Move all backends into backend directory

backend/sftp/stringlock.go

@@ -0,0 +1,47 @@
+package sftp
+
+import "sync"
+
+// stringLock locks for string IDs passed in
+type stringLock struct {
+	mu    sync.Mutex               // mutex to protect below
+	locks map[string]chan struct{} // map of locks
+}
+
+// newStringLock creates a stringLock
+func newStringLock() *stringLock {
+	return &stringLock{
+		locks: make(map[string]chan struct{}),
+	}
+}
+
+// Lock locks on the id passed in
+func (l *stringLock) Lock(ID string) {
+	l.mu.Lock()
+	for {
+		ch, ok := l.locks[ID]
+		if !ok {
+			break
+		}
+		// Wait for the channel to be closed
+		l.mu.Unlock()
+		// fs.Logf(nil, "Waiting for stringLock on %q", ID)
+		<-ch
+		l.mu.Lock()
+	}
+	l.locks[ID] = make(chan struct{})
+	l.mu.Unlock()
+}
+
+// Unlock unlocks on the id passed in.  Will panic if Lock with the
+// given id wasn't called first.
+func (l *stringLock) Unlock(ID string) {
+	l.mu.Lock()
+	ch, ok := l.locks[ID]
+	if !ok {
+		panic("stringLock: Unlock before Lock")
+	}
+	close(ch)
+	delete(l.locks, ID)
+	l.mu.Unlock()
+}