Move storagedriver package to registry/storage/driver

This change is slightly more complex than previous package maves in that the
package name changed. To address this, we simply always reference the package
driver as storagedriver to avoid compatbility issues with existing code. While
unfortunate, this can be cleaned up over time.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

docs/storage/driver/fileinfo.go

@@ -0,0 +1,79 @@
+package driver
+
+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
+}