Move shared checks to base storage driver

This changeset moves common checks from driver implementations into base
package. The Base type can be embedded in a driver implementation, ensuring
that incoming arguments are checked for validity.

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

storagedriver/base/base.go

@@ -0,0 +1,141 @@
+// Package base provides a base implementation of the storage driver that can
+// be used to implement common checks. The goal is to increase the amount of
+// code sharing.
+//
+// The canonical approach to use this class is to embed in the exported driver
+// struct such that calls are proxied through this implementation. First,
+// declare the internal driver, as follows:
+//
+// 	type driver struct { ... internal ...}
+//
+// The resulting type should implement StorageDriver such that it can be the
+// target of a Base struct. The exported type can then be declared as follows:
+//
+// 	type Driver struct {
+// 		Base
+// 	}
+//
+// Because Driver embeds Base, it effectively implements Base. If the driver
+// needs to intercept a call, before going to base, Driver should implement
+// that method. Effectively, Driver can intercept calls before coming in and
+// driver implements the actual logic.
+//
+// To further shield the embed from other packages, it is recommended to
+// employ a private embed struct:
+//
+// 	type baseEmbed struct {
+// 		base.Base
+// 	}
+//
+// Then, declare driver to embed baseEmbed, rather than Base directly:
+//
+// 	type Driver struct {
+// 		baseEmbed
+// 	}
+//
+// The type now implements StorageDriver, proxying through Base, without
+// exporting an unnessecary field.
+package base
+
+import (
+	"io"
+
+	"github.com/docker/distribution/storagedriver"
+)
+
+// Base provides a wrapper around a storagedriver implementation that provides
+// common path and bounds checking.
+type Base struct {
+	storagedriver.StorageDriver
+}
+
+// GetContent wraps GetContent of underlying storage driver.
+func (base *Base) GetContent(path string) ([]byte, error) {
+	if !storagedriver.PathRegexp.MatchString(path) {
+		return nil, storagedriver.InvalidPathError{Path: path}
+	}
+
+	return base.StorageDriver.GetContent(path)
+}
+
+// PutContent wraps PutContent of underlying storage driver.
+func (base *Base) PutContent(path string, content []byte) error {
+	if !storagedriver.PathRegexp.MatchString(path) {
+		return storagedriver.InvalidPathError{Path: path}
+	}
+
+	return base.StorageDriver.PutContent(path, content)
+}
+
+// ReadStream wraps ReadStream of underlying storage driver.
+func (base *Base) ReadStream(path string, offset int64) (io.ReadCloser, error) {
+	if offset < 0 {
+		return nil, storagedriver.InvalidOffsetError{Path: path, Offset: offset}
+	}
+
+	if !storagedriver.PathRegexp.MatchString(path) {
+		return nil, storagedriver.InvalidPathError{Path: path}
+	}
+
+	return base.StorageDriver.ReadStream(path, offset)
+}
+
+// WriteStream wraps WriteStream of underlying storage driver.
+func (base *Base) WriteStream(path string, offset int64, reader io.Reader) (nn int64, err error) {
+	if offset < 0 {
+		return 0, storagedriver.InvalidOffsetError{Path: path, Offset: offset}
+	}
+
+	if !storagedriver.PathRegexp.MatchString(path) {
+		return 0, storagedriver.InvalidPathError{Path: path}
+	}
+
+	return base.StorageDriver.WriteStream(path, offset, reader)
+}
+
+// Stat wraps Stat of underlying storage driver.
+func (base *Base) Stat(path string) (storagedriver.FileInfo, error) {
+	if !storagedriver.PathRegexp.MatchString(path) {
+		return nil, storagedriver.InvalidPathError{Path: path}
+	}
+
+	return base.StorageDriver.Stat(path)
+}
+
+// List wraps List of underlying storage driver.
+func (base *Base) List(path string) ([]string, error) {
+	if !storagedriver.PathRegexp.MatchString(path) && path != "/" {
+		return nil, storagedriver.InvalidPathError{Path: path}
+	}
+
+	return base.StorageDriver.List(path)
+}
+
+// Move wraps Move of underlying storage driver.
+func (base *Base) Move(sourcePath string, destPath string) error {
+	if !storagedriver.PathRegexp.MatchString(sourcePath) {
+		return storagedriver.InvalidPathError{Path: sourcePath}
+	} else if !storagedriver.PathRegexp.MatchString(destPath) {
+		return storagedriver.InvalidPathError{Path: destPath}
+	}
+
+	return base.StorageDriver.Move(sourcePath, destPath)
+}
+
+// Delete wraps Delete of underlying storage driver.
+func (base *Base) Delete(path string) error {
+	if !storagedriver.PathRegexp.MatchString(path) {
+		return storagedriver.InvalidPathError{Path: path}
+	}
+
+	return base.StorageDriver.Delete(path)
+}
+
+// URLFor wraps URLFor of underlying storage driver.
+func (base *Base) URLFor(path string, options map[string]interface{}) (string, error) {
+	if !storagedriver.PathRegexp.MatchString(path) {
+		return "", storagedriver.InvalidPathError{Path: path}
+	}
+
+	return base.StorageDriver.URLFor(path, options)
+}