Replace containerStore.{Lock,Unlock} with {startWriting,stopWriting}

This integrates ReloadIfChanges, and makes it clearer that the responsibility
for maintaining locking details is with the containerStore; we can change it
in a single place.

Should not change behavior.

Signed-off-by: Miloslav Trmač <mitr@redhat.com>

storage/containers.go

@@ -72,17 +72,12 @@ type rwContainerStore interface {
 	containerBigDataStore
 	flaggableStore
 
-	// Acquire a writer lock.
-	// The default unix implementation panics if:
-	// - opening the lockfile failed
-	// - tried to lock a read-only lock-file
-	Lock()
+	// startWriting makes sure the store is fresh, and locks it for writing.
+	// If this succeeds, the caller MUST call stopWriting().
+	startWriting() error
 
-	// Unlock the lock.
-	// The default unix implementation panics if:
-	// - unlocking an unlocked lock
-	// - if the lock counter is corrupted
-	Unlock()
+	// stopWriting releases locks obtained by startWriting.
+	stopWriting()
 
 	// startReading makes sure the store is fresh, and locks it for reading.
 	// If this succeeds, the caller MUST call stopReading().
@@ -209,6 +204,30 @@ func (c *Container) MountOpts() []string {
 	}
 }
 
+// startWriting makes sure the store is fresh, and locks it for writing.
+// If this succeeds, the caller MUST call stopWriting().
+func (r *containerStore) startWriting() error {
+	r.lockfile.Lock()
+	succeeded := false
+	defer func() {
+		if !succeeded {
+			r.lockfile.Unlock()
+		}
+	}()
+
+	if err := r.ReloadIfChanged(); err != nil {
+		return err
+	}
+
+	succeeded = true
+	return nil
+}
+
+// stopWriting releases locks obtained by startWriting.
+func (r *containerStore) stopWriting() {
+	r.lockfile.Unlock()
+}
+
 // startReading makes sure the store is fresh, and locks it for reading.
 // If this succeeds, the caller MUST call stopReading().
 func (r *containerStore) startReading() error {

storage/store.go

@@ -1119,11 +1119,10 @@ func (s *store) writeToContainerStore(fn func(store rwContainerStore) error) err
 		return err
 	}
 
-	store.Lock()
-	defer store.Unlock()
-	if err := store.ReloadIfChanged(); err != nil {
+	if err := store.startWriting(); err != nil {
 		return err
 	}
+	defer store.stopWriting()
 	return fn(store)
 }
 
@@ -1154,11 +1153,10 @@ func (s *store) writeToAllStores(fn func(rlstore rwLayerStore, ristore rwImageSt
 	if err := ristore.ReloadIfChanged(); err != nil {
 		return err
 	}
-	rcstore.Lock()
-	defer rcstore.Unlock()
-	if err := rcstore.ReloadIfChanged(); err != nil {
+	if err := rcstore.startWriting(); err != nil {
 		return err
 	}
+	defer rcstore.stopWriting()
 
 	return fn(rlstore, ristore, rcstore)
 }
@@ -1195,11 +1193,10 @@ func (s *store) PutLayer(id, parent string, names []string, mountLabel string, w
 	if err := rlstore.ReloadIfChanged(); err != nil {
 		return nil, -1, err
 	}
-	rcstore.Lock()
-	defer rcstore.Unlock()
-	if err := rcstore.ReloadIfChanged(); err != nil {
+	if err := rcstore.startWriting(); err != nil {
 		return nil, -1, err
 	}
+	defer rcstore.stopWriting()
 	if options == nil {
 		options = &LayerOptions{}
 	}