Avoid deadlock if Stop is called before Serve

Make a tiny state machine to detect the transition init -> stopped (not via serving) Fixes jaegertracing#2601 Related jaegertracing#103 Signed-off-by: Carl Henrik Lunde <[email protected]>
chlunde · Oct 30, 2020 · e4bc0af · e4bc0af
1 parent ec03a2d
commit e4bc0af
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Showing 2 changed files with 19 additions and 17 deletions.
diff --git a/cmd/agent/app/agent_test.go b/cmd/agent/app/agent_test.go
@@ -128,15 +128,6 @@ func withRunningAgent(t *testing.T, testcase func(string, chan error)) {
 
 	testcase(agent.HTTPAddr(), ch)
 
-	// TODO (wjang) We sleep because the processors in the agent might not have had time to
-	// start up yet. If we were to call Stop() before the processors have time to startup,
-	// it'll panic because of a DATA RACE between wg.Add() and wg.Wait() in thrift_processor.
-	// A real fix for this issue would be to add semaphores to tbuffered_server, thrift_processor,
-	// and agent itself. Given all this extra overhead and testing required to get this to work
-	// "elegantly", I opted to just sleep here given how unlikely this situation will occur in
-	// production.
-	time.Sleep(2 * time.Second)
-
 	agent.Stop()
 	assert.NoError(t, <-ch)
 
@@ -181,10 +172,6 @@ func TestStartStopRace(t *testing.T) {
 		t.Fatalf("error from agent.Run(): %s", err)
 	}
 
-	// FIXME https://github.com/jaegertracing/jaeger/issues/2601
-	t.Log("give some time for processors to start")
-	time.Sleep(500 * time.Millisecond)
-
 	t.Log("stopping agent")
 	agent.Stop()
 

diff --git a/cmd/agent/app/servers/tbuffered_server.go b/cmd/agent/app/servers/tbuffered_server.go
@@ -59,6 +59,16 @@ type TBufferedServer struct {
 	}
 }
 
+// state values for TBufferedServer.serving
+//
+// init -> serving -> stopped
+// init -> stopped (might happen in unit tests)
+const (
+	stateStopped = iota
+	stateServing
+	stateInit
+)
+
 // NewTBufferedServer creates a TBufferedServer
 func NewTBufferedServer(
 	transport ThriftTransport,
@@ -79,14 +89,20 @@ func NewTBufferedServer(
 		maxQueueSize:  maxQueueSize,
 		maxPacketSize: maxPacketSize,
 		readBufPool:   readBufPool,
+		serving:       stateInit,
 	}
+
 	metrics.MustInit(&res.metrics, mFactory, nil)
 	return res, nil
 }
 
 // Serve initiates the readers and starts serving traffic
 func (s *TBufferedServer) Serve() {
-	atomic.StoreUint32(&s.serving, 1)
+	defer close(s.dataChan)
+	if !atomic.CompareAndSwapUint32(&s.serving, stateInit, stateServing) {
+		return // Stop already called
+	}
+
 	for s.IsServing() {
 		readBuf := s.readBufPool.Get().(*ReadBuf)
 		n, err := s.transport.Read(readBuf.bytes)
@@ -104,7 +120,6 @@ func (s *TBufferedServer) Serve() {
 			s.metrics.ReadError.Inc(1)
 		}
 	}
-	close(s.dataChan)
 }
 
 func (s *TBufferedServer) updateQueueSize(delta int64) {
@@ -114,13 +129,13 @@ func (s *TBufferedServer) updateQueueSize(delta int64) {
 
 // IsServing indicates whether the server is currently serving traffic
 func (s *TBufferedServer) IsServing() bool {
-	return atomic.LoadUint32(&s.serving) == 1
+	return atomic.LoadUint32(&s.serving) == stateServing
 }
 
 // Stop stops the serving of traffic and waits until the queue is
 // emptied by the readers
 func (s *TBufferedServer) Stop() {
-	atomic.StoreUint32(&s.serving, 0)
+	atomic.StoreUint32(&s.serving, stateStopped)
 	_ = s.transport.Close()
 }