Export ServeJSON for serving error codes

This changeset provides a common http handler for serving errcodes. This should
unify http responses across webservices in the face of errors.

Several type assertions have been added, as well, to ensure the error interface
is implemented.

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

docs/handlers/helpers.go

@@ -1,43 +1,10 @@
 package handlers
 
 import (
-	"encoding/json"
 	"io"
 	"net/http"
-
-	"github.com/docker/distribution/registry/api/errcode"
 )
 
-// serveJSON marshals v and sets the content-type header to
-// 'application/json'. If a different status code is required, call
-// ResponseWriter.WriteHeader before this function.
-func serveJSON(w http.ResponseWriter, v interface{}) error {
-	w.Header().Set("Content-Type", "application/json; charset=utf-8")
-	sc := http.StatusInternalServerError
-
-	if errs, ok := v.(errcode.Errors); ok && len(errs) > 0 {
-		if err, ok := errs[0].(errcode.ErrorCoder); ok {
-			if sc2 := err.ErrorCode().Descriptor().HTTPStatusCode; sc2 != 0 {
-				sc = sc2
-			}
-		}
-	} else if err, ok := v.(errcode.ErrorCoder); ok {
-		if sc2 := err.ErrorCode().Descriptor().HTTPStatusCode; sc2 != 0 {
-			sc = sc2
-		}
-	}
-
-	w.WriteHeader(sc)
-
-	enc := json.NewEncoder(w)
-
-	if err := enc.Encode(v); err != nil {
-		return err
-	}
-
-	return nil
-}
-
 // closeResources closes all the provided resources after running the target
 // handler.
 func closeResources(handler http.Handler, closers ...io.Closer) http.Handler {