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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 http://mozilla.org/MPL/2.0/. */
+
+#ifndef js_UbiNodeShortestPaths_h
+#define js_UbiNodeShortestPaths_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 {
+ private:
+ Node predecessor_;
+ EdgeName name_;
+
+ public:
+ using Ptr = js::UniquePtr<BackEdge>;
+
+ BackEdge() : predecessor_(), name_(nullptr) {}
+
+ [[nodiscard]] bool init(const Node& predecessor, Edge& edge) {
+ MOZ_ASSERT(!predecessor_);
+ MOZ_ASSERT(!name_);
+
+ predecessor_ = predecessor;
+ name_ = std::move(edge.name);
+ 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) {
+ this->~BackEdge();
+ 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 {
+ private:
+ // Types, type aliases, and data members.
+
+ using BackEdgeVector = JS::ubi::Vector<BackEdge::Ptr>;
+ using NodeToBackEdgeVectorMap =
+ js::HashMap<Node, BackEdgeVector, js::DefaultHasher<Node>,
+ js::SystemAllocPolicy>;
+
+ 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() *
+ shortestPaths.maxNumPaths_),
+ totalPathsRecorded(0) {}
+
+ bool operator()(Traversal& traversal, const JS::ubi::Node& origin,
+ JS::ubi::Edge& edge, BackEdge* back, bool first) {
+ MOZ_ASSERT(back);
+ MOZ_ASSERT(origin == shortestPaths.root_ ||
+ traversal.visited.has(origin));
+ 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;
+ }
+ paths.infallibleAppend(std::move(cloned));
+ if (!shortestPaths.paths_.putNew(edge.referent, std::move(paths))) {
+ return false;
+ }
+ totalPathsRecorded++;
+ } else {
+ auto ptr = shortestPaths.paths_.lookup(edge.referent);
+ MOZ_ASSERT(ptr,
+ "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;
+ }
+ ptr->value().infallibleAppend(std::move(thisBackEdge));
+ totalPathsRecorded++;
+ }
+ }
+
+ MOZ_ASSERT(totalPathsRecorded <= totalMaxPathsToRecord);
+ if (totalPathsRecorded == totalMaxPathsToRecord) {
+ traversal.stop();
+ }
+
+ 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:
+ // Private methods.
+
+ ShortestPaths(uint32_t maxNumPaths, const Node& root, NodeSet&& targets)
+ : maxNumPaths_(maxNumPaths),
+ root_(root),
+ targets_(std::move(targets)),
+ paths_(targets_.count()),
+ backEdges_() {
+ MOZ_ASSERT(maxNumPaths_ > 0);
+ MOZ_ASSERT(root_);
+ }
+
+ public:
+ // Public methods.
+
+ ShortestPaths(ShortestPaths&& rhs)
+ : maxNumPaths_(rhs.maxNumPaths_),
+ root_(rhs.root_),
+ targets_(std::move(rhs.targets_)),
+ paths_(std::move(rhs.paths_)),
+ backEdges_(std::move(rhs.backEdges_)) {
+ MOZ_ASSERT(this != &rhs, "self-move is not allowed");
+ }
+
+ ShortestPaths& operator=(ShortestPaths&& rhs) {
+ this->~ShortestPaths();
+ 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);
+
+ 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) {
+ MOZ_ASSERT(targets_.has(target));
+
+ 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()) {
+ path.clear();
+
+ if (!path.append(backEdge.get())) {
+ return false;
+ }
+
+ Node here = backEdge->predecessor();
+ MOZ_ASSERT(here);
+
+ while (here != root_) {
+ auto p = backEdges_.lookup(here);
+ MOZ_ASSERT(p);
+ if (!path.append(&p->value())) {
+ return false;
+ }
+ here = p->value().predecessor();
+ MOZ_ASSERT(here);
+ }
+
+ path.reverse();
+
+ 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);
+#endif
+
+} // namespace ubi
+} // namespace JS
+
+#endif // js_UbiNodeShortestPaths_h