Add the URLFor optional method to the storagedriver api

We now also have a storagedriver error variable for identifying
api calls that are not implemented by drivers (the URLFor method
is not implemented by either the filesystem or inmemory drivers)

storagedriver/filesystem/driver.go

@@ -265,6 +265,12 @@ func (d *Driver) Delete(subPath string) error {
 	return err
 }
 
+// URLFor returns a URL which may be used to retrieve the content stored at the given path.
+// May return an UnsupportedMethodErr in certain StorageDriver implementations.
+func (d *Driver) URLFor(path string) (string, error) {
+	return "", storagedriver.ErrUnsupportedMethod
+}
+
 // fullPath returns the absolute path of a key within the Driver's storage.
 func (d *Driver) fullPath(subPath string) string {
 	return path.Join(d.rootDirectory, subPath)

storagedriver/inmemory/driver.go

@@ -251,3 +251,9 @@ func (d *Driver) Delete(path string) error {
 		return err
 	}
 }
+
+// URLFor returns a URL which may be used to retrieve the content stored at the given path.
+// May return an UnsupportedMethodErr in certain StorageDriver implementations.
+func (d *Driver) URLFor(path string) (string, error) {
+	return "", storagedriver.ErrUnsupportedMethod
+}

storagedriver/s3/s3.go

@@ -580,6 +580,16 @@ func (d *Driver) Delete(path string) error {
 	return nil
 }
 
+// URLFor returns a URL which may be used to retrieve the content stored at the given path.
+// May return an UnsupportedMethodErr in certain StorageDriver implementations.
+func (d *Driver) URLFor(path string) (string, error) {
+	if !storagedriver.PathRegexp.MatchString(path) {
+		return "", storagedriver.InvalidPathError{Path: path}
+	}
+
+	return d.Bucket.SignedURL(d.s3Path(path), time.Now().Add(24*time.Hour)), nil
+}
+
 func (d *Driver) s3Path(path string) string {
 	return strings.TrimLeft(strings.TrimRight(d.rootDirectory, "/")+path, "/")
 }

storagedriver/storagedriver.go

@@ -1,6 +1,7 @@
 package storagedriver
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"regexp"
@@ -69,6 +70,10 @@ type StorageDriver interface {
 
 	// Delete recursively deletes all objects stored at "path" and its subpaths.
 	Delete(path string) error
+
+	// URLFor returns a URL which may be used to retrieve the content stored at the given path.
+	// May return an UnsupportedMethodErr in certain StorageDriver implementations.
+	URLFor(path string) (string, error)
 }
 
 // PathRegexp is the regular expression which each file path must match.
@@ -78,6 +83,9 @@ type StorageDriver interface {
 // a period, underscore, or hyphen.
 var PathRegexp = regexp.MustCompile(`^(/[a-z0-9]+([._-][a-z0-9]+)*)+$`)
 
+// UnsupportedMethodErr may be returned in the case where a StorageDriver implementation does not support an optional method.
+var ErrUnsupportedMethod = errors.New("Unsupported method")
+
 // PathNotFoundError is returned when operating on a nonexistent path.
 type PathNotFoundError struct {
 	Path string

storagedriver/testsuites/testsuites.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"io/ioutil"
 	"math/rand"
+	"net/http"
 	"os"
 	"path"
 	"sort"
@@ -580,6 +581,32 @@ func (suite *DriverSuite) TestDelete(c *check.C) {
 	c.Assert(err, check.FitsTypeOf, storagedriver.PathNotFoundError{})
 }
 
+// TestURLFor checks that the URLFor method functions properly, but only if it
+// is implemented
+func (suite *DriverSuite) TestURLFor(c *check.C) {
+	filename := randomPath(32)
+	contents := randomContents(32)
+
+	defer suite.StorageDriver.Delete(firstPart(filename))
+
+	err := suite.StorageDriver.PutContent(filename, contents)
+	c.Assert(err, check.IsNil)
+
+	url, err := suite.StorageDriver.URLFor(filename)
+	if err == storagedriver.ErrUnsupportedMethod {
+		return
+	}
+	c.Assert(err, check.IsNil)
+
+	response, err := http.Get(url)
+	c.Assert(err, check.IsNil)
+	defer response.Body.Close()
+
+	read, err := ioutil.ReadAll(response.Body)
+	c.Assert(err, check.IsNil)
+	c.Assert(read, check.DeepEquals, contents)
+}
+
 // TestDeleteNonexistent checks that removing a nonexistent key fails.
 func (suite *DriverSuite) TestDeleteNonexistent(c *check.C) {
 	filename := randomPath(32)