Add function to set custom auth scope in context

Currently auth.docker.io uses a custom auth scope for (docker) plugins
`repository(plugin):<repo>:<perms>`.
This makes it impossible to use containerd distribution tooling to fetch
plugins without also supplying a totally custom authorizer.

This changes allows clients to set the correct scope on the context.
It's a little bit nasty but "works".

I'm also a bit suspect of some a couple of these unexported context
functrions. Before the primary one used `contextWithRepositoryScope`
overwrites any scope value and there is another one that appends the
scope value.
With this change they both append...

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

remotes/docker/scope.go

@@ -51,19 +51,25 @@ func contextWithRepositoryScope(ctx context.Context, refspec reference.Spec, pus
 	if err != nil {
 		return nil, err
 	}
-	return context.WithValue(ctx, tokenScopesKey{}, []string{s}), nil
+	return WithScope(ctx, s), nil
+}
+
+// WithScope appends a custom registry auth scope to the context.
+func WithScope(ctx context.Context, scope string) context.Context {
+	var scopes []string
+	if v := ctx.Value(tokenScopesKey{}); v != nil {
+		scopes = v.([]string)
+		scopes = append(scopes, scope)
+	} else {
+		scopes = []string{scope}
+	}
+	return context.WithValue(ctx, tokenScopesKey{}, scopes)
 }
 
 // contextWithAppendPullRepositoryScope is used to append repository pull
 // scope into existing scopes indexed by the tokenScopesKey{}.
 func contextWithAppendPullRepositoryScope(ctx context.Context, repo string) context.Context {
-	var scopes []string
-
-	if v := ctx.Value(tokenScopesKey{}); v != nil {
-		scopes = append(scopes, v.([]string)...)
-	}
-	scopes = append(scopes, fmt.Sprintf("repository:%s:pull", repo))
-	return context.WithValue(ctx, tokenScopesKey{}, scopes)
+	return WithScope(ctx, fmt.Sprintf("repository:%s:pull", repo))
 }
 
 // getTokenScopes returns deduplicated and sorted scopes from ctx.Value(tokenScopesKey{}) and common scopes.

remotes/docker/scope_test.go

@@ -22,6 +22,7 @@ import (
 
 	"github.com/containerd/containerd/reference"
 	"gotest.tools/assert"
+	"gotest.tools/assert/cmp"
 )
 
 func TestRepositoryScope(t *testing.T) {
@@ -94,3 +95,14 @@ func TestGetTokenScopes(t *testing.T) {
 		assert.DeepEqual(t, tc.expected, actual)
 	}
 }
+
+func TestCustomScope(t *testing.T) {
+	scope := "whatever:foo/bar:pull"
+	ctx := WithScope(context.Background(), scope)
+	ctx = contextWithAppendPullRepositoryScope(ctx, "foo/bar")
+
+	scopes := getTokenScopes(ctx, []string{})
+	assert.Assert(t, cmp.Len(scopes, 2))
+	assert.Check(t, cmp.Equal(scopes[0], "repository:foo/bar:pull"))
+	assert.Check(t, cmp.Equal(scopes[1], scope))
+}