superset of data necessary to perform all async IO.
@@ -347,12 +357,12 @@ func (fd *FD) Init(net string, pollable bool) (string, error) {
 	if err != nil {
 		return "", err
 	}
-	if hasLoadSetFileCompletionNotificationModes {
+	if useSetFileCompletionNotificationModes {
 		// We do not use events, so we can skip them always.
 		flags := uint8(syscall.FILE_SKIP_SET_EVENT_ON_HANDLE)
 		// It's not safe to skip completion notifications for UDP:
 		// http://blogs.technet.com/b/winserverperformance/archive/2008/06/26/designing-applications-for-high-performance-part-iii.aspx
-		if skipSyncNotif && (net == "tcp" || net == "file") {
+		if net == "tcp" || net == "file" {
 			flags |= syscall.FILE_SKIP_COMPLETION_PORT_ON_SUCCESS
 		}
 		err := syscall.SetFileCompletionNotificationModes(fd.Sysfd, flags)