Adds StorageDriverFactory, unifying creation of StorageDrivers

Custom storage drivers can register a factory to create the driver by
name, similar to the database/sql package's Register and Open
factory.Create returns an in-process driver if registered or an IPC
driver if one can be found, erroring otherwise
This standardizes parameter passing for creation of storage drivers

Also adds documentation for storagedriver package and children

storagedriver/ipc/ipc.go

@@ -10,12 +10,16 @@ import (
 	"github.com/docker/libchan"
 )
 
+// Defines a remote method call request
+// A return value struct is to be sent over the ResponseChannel
 type Request struct {
 	Type            string
 	Parameters      map[string]interface{}
 	ResponseChannel libchan.Sender
 }
 
+// A simple wrapper around an io.ReadCloser that implements the io.ReadWriteCloser interface
+// Writes are disallowed and will return an error if ever called
 type noWriteReadWriteCloser struct {
 	io.ReadCloser
 }
@@ -24,6 +28,8 @@ func (r noWriteReadWriteCloser) Write(p []byte) (n int, err error) {
 	return 0, errors.New("Write unsupported")
 }
 
+// Wraps an io.Reader as an io.ReadWriteCloser with a nop Close and unsupported Write method
+// Has no effect when an io.ReadWriteCloser is passed in
 func WrapReader(reader io.Reader) io.ReadWriteCloser {
 	if readWriteCloser, ok := reader.(io.ReadWriteCloser); ok {
 		return readWriteCloser
@@ -39,6 +45,7 @@ type responseError struct {
 	Message string
 }
 
+// Wraps an error in a serializable struct containing the error's type and message
 func ResponseError(err error) *responseError {
 	if err == nil {
 		return nil
@@ -53,29 +60,37 @@ func (err *responseError) Error() string {
 	return fmt.Sprintf("%s: %s", err.Type, err.Message)
 }
 
+// IPC method call response object definitions
+
+// Response for a ReadStream request
 type ReadStreamResponse struct {
 	Reader io.ReadWriteCloser
 	Error  *responseError
 }
 
+// Response for a WriteStream request
 type WriteStreamResponse struct {
 	Error *responseError
 }
 
+// Response for a ResumeWritePosition request
 type ResumeWritePositionResponse struct {
 	Position uint64
 	Error    *responseError
 }
 
+// Response for a List request
 type ListResponse struct {
 	Keys  []string
 	Error *responseError
 }
 
+// Response for a Move request
 type MoveResponse struct {
 	Error *responseError
 }
 
+// Response for a Delete request
 type DeleteResponse struct {
 	Error *responseError
 }