single-thread staging to avoid potential double commit

Change-Id: I309b4440c93858d583d7d719c2f83a9fa3a124fb

adapter/analytics/analytics.go

@@ -67,6 +67,7 @@ type manager struct {
 	secret             string
 	sendChannelSize    int
 	closeWait          sync.WaitGroup
+	stageLock          sync.Mutex
 }
 
 // Start starts the manager.

adapter/analytics/staging.go

@@ -30,6 +30,9 @@ import (
 
 // stageUpload moves anything in the temp dir to the staging dir
 func (m *manager) stageUpload() error {
+	m.stageLock.Lock()
+	defer m.stageLock.Unlock()
+
 	subDirs, err := ioutil.ReadDir(m.tempDir)
 	if err != nil {
 		return fmt.Errorf("ReadDir(%s): %s", m.tempDir, err)