overlay: lock staging directories

lock any staging directory while it is being used so that another
process cannot delete it.

Now the Cleanup() function deletes only the staging directories that
are not locked by any other user.

Closes: #1915

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>

drivers/overlay/overlay.go

@@ -28,6 +28,7 @@ import (
 	"github.com/containers/storage/pkg/fsutils"
 	"github.com/containers/storage/pkg/idmap"
 	"github.com/containers/storage/pkg/idtools"
+	"github.com/containers/storage/pkg/lockfile"
 	"github.com/containers/storage/pkg/mount"
 	"github.com/containers/storage/pkg/parsers"
 	"github.com/containers/storage/pkg/system"
@@ -127,6 +128,8 @@ type Driver struct {
 	usingMetacopy    bool
 	usingComposefs   bool
 
+	stagingDirsLocks map[string]*lockfile.LockFile
+
 	supportsIDMappedMounts *bool
 }
 
@@ -460,6 +463,7 @@ func Init(home string, options graphdriver.Options) (graphdriver.Driver, error)
 		supportsVolatile: supportsVolatile,
 		usingComposefs:   opts.useComposefs,
 		options:          *opts,
+		stagingDirsLocks: make(map[string]*lockfile.LockFile),
 	}
 
 	d.naiveDiff = graphdriver.NewNaiveDiffDriver(d, graphdriver.NewNaiveLayerIDMapUpdater(d))
@@ -880,10 +884,43 @@ func (d *Driver) Metadata(id string) (map[string]string, error) {
 // is being shutdown. For now, we just have to unmount the bind mount
 // we had created.
 func (d *Driver) Cleanup() error {
-	_ = os.RemoveAll(filepath.Join(d.home, stagingDir))
+	anyPresent := d.pruneStagingDirectories()
+	if anyPresent {
+		return nil
+	}
 	return mount.Unmount(d.home)
 }
 
+// pruneStagingDirectories cleans up any staging directory that was leaked.
+// It returns whether any staging directory is still present.
+func (d *Driver) pruneStagingDirectories() bool {
+	for _, lock := range d.stagingDirsLocks {
+		lock.Unlock()
+	}
+	d.stagingDirsLocks = make(map[string]*lockfile.LockFile)
+
+	anyPresent := false
+
+	dirs, err := os.ReadDir(filepath.Join(d.home, stagingDir))
+	if err == nil {
+		for _, dir := range dirs {
+			stagingDirToRemove := filepath.Join(d.home, stagingDir, dir.Name())
+			lock, err := lockfile.GetLockFile(filepath.Join(stagingDirToRemove, "staging.lock"))
+			if err != nil {
+				anyPresent = true
+				continue
+			}
+			if err := lock.TryLock(); err != nil {
+				anyPresent = true
+				continue
+			}
+			_ = os.RemoveAll(stagingDirToRemove)
+			lock.Unlock()
+		}
+	}
+	return anyPresent
+}
+
 // LookupAdditionalLayer looks up additional layer store by the specified
 // digest and ref and returns an object representing that layer.
 // This API is experimental and can be changed without bumping the major version number.
@@ -2029,7 +2066,14 @@ func (d *Driver) DiffGetter(id string) (graphdriver.FileGetCloser, error) {
 
 // CleanupStagingDirectory cleanups the staging directory.
 func (d *Driver) CleanupStagingDirectory(stagingDirectory string) error {
-	return os.RemoveAll(filepath.Dir(stagingDirectory))
+	parentStagingDir := filepath.Dir(stagingDirectory)
+
+	if lock, ok := d.stagingDirsLocks[parentStagingDir]; ok {
+		delete(d.stagingDirsLocks, parentStagingDir)
+		lock.Unlock()
+	}
+
+	return os.RemoveAll(parentStagingDir)
 }
 
 func supportsDataOnlyLayersCached(home, runhome string) (bool, error) {
@@ -2080,6 +2124,13 @@ func (d *Driver) ApplyDiffWithDiffer(id, parent string, options *graphdriver.App
 		if err := os.Mkdir(applyDir, perms); err != nil {
 			return graphdriver.DriverWithDifferOutput{}, err
 		}
+
+		lock, err := lockfile.GetLockFile(filepath.Join(layerDir, "staging.lock"))
+		if err != nil {
+			return graphdriver.DriverWithDifferOutput{}, err
+		}
+		d.stagingDirsLocks[layerDir] = lock
+		lock.Lock()
 	} else {
 		var err error
 		applyDir, err = d.getDiffPath(id)
@@ -2113,9 +2164,18 @@ func (d *Driver) ApplyDiffWithDiffer(id, parent string, options *graphdriver.App
 // ApplyDiffFromStagingDirectory applies the changes using the specified staging directory.
 func (d *Driver) ApplyDiffFromStagingDirectory(id, parent string, diffOutput *graphdriver.DriverWithDifferOutput, options *graphdriver.ApplyDiffWithDifferOpts) error {
 	stagingDirectory := diffOutput.Target
-	if filepath.Dir(filepath.Dir(stagingDirectory)) != d.getStagingDir(id) {
+	parentStagingDir := filepath.Dir(stagingDirectory)
+	if filepath.Dir(parentStagingDir) != d.getStagingDir(id) {
 		return fmt.Errorf("%q is not a staging directory", stagingDirectory)
 	}
+
+	defer func() {
+		if lock, ok := d.stagingDirsLocks[parentStagingDir]; ok {
+			delete(d.stagingDirsLocks, parentStagingDir)
+			lock.Unlock()
+		}
+	}()
+
 	diffPath, err := d.getDiffPath(id)
 	if err != nil {
 		return err