Replace StorageLayer.CurrentSize interface call with Stat

To support single-flight Size and ModTime queries against backend storage file,
we are replacing the CurrentSize call with a Stat call. A FileInfo interface is
provided for backends to provide a type, with a default implementation called
FileInfoInternal, for use by driver implementations.

More work needs to follow this change to update all the driver implementations.

storagedriver/fileinfo.go

@@ -0,0 +1,79 @@
+package storagedriver
+
+import "time"
+
+// FileInfo returns information about a given path. Inspired by os.FileInfo,
+// it elides the base name method for a full path instead.
+type FileInfo interface {
+	// Path provides the full path of the target of this file info.
+	Path() string
+
+	// Size returns current length in bytes of the file. The return value can
+	// be used to write to the end of the file at path. The value is
+	// meaningless if IsDir returns true.
+	Size() int64
+
+	// ModTime returns the modification time for the file. For backends that
+	// don't have a modification time, the creation time should be returned.
+	ModTime() time.Time
+
+	// IsDir returns true if the path is a directory.
+	IsDir() bool
+}
+
+// NOTE(stevvooe): The next two types, FileInfoFields and FileInfoInternal
+// should only be used by storagedriver implementations. They should moved to
+// a "driver" package, similar to database/sql.
+
+// FileInfoFields provides the exported fields for implementing FileInfo
+// interface in storagedriver implementations. It should be used with
+// InternalFileInfo.
+type FileInfoFields struct {
+	// Path provides the full path of the target of this file info.
+	Path string
+
+	// Size is current length in bytes of the file. The value of this field
+	// can be used to write to the end of the file at path. The value is
+	// meaningless if IsDir is set to true.
+	Size int64
+
+	// ModTime returns the modification time for the file. For backends that
+	// don't have a modification time, the creation time should be returned.
+	ModTime time.Time
+
+	// IsDir returns true if the path is a directory.
+	IsDir bool
+}
+
+// FileInfoInternal implements the FileInfo interface. This should only be
+// used by storagedriver implementations that don't have a specialized
+// FileInfo type.
+type FileInfoInternal struct {
+	FileInfoFields
+}
+
+var _ FileInfo = FileInfoInternal{}
+var _ FileInfo = &FileInfoInternal{}
+
+// Path provides the full path of the target of this file info.
+func (fi FileInfoInternal) Path() string {
+	return fi.FileInfoFields.Path
+}
+
+// Size returns current length in bytes of the file. The return value can
+// be used to write to the end of the file at path. The value is
+// meaningless if IsDir returns true.
+func (fi FileInfoInternal) Size() int64 {
+	return fi.FileInfoFields.Size
+}
+
+// ModTime returns the modification time for the file. For backends that
+// don't have a modification time, the creation time should be returned.
+func (fi FileInfoInternal) ModTime() time.Time {
+	return fi.FileInfoFields.ModTime
+}
+
+// IsDir returns true if the path is a directory.
+func (fi FileInfoInternal) IsDir() bool {
+	return fi.FileInfoFields.IsDir
+}