Fix gathering candidates race condition

Canceling previous gathering does not happen synchronously. If it takes
a little bit longer, it can close newly created `gatherCandidateDone`
channel. If we pass channel to the `gatherCandidates` function, it will
always close its done channel and only the latest done channel will be
available in `gatherCandidateDone`.

gather.go

@@ -51,17 +51,18 @@ func (a *Agent) GatherCandidates() error {
 		a.gatherCandidateCancel() // Cancel previous gathering routine
 		ctx, cancel := context.WithCancel(ctx)
 		a.gatherCandidateCancel = cancel
-		a.gatherCandidateDone = make(chan struct{})
+		done := make(chan struct{})
+		a.gatherCandidateDone = done
 
-		go a.gatherCandidates(ctx)
+		go a.gatherCandidates(ctx, done)
 	}); runErr != nil {
 		return runErr
 	}
 	return gatherErr
 }
 
-func (a *Agent) gatherCandidates(ctx context.Context) {
-	defer close(a.gatherCandidateDone)
+func (a *Agent) gatherCandidates(ctx context.Context, done chan struct{}) {
+	defer close(done)
 	if err := a.setGatheringState(GatheringStateGathering); err != nil { //nolint:contextcheck
 		a.log.Warnf("failed to set gatheringState to GatheringStateGathering: %v", err)
 		return

gather_test.go

@@ -87,6 +87,34 @@ func TestListenUDP(t *testing.T) {
 	assert.NoError(t, a.Close())
 }
 
+func TestGatherConcurrency(t *testing.T) {
+	report := test.CheckRoutines(t)
+	defer report()
+
+	lim := test.TimeOut(time.Second * 30)
+	defer lim.Stop()
+
+	a, err := NewAgent(&AgentConfig{
+		NetworkTypes:    []NetworkType{NetworkTypeUDP4, NetworkTypeUDP6},
+		IncludeLoopback: true,
+	})
+	assert.NoError(t, err)
+
+	candidateGathered, candidateGatheredFunc := context.WithCancel(context.Background())
+	assert.NoError(t, a.OnCandidate(func(c Candidate) {
+		candidateGatheredFunc()
+	}))
+
+	// tesing for panic
+	for i := 0; i < 10; i++ {
+		_ = a.GatherCandidates()
+	}
+
+	<-candidateGathered.Done()
+
+	assert.NoError(t, a.Close())
+}
+
 func TestLoopbackCandidate(t *testing.T) {
 	report := test.CheckRoutines(t)
 	defer report()