From e4c4409a6ea087ade972b61b6c8e0b8e8eeff37d 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            | 12 ++++++++++++
 integration/v3_watch_test.go | 32 ++++++++++++++++++++++++++++++++
 2 files changed, 44 insertions(+)

diff --git a/clientv3/watch.go b/clientv3/watch.go
index 4a3b8cccda0..290094c60af 100644
--- a/clientv3/watch.go
+++ b/clientv3/watch.go
@@ -637,6 +637,18 @@ 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}
+				wc.Send(req)
+			}
 			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 c91f4df6503..2aca66605b1 100644
--- a/integration/v3_watch_test.go
+++ b/integration/v3_watch_test.go
@@ -1201,3 +1201,35 @@ 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)
+	}
+}