vfs: add Busy to tell if a VFS still has data to write

Busy returns true if the VFS has files open for write or files in the
VFS cache which are open or waiting to be uploaded. It is for code
which wants to shut a VFS down only once that wouldn't lose any work.

vfs/vfs.go

@@ -457,6 +457,25 @@ func (vfs *VFS) FlushDirCache() {
 	vfs.root.ForgetAll()
 }
 
+// countInUse returns the number of files open for write and the
+// number of cached files which are open or waiting to be uploaded.
+func (vfs *VFS) countInUse() (writers, cacheInUse int) {
+	writers = vfs.root.countActiveWriters()
+	if vfs.cache != nil {
+		cacheInUse = vfs.cache.TotalInUse()
+	}
+	return writers, cacheInUse
+}
+
+// Busy returns true if the VFS has files open for write or cached
+// files which are open or waiting to be uploaded.
+//
+// It may block while a directory is being read from the remote.
+func (vfs *VFS) Busy() bool {
+	writers, cacheInUse := vfs.countInUse()
+	return writers != 0 || cacheInUse != 0
+}
+
 // WaitForWriters sleeps until all writers have finished or
 // time.Duration has elapsed
 func (vfs *VFS) WaitForWriters(timeout time.Duration) {
@@ -468,11 +487,7 @@ func (vfs *VFS) WaitForWriters(timeout time.Duration) {
 	defer tick.Stop()
 	tick.Stop()
 	for {
-		writers := vfs.root.countActiveWriters()
-		cacheInUse := 0
-		if vfs.cache != nil {
-			cacheInUse = vfs.cache.TotalInUse()
-		}
+		writers, cacheInUse := vfs.countInUse()
 		if writers == 0 && cacheInUse == 0 {
 			return
 		}

vfs/vfs_test.go

@@ -190,6 +190,28 @@ func TestVFSNewShuttingDown(t *testing.T) {
 	vfs2.Shutdown()
 }
 
+// TestVFSBusy checks Busy reports files open for write and cached
+// files which haven't been uploaded yet.
+func TestVFSBusy(t *testing.T) {
+	opt := vfscommon.Opt
+	opt.CacheMode = vfscommon.CacheModeWrites
+	opt.WriteBack = 10 * writeBackDelay
+	_, vfs := newTestVFSOpt(t, &opt)
+	assert.False(t, vfs.Busy())
+
+	fh, err := vfs.Create("file.txt")
+	require.NoError(t, err)
+	assert.True(t, vfs.Busy(), "not busy with a file open for write")
+
+	_, err = fh.WriteString("hello")
+	require.NoError(t, err)
+	require.NoError(t, fh.Close())
+	assert.True(t, vfs.Busy(), "not busy with a file waiting to be uploaded")
+
+	vfs.WaitForWriters(waitForWritersDelay)
+	assert.False(t, vfs.Busy(), "busy after the file was uploaded")
+}
+
 // TestVFSNewWithOpts sees if the New command works properly
 func TestVFSNewWithOpts(t *testing.T) {
 	var opt = vfscommon.Opt