GODRIVER-1750 Ensure contexts are always cancelled during server moni…

…toring (mongodb#654)

x/mongo/driver/topology/server.go

@@ -617,6 +617,10 @@ func (s *Server) setupHeartbeatConnection() error {
 
 	// Take the lock when assigning the context and connection because they're accessed by cancelCheck.
 	s.heartbeatLock.Lock()
+	if s.heartbeatCtxCancel != nil {
+		// Ensure the previous context is cancelled to avoid a leak.
+		s.heartbeatCtxCancel()
+	}
 	s.heartbeatCtx, s.heartbeatCtxCancel = context.WithCancel(s.globalCtx)
 	s.conn = conn
 	s.heartbeatLock.Unlock()

x/mongo/driver/topology/server_test.go

@@ -627,6 +627,42 @@ func TestServer(t *testing.T) {
 		assert.Equal(t, s.cfg.heartbeatTimeout, conn.readTimeout, "expected readTimeout to be: %v, got: %v", s.cfg.heartbeatTimeout, conn.readTimeout)
 		assert.Equal(t, s.cfg.heartbeatTimeout, conn.writeTimeout, "expected writeTimeout to be: %v, got: %v", s.cfg.heartbeatTimeout, conn.writeTimeout)
 	})
+	t.Run("heartbeat contexts are not leaked", func(t *testing.T) {
+		// The context created for heartbeats should be cancelled when it is no longer needed to avoid leaks.
+
+		server, err := ConnectServer(
+			address.Address("invalid"),
+			nil,
+			primitive.NewObjectID(),
+			withMonitoringDisabled(func(bool) bool {
+				return true
+			}),
+		)
+		assert.Nil(t, err, "ConnectServer error: %v", err)
+
+		// Expect check to return an error in the server description because the server address doesn't exist. This is
+		// OK because we just want to ensure the heartbeat context is created.
+		desc, err := server.check()
+		assert.Nil(t, err, "check error: %v", err)
+		assert.NotNil(t, desc.LastError, "expected server description to contain an error, got nil")
+		assert.NotNil(t, server.heartbeatCtx, "expected heartbeatCtx to be non-nil, got nil")
+		assert.Nil(t, server.heartbeatCtx.Err(), "expected heartbeatCtx error to be nil, got %v", server.heartbeatCtx.Err())
+
+		// Override heartbeatCtxCancel with a wrapper that records whether or not it was called.
+		oldCancelFn := server.heartbeatCtxCancel
+		var previousCtxCancelled bool
+		server.heartbeatCtxCancel = func() {
+			previousCtxCancelled = true
+			oldCancelFn()
+		}
+
+		// The second check call should attempt to create a new heartbeat connection and should cancel the previous
+		// heartbeatCtx during the process.
+		desc, err = server.check()
+		assert.Nil(t, err, "check error: %v", err)
+		assert.NotNil(t, desc.LastError, "expected server description to contain an error, got nil")
+		assert.True(t, previousCtxCancelled, "expected check to cancel previous context but did not")
+	})
 }
 
 func includesMetadata(t *testing.T, wm []byte) bool {