1 files changed, 72 insertions, 0 deletions
diff --git a/pkg/v1/remote/check.go b/pkg/v1/remote/check.go
new file mode 100644
index 0000000..b4395c2
--- /dev/null
+++ b/pkg/v1/remote/check.go
@@ -0,0 +1,72 @@
+// Copyright 2019 Google LLC All Rights Reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//    http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package remote
+
+import (
+	"context"
+	"fmt"
+	"net/http"
+
+	"github.com/google/go-containerregistry/pkg/authn"
+	"github.com/google/go-containerregistry/pkg/name"
+	"github.com/google/go-containerregistry/pkg/v1/remote/transport"
+)
+
+// CheckPushPermission returns an error if the given keychain cannot authorize
+// a push operation to the given ref.
+//
+// This can be useful to check whether the caller has permission to push an
+// image before doing work to construct the image.
+//
+// TODO(#412): Remove the need for this method.
+func CheckPushPermission(ref name.Reference, kc authn.Keychain, t http.RoundTripper) error {
+	auth, err := kc.Resolve(ref.Context().Registry)
+	if err != nil {
+		return fmt.Errorf("resolving authorization for %v failed: %w", ref.Context().Registry, err)
+	}
+
+	scopes := []string{ref.Scope(transport.PushScope)}
+	tr, err := transport.NewWithContext(context.TODO(), ref.Context().Registry, auth, t, scopes)
+	if err != nil {
+		return fmt.Errorf("creating push check transport for %v failed: %w", ref.Context().Registry, err)
+	}
+	// TODO(jasonhall): Against GCR, just doing the token handshake is
+	// enough, but this doesn't extend to Dockerhub
+	// (https://github.com/docker/hub-feedback/issues/1771), so we actually
+	// need to initiate an upload to tell whether the credentials can
+	// authorize a push. Figure out how to return early here when we can,
+	// to avoid a roundtrip for spec-compliant registries.
+	w := writer{
+		repo:   ref.Context(),
+		client: &http.Client{Transport: tr},
+	}
+	loc, _, err := w.initiateUpload(context.Background(), "", "", "")
+	if loc != "" {
+		// Since we're only initiating the upload to check whether we
+		// can, we should attempt to cancel it, in case initiating
+		// reserves some resources on the server. We shouldn't wait for
+		// cancelling to complete, and we don't care if it fails.
+		go w.cancelUpload(loc)
+	}
+	return err
+}
+
+func (w *writer) cancelUpload(loc string) {
+	req, err := http.NewRequest(http.MethodDelete, loc, nil)
+	if err != nil {
+		return
+	}
+	_, _ = w.client.Do(req)
+}