clientV3watch: Watch Close should close successfully

Closing of watch by client will cancel the watch grpc stream and
can produce a context canceled error. However, since client
simply wanted to close the watcher the error can create confusion
that something went wrong instead of a successful close. Ensure
that Close do not return error.

Fixed #10340

clientv3/integration/watch_test.go

@@ -1133,3 +1133,24 @@ func TestWatchCancelDisconnected(t *testing.T) {
 		t.Fatal("took too long to cancel disconnected watcher")
 	}
 }
+
+// TestWatchClose ensures that close does not return error
+func TestWatchClose(t *testing.T) {
+	runWatchTest(t, testWatchClose)
+}
+
+func testWatchClose(t *testing.T, wctx *watchctx) {
+	ctx, cancel := context.WithCancel(context.Background())
+	wch := wctx.w.Watch(ctx, "a")
+	cancel()
+	if wch == nil {
+		t.Fatalf("expected watcher channel, got nil")
+	}
+	if wctx.w.Close() != nil {
+		t.Fatalf("watch did not close successfully")
+	}
+	wresp, ok := <-wch
+	if ok {
+		t.Fatalf("read wch got %v; expected closed channel", wresp)
+	}
+}

clientv3/watch.go

@@ -371,6 +371,10 @@ func (w *watcher) Close() (err error) {
 			err = werr
 		}
 	}
+	// Consider context.Canceled as a successful close
+	if err == context.Canceled {
+		err = nil
+	}
 	return err
 }