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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef js_UbiNodeShortestPaths_h
#define js_UbiNodeShortestPaths_h
#include "mozilla/CheckedInt.h"
#include "mozilla/Maybe.h"
#include <utility>
#include "js/AllocPolicy.h"
#include "js/GCAPI.h"
#include "js/UbiNode.h"
#include "js/UbiNodeBreadthFirst.h"
#include "js/UniquePtr.h"
namespace JS {
namespace ubi {
* A back edge along a path in the heap graph.
struct JS_PUBLIC_API BackEdge {
Node predecessor_;
EdgeName name_;
using Ptr = js::UniquePtr<BackEdge>;
BackEdge() : predecessor_(), name_(nullptr) {}
[[nodiscard]] bool init(const Node& predecessor, Edge& edge) {
predecessor_ = predecessor;
name_ = std::move(;
return true;
BackEdge(const BackEdge&) = delete;
BackEdge& operator=(const BackEdge&) = delete;
BackEdge(BackEdge&& rhs)
: predecessor_(rhs.predecessor_), name_(std::move(rhs.name_)) {
MOZ_ASSERT(&rhs != this);
BackEdge& operator=(BackEdge&& rhs) {
new (this) BackEdge(std::move(rhs));
return *this;
Ptr clone() const;
const EdgeName& name() const { return name_; }
EdgeName& name() { return name_; }
const JS::ubi::Node& predecessor() const { return predecessor_; }
* A path is a series of back edges from which we discovered a target node.
using Path = JS::ubi::Vector<BackEdge*>;
* The `JS::ubi::ShortestPaths` type represents a collection of up to N shortest
* retaining paths for each of a target set of nodes, starting from the same
* root node.
struct JS_PUBLIC_API ShortestPaths {
// Types, type aliases, and data members.
using BackEdgeVector = JS::ubi::Vector<BackEdge::Ptr>;
using NodeToBackEdgeVectorMap =
js::HashMap<Node, BackEdgeVector, js::DefaultHasher<Node>,
struct Handler;
using Traversal = BreadthFirst<Handler>;
* A `JS::ubi::BreadthFirst` traversal handler that records back edges for
* how we reached each node, allowing us to reconstruct the shortest
* retaining paths after the traversal.
struct Handler {
using NodeData = BackEdge;
ShortestPaths& shortestPaths;
size_t totalMaxPathsToRecord;
size_t totalPathsRecorded;
explicit Handler(ShortestPaths& shortestPaths)
: shortestPaths(shortestPaths),
totalMaxPathsToRecord(shortestPaths.targets_.count() *
totalPathsRecorded(0) {}
bool operator()(Traversal& traversal, const JS::ubi::Node& origin,
JS::ubi::Edge& edge, BackEdge* back, bool first) {
MOZ_ASSERT(origin == shortestPaths.root_ ||
MOZ_ASSERT(totalPathsRecorded < totalMaxPathsToRecord);
if (first && !back->init(origin, edge)) {
return false;
if (!shortestPaths.targets_.has(edge.referent)) {
return true;
// If `first` is true, then we moved the edge's name into `back` in
// the above call to `init`. So clone that back edge to get the
// correct edge name. If `first` is not true, then our edge name is
// still in `edge`. This accounts for the asymmetry between
// `back->clone()` in the first branch, and the `init` call in the
// second branch.
if (first) {
BackEdgeVector paths;
if (!paths.reserve(shortestPaths.maxNumPaths_)) {
return false;
auto cloned = back->clone();
if (!cloned) {
return false;
if (!shortestPaths.paths_.putNew(edge.referent, std::move(paths))) {
return false;
} else {
auto ptr = shortestPaths.paths_.lookup(edge.referent);
"This isn't the first time we have seen the target node "
"`edge.referent`. "
"We should have inserted it into shortestPaths.paths_ the "
"first time we "
"saw it.");
if (ptr->value().length() < shortestPaths.maxNumPaths_) {
auto thisBackEdge = js::MakeUnique<BackEdge>();
if (!thisBackEdge || !thisBackEdge->init(origin, edge)) {
return false;
MOZ_ASSERT(totalPathsRecorded <= totalMaxPathsToRecord);
if (totalPathsRecorded == totalMaxPathsToRecord) {
return true;
// The maximum number of paths to record for each node.
uint32_t maxNumPaths_;
// The root node we are starting the search from.
Node root_;
// The set of nodes we are searching for paths to.
NodeSet targets_;
// The resulting paths.
