Syncrhonize uploads until one is successful, then allow concurrent

To prevent being asked for credentials n times (where n = # of
concurrent uploads), we process the upload queue synchronously until one
of the uploads succeeds, and then allow concurrent uploads.

This could be a bit faster if the condition is fired after the first
successful API hit, rather than the first successful total upload, but
some deeper refactoring will need to be done.

lfs/upload_queue.go

@@ -9,6 +9,10 @@ import (
 	"sync/atomic"
 )
 
+var (
+	clientAuthorized = 0
+)
+
 // Uploadable describes a file that can be uploaded.
 type Uploadable struct {
 	OIDPath  string
@@ -55,15 +59,17 @@ type UploadQueue struct {
 	files    int
 	finished int64
 	size     int64
+	authCond *sync.Cond
 }
 
 // NewUploadQueue builds an UploadQueue, allowing `workers` concurrent uploads.
 func NewUploadQueue(workers, files int) *UploadQueue {
 	return &UploadQueue{
-		uploadc: make(chan *Uploadable, files),
-		errorc:  make(chan *WrappedError),
-		workers: workers,
-		files:   files,
+		uploadc:  make(chan *Uploadable, files),
+		errorc:   make(chan *WrappedError),
+		workers:  workers,
+		files:    files,
+		authCond: sync.NewCond(&sync.Mutex{}),
 	}
 }
 
@@ -88,9 +94,19 @@ func (q *UploadQueue) Process() {
 		}
 	}()
 
+	workersReady := make(chan int, q.workers)
+
 	for i := 0; i < q.workers; i++ {
 		go func(n int) {
+			workersReady <- 1
+
 			for upload := range q.uploadc {
+				q.authCond.L.Lock()
+				if clientAuthorized == 0 {
+					q.authCond.Wait()
+				}
+				q.authCond.L.Unlock()
+
 				cb := func(total, read int64, current int) error {
 					bar.Add(current)
 					if upload.CB != nil {
@@ -102,7 +118,14 @@ func (q *UploadQueue) Process() {
 				err := Upload(upload.OIDPath, upload.Filename, cb)
 				if err != nil {
 					q.errorc <- err
+					// This one failed, send a Signal() to wake up the next one
+					q.authCond.Signal()
+				} else {
+					// This one succeeded, set the auth condition to 1 and Broadcast()
+					clientAuthorized = 1
+					q.authCond.Broadcast()
 				}
+
 				f := atomic.AddInt64(&q.finished, 1)
 				bar.Prefix(fmt.Sprintf("(%d of %d files) ", f, q.files))
 				q.wg.Done()
@@ -111,6 +134,8 @@ func (q *UploadQueue) Process() {
 	}
 
 	close(q.uploadc)
+	<-workersReady
+	q.authCond.Signal() // Signal the first goroutine to run
 	q.wg.Wait()
 	close(q.errorc)