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/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)
+	}
+}