vfs: fix a VFS being reused while it is being shut down

When the last user of a VFS shut it down at the same time as a new
user of the same remote asked for one, the new user could be given the
VFS being shut down, with its cache and background tasks stopped.

Only a VFS which is still in use is now reused, otherwise a new one is
made.

(cherry picked from commit c62aa2adc9)

vfs/vfs.go

@@ -234,9 +234,10 @@ func New(ctx context.Context, f fs.Fs, opt *vfscommon.Options) *VFS {
 	defer activeMu.Unlock()
 	configName := fs.ConfigString(f)
 	for _, activeVFS := range active[configName] {
-		if vfs.Opt == activeVFS.Opt {
+		// A VFS whose last reference has gone is being shut down
+		// but may not have removed itself from the cache yet.
+		if vfs.Opt == activeVFS.Opt && activeVFS.Hold() {
 			fs.Debugf(f, "Reusing VFS from active cache")
-			activeVFS.inUse.Add(1)
 			cancel()
 			return activeVFS
 		}

vfs/vfs_test.go

@@ -174,6 +174,22 @@ func TestVFSHold(t *testing.T) {
 	assert.False(t, vfs.Hold(), "VFS held after being shut down")
 }
 
+// TestVFSNewShuttingDown checks New doesn't reuse a VFS which is being
+// shut down but hasn't yet removed itself from the active cache.
+func TestVFSNewShuttingDown(t *testing.T) {
+	r := fstest.NewRun(t)
+	vfs := New(context.Background(), r.Fremote, nil)
+
+	// The state Shutdown leaves the VFS in while it waits for activeMu
+	vfs.inUse.Store(0)
+	vfs2 := New(context.Background(), r.Fremote, nil)
+	assert.NotSame(t, vfs, vfs2, "reused a VFS being shut down")
+	assert.Equal(t, int32(0), vfs.inUse.Load())
+
+	vfs.Shutdown()
+	vfs2.Shutdown()
+}
+
 // TestVFSNewWithOpts sees if the New command works properly
 func TestVFSNewWithOpts(t *testing.T) {
 	var opt = vfscommon.Opt