Fixing releaseableLayer handling of layer streams and mounts.

releaseableLayer includes automatic handling for creating a read/write layer and mounting it on a call to Mount(), but then does not correspondingly unmount the layer before trying to delete it, which will fail for some graphdrivers. Commit on a releaseable layer also leaks the tarstream for the layer. To fix this, the stream close is deferred in Commit and releaseRWLayer now correctly handles unmounting the layer before trying to delete it.  In addition, the changes include better error handling in Release() to make sure that errors are returned to the caller for failures on read/write layers instead of being ignored.# Please enter the commit message for your changes. Lines starting

Signed-off-by: Stefan Wernli <swernli@ntdev.microsoft.com>

daemon/build.go

@@ -27,6 +27,7 @@ type releaseableLayer struct {
 
 func (rl *releaseableLayer) Mount() (string, error) {
 	var err error
+	var mountPath string
 	var chainID layer.ChainID
 	if rl.roLayer != nil {
 		chainID = rl.roLayer.ChainID()
@@ -38,7 +39,19 @@ func (rl *releaseableLayer) Mount() (string, error) {
 		return "", errors.Wrap(err, "failed to create rwlayer")
 	}
 
-	return rl.rwLayer.Mount("")
+	mountPath, err = rl.rwLayer.Mount("")
+	if err != nil {
+		// Clean up the layer if we fail to mount it here.
+		metadata, err := rl.layerStore.ReleaseRWLayer(rl.rwLayer)
+		layer.LogReleaseMetadata(metadata)
+		if err != nil {
+			logrus.Errorf("Failed to release RWLayer: %s", err)
+		}
+		rl.rwLayer = nil
+		return "", err
+	}
+
+	return mountPath, nil
 }
 
 func (rl *releaseableLayer) Commit(platform string) (builder.ReleaseableLayer, error) {
@@ -51,6 +64,7 @@ func (rl *releaseableLayer) Commit(platform string) (builder.ReleaseableLayer, e
 	if err != nil {
 		return nil, err
 	}
+	defer stream.Close()
 
 	newLayer, err := rl.layerStore.Register(stream, chainID, layer.Platform(platform))
 	if err != nil {
@@ -75,20 +89,32 @@ func (rl *releaseableLayer) Release() error {
 	if rl.released {
 		return nil
 	}
+	if err := rl.releaseRWLayer(); err != nil {
+		// Best effort attempt at releasing read-only layer before returning original error.
+		rl.releaseROLayer()
+		return err
+	}
+	if err := rl.releaseROLayer(); err != nil {
+		return err
+	}
 	rl.released = true
-	rl.releaseRWLayer()
-	return rl.releaseROLayer()
+	return nil
 }
 
 func (rl *releaseableLayer) releaseRWLayer() error {
 	if rl.rwLayer == nil {
 		return nil
 	}
+	if err := rl.rwLayer.Unmount(); err != nil {
+		logrus.Errorf("Failed to unmount RWLayer: %s", err)
+		return err
+	}
 	metadata, err := rl.layerStore.ReleaseRWLayer(rl.rwLayer)
 	layer.LogReleaseMetadata(metadata)
 	if err != nil {
 		logrus.Errorf("Failed to release RWLayer: %s", err)
 	}
+	rl.rwLayer = nil
 	return err
 }
 
@@ -98,6 +124,10 @@ func (rl *releaseableLayer) releaseROLayer() error {
 	}
 	metadata, err := rl.layerStore.Release(rl.roLayer)
 	layer.LogReleaseMetadata(metadata)
+	if err != nil {
+		logrus.Errorf("Failed to release ROLayer: %s", err)
+	}
+	rl.roLayer = nil
 	return err
 }