net/{neterror,dns/resolver}: move PacketWasTruncated to neterror from DNS code

And delete the unused code in net/dns/resolver/neterr_*.go.

Change-Id: Ibe62c486bacce2733eb9968c96a98cbbdb2758bd
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

net/dns/resolver/forwarder.go

@@ -25,6 +25,7 @@ import (
 	dns "golang.org/x/net/dns/dnsmessage"
 	"inet.af/netaddr"
 	"tailscale.com/hostinfo"
+	"tailscale.com/net/neterror"
 	"tailscale.com/net/netns"
 	"tailscale.com/net/tsdial"
 	"tailscale.com/types/dnstype"
@@ -482,7 +483,7 @@ func (f *forwarder) send(ctx context.Context, fq *forwardQuery, rr resolverAndDe
 		if err := ctx.Err(); err != nil {
 			return nil, err
 		}
-		if packetWasTruncated(err) {
+		if neterror.PacketWasTruncated(err) {
 			err = nil
 		} else {
 			metricDNSFwdUDPErrorRead.Add(1)

net/dns/resolver/neterr_darwin.go

@@ -1,30 +0,0 @@
-// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package resolver
-
-import (
-	"errors"
-	"syscall"
-)
-
-// Avoid allocation when calling errors.Is below
-// by converting syscall.Errno to error here.
-var (
-	networkDown        error = syscall.ENETDOWN
-	networkUnreachable error = syscall.ENETUNREACH
-)
-
-func networkIsDown(err error) bool {
-	return errors.Is(err, networkDown)
-}
-
-func networkIsUnreachable(err error) bool {
-	return errors.Is(err, networkUnreachable)
-}
-
-// packetWasTruncated returns true if err indicates truncation but the RecvFrom
-// that generated err was otherwise successful. It always returns false on this
-// platform.
-func packetWasTruncated(err error) bool { return false }

net/dns/resolver/neterr_other.go

@@ -1,16 +0,0 @@
-// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-//go:build !darwin && !windows
-// +build !darwin,!windows
-
-package resolver
-
-func networkIsDown(err error) bool        { return false }
-func networkIsUnreachable(err error) bool { return false }
-
-// packetWasTruncated returns true if err indicates truncation but the RecvFrom
-// that generated err was otherwise successful. It always returns false on this
-// platform.
-func packetWasTruncated(err error) bool { return false }

net/dns/resolver/neterr_windows.go

@@ -1,43 +0,0 @@
-// Copyright (c) 2020 Tailscale Inc & AUTHORS All rights reserved.
-// Use of this source code is governed by a BSD-style
-// license that can be found in the LICENSE file.
-
-package resolver
-
-import (
-	"errors"
-	"net"
-	"os"
-
-	"golang.org/x/sys/windows"
-)
-
-func networkIsDown(err error) bool {
-	if oe, ok := err.(*net.OpError); ok && oe.Op == "write" {
-		if se, ok := oe.Err.(*os.SyscallError); ok {
-			if se.Syscall == "wsasendto" && se.Err == windows.WSAENETUNREACH {
-				return true
-			}
-		}
-	}
-	return false
-}
-
-func networkIsUnreachable(err error) bool {
-	// TODO(bradfitz,josharian): something here? what is the
-	// difference between down and unreachable? Add comments.
-	return false
-}
-
-// packetWasTruncated returns true if err indicates truncation but the RecvFrom
-// that generated err was otherwise successful. On Windows, Go's UDP RecvFrom
-// calls WSARecvFrom which returns the WSAEMSGSIZE error code when the received
-// datagram is larger than the provided buffer. When that happens, both a valid
-// size and an error are returned (as per the partial fix for golang/go#14074).
-// If the WSAEMSGSIZE error is returned, then we ignore the error to get
-// semantics similar to the POSIX operating systems. One caveat is that it
-// appears that the source address is not returned when WSAEMSGSIZE occurs, but
-// we do not currently look at the source address.
-func packetWasTruncated(err error) bool {
-	return errors.Is(err, windows.WSAEMSGSIZE)
-}

net/neterror/neterror.go

@@ -40,3 +40,21 @@ func TreatAsLostUDP(err error) bool {
 	}
 	return false
 }
+
+var packetWasTruncated func(error) bool // non-nil on Windows at least
+
+// PacketWasTruncated reports whether err indicates truncation but the RecvFrom
+// that generated err was otherwise successful. On Windows, Go's UDP RecvFrom
+// calls WSARecvFrom which returns the WSAEMSGSIZE error code when the received
+// datagram is larger than the provided buffer. When that happens, both a valid
+// size and an error are returned (as per the partial fix for golang/go#14074).
+// If the WSAEMSGSIZE error is returned, then we ignore the error to get
+// semantics similar to the POSIX operating systems. One caveat is that it
+// appears that the source address is not returned when WSAEMSGSIZE occurs, but
+// we do not currently look at the source address.
+func PacketWasTruncated(err error) bool {
+	if packetWasTruncated == nil {
+		return false
+	}
+	return packetWasTruncated(err)
+}

net/neterror/neterror_windows.go

@@ -0,0 +1,17 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package neterror
+
+import (
+	"errors"
+
+	"golang.org/x/sys/windows"
+)
+
+func init() {
+	packetWasTruncated = func(err error) bool {
+		return errors.Is(err, windows.WSAEMSGSIZE)
+	}
+}