Only use ModifiedSince, not TouchedSince, on store.graphLock

At least containers#926 suggests that
using timestamps "seems to fail", without elaborating further.

At the very least, ModifiedSince is the more general check, because it
can work across shared filesystems or on filesystems with bad timestamp
granularity, it's about as costly (a single syscall, pread vs. fstat),
and we can now completely eliminate tracking store.lastLoaded.

The more common code shape will also help factor the common code out further.

Signed-off-by: Miloslav Trmač <[email protected]>

store.go

@@ -602,7 +602,6 @@ type store struct {
 	transientStore  bool
 
 	// The following fields can only be accessed with graphLock held.
-	lastLoaded         time.Time
 	graphLockLastWrite lockfile.LastWrite
 	graphDriver        drivers.Driver
 	layerStore         rwLayerStore
@@ -919,10 +918,14 @@ func (s *store) getGraphDriver() (drivers.Driver, error) {
 func (s *store) GraphDriver() (drivers.Driver, error) {
 	s.graphLock.Lock()
 	defer s.graphLock.Unlock()
-	if s.graphLock.TouchedSince(s.lastLoaded) {
+	lastWrite, modified, err := s.graphLock.ModifiedSince(s.graphLockLastWrite)
+	if err != nil {
+		return nil, err
+	}
+	if modified {
 		s.graphDriver = nil
 		s.layerStore = nil
-		s.lastLoaded = time.Now()
+		s.graphLockLastWrite = lastWrite
 	}
 	return s.getGraphDriver()
 }
@@ -932,10 +935,14 @@ func (s *store) GraphDriver() (drivers.Driver, error) {
 func (s *store) getLayerStore() (rwLayerStore, error) {
 	s.graphLock.Lock()
 	defer s.graphLock.Unlock()
-	if s.graphLock.TouchedSince(s.lastLoaded) {
+	lastWrite, modified, err := s.graphLock.ModifiedSince(s.graphLockLastWrite)
+	if err != nil {
+		return nil, err
+	}
+	if modified {
 		s.graphDriver = nil
 		s.layerStore = nil
-		s.lastLoaded = time.Now()
+		s.graphLockLastWrite = lastWrite
 	}
 	if s.layerStore != nil {
 		return s.layerStore, nil
@@ -2578,7 +2585,6 @@ func (s *store) mount(id string, options drivers.MountOpts) (string, error) {
 		if err != nil {
 			return "", err
 		}
-		s.lastLoaded = time.Now()
 	}
 
 	if options.UidMaps != nil || options.GidMaps != nil {
@@ -2726,7 +2732,6 @@ func (s *store) Diff(from, to string, options *DiffOptions) (io.ReadCloser, erro
 		if err != nil {
 			return nil, err
 		}
-		s.lastLoaded = time.Now()
 	}
 
 	for _, s := range layerStores {