Make Storage Driver API calls context aware.

- Change driver interface to take a context as its first argument
     - Make newFileReader take a context as its first argument
     - Make newFileWriter take a context as its first argument
     - Make blobstore exists and delete take a context as a first argument
     - Pass the layerreader's context to the storage layer
     - Pass the app's context to purgeuploads
     - Store the app's context into the blobstore (was previously null)
     - Pass the trace'd context to the storage drivers

Signed-off-by: Richard Scothern <richard.scothern@gmail.com>

docs/storage/driver/storagedriver.go

@@ -7,6 +7,8 @@ import (
 	"regexp"
 	"strconv"
 	"strings"
+
+	"github.com/docker/distribution/context"
 )
 
 // Version is a string representing the storage driver version, of the form
@@ -42,45 +44,45 @@ type StorageDriver interface {
 
 	// GetContent retrieves the content stored at "path" as a []byte.
 	// This should primarily be used for small objects.
-	GetContent(path string) ([]byte, error)
+	GetContent(ctx context.Context, path string) ([]byte, error)
 
 	// PutContent stores the []byte content at a location designated by "path".
 	// This should primarily be used for small objects.
-	PutContent(path string, content []byte) error
+	PutContent(ctx context.Context, path string, content []byte) error
 
 	// ReadStream retrieves an io.ReadCloser for the content stored at "path"
 	// with a given byte offset.
 	// May be used to resume reading a stream by providing a nonzero offset.
-	ReadStream(path string, offset int64) (io.ReadCloser, error)
+	ReadStream(ctx context.Context, path string, offset int64) (io.ReadCloser, error)
 
 	// WriteStream stores the contents of the provided io.ReadCloser at a
 	// location designated by the given path.
 	// May be used to resume writing a stream by providing a nonzero offset.
 	// The offset must be no larger than the CurrentSize for this path.
-	WriteStream(path string, offset int64, reader io.Reader) (nn int64, err error)
+	WriteStream(ctx context.Context, path string, offset int64, reader io.Reader) (nn int64, err error)
 
 	// Stat retrieves the FileInfo for the given path, including the current
 	// size in bytes and the creation time.
-	Stat(path string) (FileInfo, error)
+	Stat(ctx context.Context, path string) (FileInfo, error)
 
 	// List returns a list of the objects that are direct descendants of the
 	//given path.
-	List(path string) ([]string, error)
+	List(ctx context.Context, path string) ([]string, error)
 
 	// Move moves an object stored at sourcePath to destPath, removing the
 	// original object.
 	// Note: This may be no more efficient than a copy followed by a delete for
 	// many implementations.
-	Move(sourcePath string, destPath string) error
+	Move(ctx context.Context, sourcePath string, destPath string) error
 
 	// Delete recursively deletes all objects stored at "path" and its subpaths.
-	Delete(path string) error
+	Delete(ctx context.Context, path string) error
 
 	// URLFor returns a URL which may be used to retrieve the content stored at
 	// the given path, possibly using the given options.
 	// May return an ErrUnsupportedMethod in certain StorageDriver
 	// implementations.
-	URLFor(path string, options map[string]interface{}) (string, error)
+	URLFor(ctx context.Context, path string, options map[string]interface{}) (string, error)
 }
 
 // PathRegexp is the regular expression which each file path must match. A