Don't use named return values in layerStore.Put

We will need want to refer to "layer" in a defer
block, in order to delete that layer. That doesn't work
with "layer" being a named return value, because a
(return nil, -1, ...) sets "layer" to nil.

So, turn "layer" into a local variable, and use an unnamed
return value. And beause all return values must be named,
or unnamed, consistently, turn "size" and "err" also into
local variables.

Then decrease the scope of the "size" and "err" local variables
to simplify understanding the code a bit.

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

layers.go

@@ -683,11 +683,10 @@ func (r *layerStore) PutAdditionalLayer(id string, parentLayer *Layer, names []s
 	return copyLayer(layer), nil
 }
 
-func (r *layerStore) Put(id string, parentLayer *Layer, names []string, mountLabel string, options map[string]string, moreOptions *LayerOptions, writeable bool, flags map[string]interface{}, diff io.Reader) (layer *Layer, size int64, err error) {
+func (r *layerStore) Put(id string, parentLayer *Layer, names []string, mountLabel string, options map[string]string, moreOptions *LayerOptions, writeable bool, flags map[string]interface{}, diff io.Reader) (*Layer, int64, error) {
 	if !r.IsReadWrite() {
 		return nil, -1, errors.Wrapf(ErrStoreIsReadOnly, "not allowed to create new layers at %q", r.layerspath())
 	}
-	size = -1
 	if err := os.MkdirAll(r.rundir, 0700); err != nil {
 		return nil, -1, err
 	}
@@ -740,24 +739,24 @@ func (r *layerStore) Put(id string, parentLayer *Layer, names []string, mountLab
 		IDMappings: idMappings,
 	}
 	if moreOptions.TemplateLayer != "" {
-		if err = r.driver.CreateFromTemplate(id, moreOptions.TemplateLayer, templateIDMappings, parent, parentMappings, &opts, writeable); err != nil {
+		if err := r.driver.CreateFromTemplate(id, moreOptions.TemplateLayer, templateIDMappings, parent, parentMappings, &opts, writeable); err != nil {
 			return nil, -1, errors.Wrapf(err, "error creating copy of template layer %q with ID %q", moreOptions.TemplateLayer, id)
 		}
 		oldMappings = templateIDMappings
 	} else {
 		if writeable {
-			if err = r.driver.CreateReadWrite(id, parent, &opts); err != nil {
+			if err := r.driver.CreateReadWrite(id, parent, &opts); err != nil {
 				return nil, -1, errors.Wrapf(err, "error creating read-write layer with ID %q", id)
 			}
 		} else {
-			if err = r.driver.Create(id, parent, &opts); err != nil {
+			if err := r.driver.Create(id, parent, &opts); err != nil {
 				return nil, -1, errors.Wrapf(err, "error creating layer with ID %q", id)
 			}
 		}
 		oldMappings = parentMappings
 	}
 	if !reflect.DeepEqual(oldMappings.UIDs(), idMappings.UIDs()) || !reflect.DeepEqual(oldMappings.GIDs(), idMappings.GIDs()) {
-		if err = r.driver.UpdateLayerIDMap(id, oldMappings, idMappings, mountLabel); err != nil {
+		if err := r.driver.UpdateLayerIDMap(id, oldMappings, idMappings, mountLabel); err != nil {
 			// We don't have a record of this layer, but at least
 			// try to clean it up underneath us.
 			if err2 := r.driver.Remove(id); err2 != nil {
@@ -767,7 +766,7 @@ func (r *layerStore) Put(id string, parentLayer *Layer, names []string, mountLab
 		}
 	}
 
-	layer = &Layer{
+	layer := &Layer{
 		ID:           id,
 		Parent:       parent,
 		Names:        names,
@@ -788,7 +787,7 @@ func (r *layerStore) Put(id string, parentLayer *Layer, names []string, mountLab
 		layer.Flags[flag] = value
 	}
 	layer.Flags[incompleteFlag] = true
-	err = r.Save()
+	err := r.Save()
 	if err != nil {
 		// We don't have a presistent record of this layer, but
 		// try to remove both the driver’s data as well as
@@ -798,6 +797,7 @@ func (r *layerStore) Put(id string, parentLayer *Layer, names []string, mountLab
 		}
 		return nil, -1, err
 	}
+	var size int64 = -1
 	if diff != nil {
 		size, err = r.applyDiffWithOptions(layer.ID, moreOptions, diff)
 		if err != nil {