net: don't forget about ongoing DNS lookup if context canceled

Only forget about it if the context timed out, as the comment says.

Fixes #20703.

Change-Id: Ie6234f1a32f85e6bfd052dc24a33aa63b8883c37
Reviewed-on: https://go-review.googlesource.com/45999
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/lookup.go

@@ -185,12 +185,15 @@ func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, err
 
 	select {
 	case <-ctx.Done():
-		// The DNS lookup timed out for some reason. Force
+		// If the DNS lookup timed out for some reason, force
 		// future requests to start the DNS lookup again
 		// rather than waiting for the current lookup to
 		// complete. See issue 8602.
-		err := mapErr(ctx.Err())
+		ctxErr := ctx.Err()
+		if ctxErr == context.DeadlineExceeded {
 			lookupGroup.Forget(host)
+		}
+		err := mapErr(ctxErr)
 		if trace != nil && trace.DNSDone != nil {
 			trace.DNSDone(nil, false, err)
 		}