engine: Add graph queryable trait

This is a trait that specifies that your resource can be queried by
others. You can either enable it plainly to add wholesale access by
everyone, or you can implement your own Allow method to limit what is
permitted.

engine/graphqueryable.go

@@ -0,0 +1,70 @@
+// Mgmt
+// Copyright (C) 2013-2019+ James Shubin and the project contributors
+// Written by James Shubin <james@shubin.ca> and the project contributors
+//
+// This program is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// This program is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+//
+// You should have received a copy of the GNU General Public License
+// along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+package engine
+
+// GraphQueryableRes is the interface that must be implemented if you want your
+// resource to be allowed to be queried from another resource in the graph. This
+// is done as a form of explicit authorization tracking so that we can consider
+// security aspects more easily. Ultimately, all resource code should be
+// trusted, but it's still a good idea to know if a particular resource is even
+// able to access information about another one, and if your resource doesn't
+// add the trait supporting this, then it won't be allowed.
+type GraphQueryableRes interface {
+	Res // implement everything in Res but add the additional requirements
+
+	// GraphQueryAllowed returns nil if you're allowed to query the graph.
+	GraphQueryAllowed(...GraphQueryableOption) error
+}
+
+// GraphQueryableOption is an option that can be used to specify the
+// authentication.
+type GraphQueryableOption func(*GraphQueryableOptions)
+
+// GraphQueryableOptions represents the different possible configurable options.
+type GraphQueryableOptions struct {
+	// Kind is the kind of the resource making the access.
+	Kind string
+	// Name is the name of the resource making the access.
+	Name string
+	// TODO: add more options if needed
+}
+
+// Apply is a helper function to apply a list of options to the struct. You
+// should initialize it with defaults you want, and then apply any you've
+// received like this.
+func (obj *GraphQueryableOptions) Apply(opts ...GraphQueryableOption) {
+	for _, optionFunc := range opts { // apply the options
+		optionFunc(obj)
+	}
+}
+
+// GraphQueryableOptionKind tells the GraphQueryAllowed function what the
+// resource kind is.
+func GraphQueryableOptionKind(kind string) GraphQueryableOption {
+	return func(gqo *GraphQueryableOptions) {
+		gqo.Kind = kind
+	}
+}
+
+// GraphQueryableOptionName tells the GraphQueryAllowed function what the
+// resource name is.
+func GraphQueryableOptionName(name string) GraphQueryableOption {
+	return func(gqo *GraphQueryableOptions) {
+		gqo.Name = name
+	}
+}