ociregistry/ociserver: allow customization of error responses

Currently there is no way to fully customize the errors sent by
the `ociserver` implementation. This change allows
that possibility by providing `ociserver.Options.WriteError`,
giving the user complete flexibility in that respect.

It's still possible to access the default implementation
by invoking `ociregistry.WriteError` or `ociregistry.MarshalError`
directly, allowing a simple way of adding functionality
to the error marshaling logic without the need to rewrite the
whole thing.

Signed-off-by: Roger Peppe <rogpeppe@gmail.com>
Change-Id: Ic680283a29a29461613c8d46d9f212400922d488
Dispatch-Trailer: {"type":"trybot","CL":1192981,"patchset":1,"ref":"refs/changes/81/1192981/1","targetBranch":"main"}

ociregistry/error.go

@@ -233,6 +233,16 @@ func (e *httpError) ResponseBody() []byte {
 	return e.body
 }
 
+// WriteError marshals the given error as JSON using [MarshalError] and
+// then writes it to w. It returns the error returned from w.Write.
+func WriteError(w http.ResponseWriter, err error) error {
+	data, httpStatus := MarshalError(err)
+	w.Header().Set("Content-Type", "application/json")
+	w.WriteHeader(httpStatus)
+	_, err = w.Write(data)
+	return err
+}
+
 // MarshalError marshals the given error as JSON according
 // to the OCI distribution specification. It also returns
 // the associated HTTP status code, or [http.StatusInternalServerError]

ociregistry/ociserver/error.go

@@ -16,18 +16,10 @@ package ociserver
 
 import (
 	"fmt"
-	"net/http"
 
 	"cuelabs.dev/go/oci/ociregistry"
 )
 
-func writeError(resp http.ResponseWriter, err error) {
-	data, httpStatus := ociregistry.MarshalError(err)
-	resp.Header().Set("Content-Type", "application/json")
-	resp.WriteHeader(httpStatus)
-	resp.Write(data)
-}
-
 func withHTTPCode(statusCode int, err error) error {
 	return ociregistry.NewHTTPError(err, statusCode, nil, nil)
 }

ociregistry/ociserver/error_test.go

@@ -26,6 +26,24 @@ import (
 	"github.com/go-quicktest/qt"
 )
 
+func TestCustomErrorWriter(t *testing.T) {
+	// Test that if an Interface method returns an HTTPError error, the
+	// HTTP status code is derived from the OCI error code in preference
+	// to the HTTPError status code.
+	r := New(&ociregistry.Funcs{}, &Options{
+		WriteError: func(w http.ResponseWriter, err error) error {
+			w.Header().Set("Some-Header", "a value")
+			return ociregistry.WriteError(w, err)
+		},
+	})
+	s := httptest.NewServer(r)
+	defer s.Close()
+	resp, err := http.Get(s.URL + "/v2/foo/manifests/sometag")
+	qt.Assert(t, qt.IsNil(err))
+	defer resp.Body.Close()
+	qt.Assert(t, qt.Equals(resp.Header.Get("Some-Header"), "a value"))
+}
+
 func TestHTTPStatusOverriddenByErrorCode(t *testing.T) {
 	// Test that if an Interface method returns an HTTPError error, the
 	// HTTP status code is derived from the OCI error code in preference

ociregistry/ociserver/registry.go

@@ -41,6 +41,12 @@ var v2 = ocispecroot.Versioned{
 
 // Options holds options for the server.
 type Options struct {
+	// WriteError is used to write error responses. It is passed the
+	// error an API call has returned and is responsible for writing
+	// it to w. If WriteError is nil, [ociregistry.WriteError] will
+	// be used.
+	WriteError func(w http.ResponseWriter, err error) error
+
 	// DisableReferrersAPI, when true, causes the registry to behave as if
 	// it does not understand the referrers API.
 	DisableReferrersAPI bool
@@ -133,6 +139,9 @@ func New(backend ociregistry.Interface, opts *Options) http.Handler {
 	if r.opts.DebugID == "" {
 		r.opts.DebugID = fmt.Sprintf("ociserver%d", atomic.AddInt32(&debugID, 1))
 	}
+	if r.opts.WriteError == nil {
+		r.opts.WriteError = ociregistry.WriteError
+	}
 	return r
 }
 
@@ -167,7 +176,7 @@ var handlers = []func(r *registry, ctx context.Context, w http.ResponseWriter, r
 
 func (r *registry) ServeHTTP(resp http.ResponseWriter, req *http.Request) {
 	if rerr := r.v2(resp, req); rerr != nil {
-		writeError(resp, rerr)
+		r.opts.WriteError(resp, rerr)
 		return
 	}
 }