From 87aa5a9e60dd9da84de1ab10b2d1562d703dba72 Mon Sep 17 00:00:00 2001
From: Jack Kleeman <jackkleeman@gmail.com>
Date: Wed, 6 May 2020 11:57:19 +0100
Subject: [PATCH] clientv3: cancel watches proactively on client context
 cancellation

Currently, watch cancel requests are only sent to the server after a
message comes through on a watch where the client has cancelled. This
means that cancelled watches that don't receive any new messages are
never cancelled; they persist for the lifetime of the client stream.
This has negative connotations for locking applications where a watch
may observe a key which might never change again after cancellation,
leading to many accumulating watches on the server.

By cancelling proactively, in most cases we simply move the cancel
request to happen earlier, and additionally we solve the case where the
cancel request would never be sent.

Fixes #9416
Heavy inspiration drawn from the solutions proposed there.
---
 clientv3/watch.go            | 18 +++++++++++++++++-
 integration/v3_watch_test.go | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 49 insertions(+), 1 deletion(-)

diff --git a/clientv3/watch.go b/clientv3/watch.go
index e2f11ad263c..36bf1a71820 100644
--- a/clientv3/watch.go
+++ b/clientv3/watch.go
@@ -624,8 +624,9 @@ func (w *watchGrpcStream) run() {
 					},
 				}
 				req := &pb.WatchRequest{RequestUnion: cr}
+				lg.Info("sending watch cancel request for failed dispatch", zap.Int64("watch-id", pbresp.WatchId))
 				if err := wc.Send(req); err != nil {
-					lg.Warningf("error when sending request: %v", err)
+					lg.Warning("failed to send watch cancel request", zap.Int64("watch-id", pbresp.WatchId), zap.Error(err))
 				}
 			}
 
@@ -649,6 +650,21 @@ func (w *watchGrpcStream) run() {
 			return
 
 		case ws := <-w.closingc:
+			if ws.id != -1 {
+				// client is closing an established watch; close it on the server proactively instead of waiting
+				// to close when the next message arrives
+				cancelSet[ws.id] = struct{}{}
+				cr := &pb.WatchRequest_CancelRequest{
+					CancelRequest: &pb.WatchCancelRequest{
+						WatchId: ws.id,
+					},
+				}
+				req := &pb.WatchRequest{RequestUnion: cr}
+				lg.Info("sending watch cancel request for closed watcher", zap.Int64("watch-id", ws.id))
+				if err := wc.Send(req); err != nil {
+					lg.Warning("failed to send watch cancel request", zap.Int64("watch-id", ws.id), zap.Error(err))
+				}
+			}
 			w.closeSubstream(ws)
 			delete(closing, ws)
 			// no more watchers on this stream, shutdown
diff --git a/integration/v3_watch_test.go b/integration/v3_watch_test.go
index 188b5883a47..de1ee4f295c 100644
--- a/integration/v3_watch_test.go
+++ b/integration/v3_watch_test.go
@@ -1213,3 +1213,35 @@ func TestV3WatchWithPrevKV(t *testing.T) {
 		}
 	}
 }
+
+// TestV3WatchCancellation ensures that watch cancellation frees up server resources.
+func TestV3WatchCancellation(t *testing.T) {
+	clus := NewClusterV3(t, &ClusterConfig{Size: 1})
+	defer clus.Terminate(t)
+
+	ctx, cancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer cancel()
+
+	cli := clus.RandClient()
+
+	// increment watcher total count and keep a stream open
+	cli.Watch(ctx, "/foo")
+
+	for i := 0; i < 1000; i++ {
+		ctx, cancel := context.WithCancel(ctx)
+		cli.Watch(ctx, "/foo")
+		cancel()
+	}
+
+	// Wait a little for cancellations to take hold
+	time.Sleep(3 * time.Second)
+
+	minWatches, err := clus.Members[0].Metric("etcd_debugging_mvcc_watcher_total")
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if minWatches != "1" {
+		t.Fatalf("expected one watch, got %s", minWatches)
+	}
+}