vfs: fix SIGHUP killing serve instead of flushing directory caches

Before, rclone serve would crash when sent a SIGHUP which contradicts
the documentation - saying it should flush the directory caches.

Moved signal handling from the mount into the vfs layer, which now
handles SIGHUP on all uses of the VFS including mount and serve.

Fixes #8607

vfs/vfs.go

@@ -179,6 +179,7 @@ type VFS struct {
 	root        *Dir
 	Opt         vfscommon.Options
 	cache       *vfscache.Cache
+	cancel      context.CancelFunc
 	cancelCache context.CancelFunc
 	usageMu     sync.Mutex
 	usageTime   time.Time
@@ -197,8 +198,10 @@ var (
 // DefaultOpt will be used
 func New(f fs.Fs, opt *vfscommon.Options) *VFS {
 	fsDir := fs.NewDir("", time.Now())
+	ctx, cancel := context.WithCancel(context.Background())
 	vfs := &VFS{
-		f: f,
+		f:      f,
+		cancel: cancel,
 	}
 	vfs.inUse.Store(1)
 
@@ -259,6 +262,9 @@ func New(f fs.Fs, opt *vfscommon.Options) *VFS {
 		go vfs.refresh()
 	}
 
+	// Handle supported signals
+	go vfs.signalHandler(ctx)
+
 	// This can take some time so do it after the Pin
 	vfs.SetCacheMode(vfs.Opt.CacheMode)
 
@@ -274,6 +280,27 @@ func (vfs *VFS) refresh() {
 	}
 }
 
+// Reload VFS cache on SIGHUP
+func (vfs *VFS) signalHandler(ctx context.Context) {
+	sigHup := make(chan os.Signal, 1)
+	NotifyOnSigHup(sigHup)
+
+	waiting := true
+	for waiting {
+		select {
+		case <-ctx.Done():
+			waiting = false
+		case <-sigHup:
+			root, err := vfs.Root()
+			if err != nil {
+				fs.Errorf(vfs.Fs(), "Error reading root: %v", err)
+			} else {
+				root.ForgetAll()
+			}
+		}
+	}
+}
+
 // Stats returns info about the VFS
 func (vfs *VFS) Stats() (out rc.Params) {
 	out = make(rc.Params)
@@ -372,6 +399,9 @@ func (vfs *VFS) Shutdown() {
 		close(vfs.pollChan)
 		vfs.pollChan = nil
 	}
+
+	// Cancel any background go routines
+	vfs.cancel()
 }
 
 // CleanUp deletes the contents of the on disk cache