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// Copyright 2018 The LUCI Authors.
// 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
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// See the License for the specific language governing permissions and
// limitations under the License.
// Package graph implements a DAG used internally to represent config objects.
// All entities in the LUCI config are represented by nodes in a graph. Nodes
// are linked with directional edges. Cycles are forbidden. Each edge is
// annotated with a name of the relation that added it (e.g. "triggered_by").
// There can be multiple edges between a given pair of nodes, e.g.
// "triggered_by" and "triggers" edges between a builder(...) and a
// trigger(...). When detecting cycles or traversing the graph, edge annotations
// are not considered. They are used only to improve error messages when
// reporting dangling edges (e.g. Builder X references undefined trigger T via
// "triggered_by").
// Each node:
// * Has a unique identifier, called key. A key is a list of
// (string kind, string id) pairs. Once constructed, a key is an opaque
// label (e.g. there's no way to explore what's inside the key through
// Starlark, to keep the API simple). Examples of keys:
// [luci.bucket("ci")]
// [luci.bucket("ci"), luci.builder("infra-builder")]
// * Has a props dict of arbitrary properties (all keys are strings, values
// are arbitrary).
// * Has a captured stack trace of where in Starlark it was defined (for error
// messages).
// * Is immutable
// Structs exposed by this package are not safe for concurrent use without
// external locking.
package graph