NodeToBackEdgeVectorMap paths_;
// Need to keep alive the traversal's back edges so we can walk them later
// when the traversal is over when recreating the shortest paths.
Traversal::NodeMap backEdges_;
// Private methods.
ShortestPaths(uint32_t maxNumPaths, const Node& root, NodeSet&& targets)
: maxNumPaths_(maxNumPaths),
backEdges_() {
MOZ_ASSERT(maxNumPaths_ > 0);
// Public methods.
ShortestPaths(ShortestPaths&& rhs)
: maxNumPaths_(rhs.maxNumPaths_),
backEdges_(std::move(rhs.backEdges_)) {
MOZ_ASSERT(this != &rhs, "self-move is not allowed");
ShortestPaths& operator=(ShortestPaths&& rhs) {
new (this) ShortestPaths(std::move(rhs));
return *this;
ShortestPaths(const ShortestPaths&) = delete;
ShortestPaths& operator=(const ShortestPaths&) = delete;
* Construct a new `JS::ubi::ShortestPaths`, finding up to `maxNumPaths`
* shortest retaining paths for each target node in `targets` starting from
* `root`.
* The resulting `ShortestPaths` instance must not outlive the
* `JS::ubi::Node` graph it was constructed from.
* - For `JS::ubi::Node` graphs backed by the live heap graph, this means
* that the `ShortestPaths`'s lifetime _must_ be contained within the
* scope of the provided `AutoCheckCannotGC` reference because a GC will
* invalidate the nodes.
* - For `JS::ubi::Node` graphs backed by some other offline structure
* provided by the embedder, the resulting `ShortestPaths`'s lifetime is
* bounded by that offline structure's lifetime.
* Returns `mozilla::Nothing()` on OOM failure. It is the caller's
* responsibility to handle and report the OOM.
static mozilla::Maybe<ShortestPaths> Create(JSContext* cx,
AutoCheckCannotGC& noGC,
uint32_t maxNumPaths,
const Node& root,
NodeSet&& targets) {
MOZ_ASSERT(targets.count() > 0);
MOZ_ASSERT(maxNumPaths > 0);
mozilla::CheckedInt<uint32_t> max = maxNumPaths;
max *= targets.count();
if (!max.isValid()) {
return mozilla::Nothing();
ShortestPaths paths(maxNumPaths, root, std::move(targets));
Handler handler(paths);
Traversal traversal(cx, handler, noGC);
traversal.wantNames = true;
if (!traversal.addStart(root) || !traversal.traverse()) {
return mozilla::Nothing();
// Take ownership of the back edges we created while traversing the
// graph so that we can follow them from `paths_` and don't
// use-after-free.
paths.backEdges_ = std::move(traversal.visited);
return mozilla::Some(std::move(paths));
* Get an iterator over each target node we searched for retaining paths
* for. The returned iterator must not outlive the `ShortestPaths`
* instance.
NodeSet::Iterator targetIter() const { return targets_.iter(); }
* Invoke the provided functor/lambda/callable once for each retaining path
* discovered for `target`. The `func` is passed a single `JS::ubi::Path&`
* argument, which contains each edge along the path ordered starting from
* the root and ending at the target, and must not outlive the scope of the
* call.
* Note that it is possible that we did not find any paths from the root to
* the given target, in which case `func` will not be invoked.
template <class Func>
[[nodiscard]] bool forEachPath(const Node& target, Func func) {
auto ptr = paths_.lookup(target);
// We didn't find any paths to this target, so nothing to do here.
if (!ptr) {
return true;
MOZ_ASSERT(ptr->value().length() <= maxNumPaths_);
Path path;
for (const auto& backEdge : ptr->value()) {
if (!path.append(backEdge.get())) {
return false;
Node here = backEdge->predecessor();
while (here != root_) {
auto p = backEdges_.lookup(here);
if (!path.append(&p->value())) {
return false;
here = p->value().predecessor();
if (!func(path)) {
return false;
return true;
#ifdef DEBUG
// A helper function to dump the first `maxNumPaths` shortest retaining paths to
// `node` from the GC roots. Useful when GC things you expect to have been
// reclaimed by the collector haven't been!
// Usage:
// JSObject* foo = ...;
// JS::ubi::dumpPaths(rt, JS::ubi::Node(foo));
JS_PUBLIC_API void dumpPaths(JSRuntime* rt, Node node,
uint32_t maxNumPaths = 10);
} // namespace ubi
} // namespace JS
#endif // js_UbiNodeShortestPaths_h