net: Forget lookups for canceled contexts

A sequential lookup using any non-canceled context has a risk of
returning the result of the previous lookup for a canceled context (i.e.
an error).

This is already prevented for timed out context by forgetting the host
immediately and extending this to also compare the error to
`context.Canceled` resolves this issue.

Fixes #22724

Change-Id: I7aafa1459a0de4dc5c4332988fbea23cbf4dba07
Reviewed-on: https://go-review.googlesource.com/77670
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

Author: tt

src/net/lookup.go

@@ -200,7 +200,7 @@ func (r *Resolver) LookupIPAddr(ctx context.Context, host string) ([]IPAddr, err
 		// rather than waiting for the current lookup to
 		// complete. See issue 8602.
 		ctxErr := ctx.Err()
-		if ctxErr == context.DeadlineExceeded {
+		if ctxErr == context.Canceled || ctxErr == context.DeadlineExceeded {
 			lookupGroup.Forget(host)
 		}
 		err := mapErr(ctxErr)

src/net/lookup_test.go

@@ -739,3 +739,25 @@ func TestLookupNonLDH(t *testing.T) {
 		t.Fatalf("lookup error = %v, want %v", err, errNoSuchHost)
 	}
 }
+
+func TestLookupContextCancel(t *testing.T) {
+	if runtime.GOOS == "nacl" {
+		t.Skip("skip on NaCl")
+	}
+
+	ctx, ctxCancel := context.WithCancel(context.Background())
+	ctxCancel()
+
+	_, err := DefaultResolver.LookupIPAddr(ctx, "google.com")
+	if err != errCanceled {
+		testenv.SkipFlakyNet(t)
+		t.Fatalf("unexpected error: %q", err)
+	}
+
+	ctx = context.Background()
+
+	_, err = DefaultResolver.LookupIPAddr(ctx, "google.com")
+	if err != nil {
+		t.Fatalf("unexpected error: %q", err)
+	}
+}