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src: add check against non-weak BaseObjects at process exit
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When a process exits cleanly, i.e. because the event loop ends up
without things to wait for, the Node.js objects that are left on
the heap should be:

 1. weak, i.e. ready for garbage collection once no longer
    referenced, or
 2. detached, i.e. scheduled for destruction once no longer
    referenced, or
 3. an unrefed libuv handle, i.e. does not keep the event loop
    alive, or
 4. an inactive libuv handle (essentially the same here)

There are a few exceptions to this rule, but generally,
if there are C++-backed Node.js objects on the heap
that do not fall into the above categories, we may be looking
at a potential memory leak. Most likely, the cause is a missing
`MakeWeak()` call on the corresponding object.

In order to avoid this kind of problem, we check the list
of BaseObjects for these criteria. In this commit, we only do so
when explicitly instructed to or when in debug mode
(where --verify-base-objects is always-on).

In particular, this avoids the kinds of memory leak issues
that were fixed in the PRs referenced below.

Refs: #35488
Refs: #35487
Refs: #35481

PR-URL: #35490
Backport-PR-URL: #38786
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Benjamin Gruenbaum <benjamingr@gmail.com>
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addaleax authored and targos committed Jun 11, 2021
1 parent 6035492 commit ea311a4
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Showing 18 changed files with 116 additions and 1 deletion.
6 changes: 6 additions & 0 deletions src/async_wrap.cc
Original file line number Diff line number Diff line change
Expand Up @@ -98,6 +98,12 @@ struct AsyncWrapObject : public AsyncWrap {
return tmpl;
}

bool IsNotIndicativeOfMemoryLeakAtExit() const override {
// We can't really know what the underlying operation does. One of the
// signs that it's time to remove this class. :)
return true;
}

SET_NO_MEMORY_INFO()
SET_MEMORY_INFO_NAME(AsyncWrapObject)
SET_SELF_SIZE(AsyncWrapObject)
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7 changes: 7 additions & 0 deletions src/base_object-inl.h
Original file line number Diff line number Diff line change
Expand Up @@ -146,6 +146,13 @@ void BaseObject::ClearWeak() {
persistent_handle_.ClearWeak();
}

bool BaseObject::IsWeakOrDetached() const {
if (persistent_handle_.IsWeak()) return true;

if (!has_pointer_data()) return false;
const PointerData* pd = const_cast<BaseObject*>(this)->pointer_data();
return pd->wants_weak_jsobj || pd->is_detached;
}

v8::Local<v8::FunctionTemplate>
BaseObject::MakeLazilyInitializedJSTemplate(Environment* env) {
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9 changes: 9 additions & 0 deletions src/base_object.h
Original file line number Diff line number Diff line change
Expand Up @@ -78,6 +78,11 @@ class BaseObject : public MemoryRetainer {
// root and will not be touched by the garbage collector.
inline void ClearWeak();

// Reports whether this BaseObject is using a weak reference or detached,
// i.e. whether is can be deleted by GC once no strong BaseObjectPtrs refer
// to it anymore.
inline bool IsWeakOrDetached() const;

// Utility to create a FunctionTemplate with one internal field (used for
// the `BaseObject*` pointer) and a constructor that initializes that field
// to `nullptr`.
Expand Down Expand Up @@ -147,6 +152,10 @@ class BaseObject : public MemoryRetainer {
virtual v8::Maybe<bool> FinalizeTransferRead(
v8::Local<v8::Context> context, v8::ValueDeserializer* deserializer);

// Indicates whether this object is expected to use a strong reference during
// a clean process exit (due to an empty event loop).
virtual bool IsNotIndicativeOfMemoryLeakAtExit() const;

virtual inline void OnGCCollect();

private:
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34 changes: 34 additions & 0 deletions src/env.cc
Original file line number Diff line number Diff line change
Expand Up @@ -1043,6 +1043,36 @@ void Environment::RemoveUnmanagedFd(int fd) {
}
}

void Environment::VerifyNoStrongBaseObjects() {
// When a process exits cleanly, i.e. because the event loop ends up without
// things to wait for, the Node.js objects that are left on the heap should
// be:
//
// 1. weak, i.e. ready for garbage collection once no longer referenced, or
// 2. detached, i.e. scheduled for destruction once no longer referenced, or
// 3. an unrefed libuv handle, i.e. does not keep the event loop alive, or
// 4. an inactive libuv handle (essentially the same here)
//
// There are a few exceptions to this rule, but generally, if there are
// C++-backed Node.js objects on the heap that do not fall into the above
// categories, we may be looking at a potential memory leak. Most likely,
// the cause is a missing MakeWeak() call on the corresponding object.
//
// In order to avoid this kind of problem, we check the list of BaseObjects
// for these criteria. Currently, we only do so when explicitly instructed to
// or when in debug mode (where --verify-base-objects is always-on).

if (!options()->verify_base_objects) return;

ForEachBaseObject([this](BaseObject* obj) {
if (obj->IsNotIndicativeOfMemoryLeakAtExit()) return;
fprintf(stderr, "Found bad BaseObject during clean exit: %s\n",
obj->MemoryInfoName().c_str());
fflush(stderr);
ABORT();
});
}

void Environment::BuildEmbedderGraph(Isolate* isolate,
EmbedderGraph* graph,
void* data) {
Expand Down Expand Up @@ -1135,4 +1165,8 @@ Local<FunctionTemplate> BaseObject::GetConstructorTemplate(Environment* env) {
return tmpl;
}

bool BaseObject::IsNotIndicativeOfMemoryLeakAtExit() const {
return IsWeakOrDetached();
}

} // namespace node
1 change: 1 addition & 0 deletions src/env.h
Original file line number Diff line number Diff line change
Expand Up @@ -833,6 +833,7 @@ class Environment : public MemoryRetainer {
void MemoryInfo(MemoryTracker* tracker) const override;

void CreateProperties();
void VerifyNoStrongBaseObjects();
// Should be called before InitializeInspector()
void InitializeDiagnostics();
#if HAVE_INSPECTOR
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7 changes: 7 additions & 0 deletions src/handle_wrap.cc
Original file line number Diff line number Diff line change
Expand Up @@ -88,6 +88,13 @@ void HandleWrap::OnGCCollect() {
}


bool HandleWrap::IsNotIndicativeOfMemoryLeakAtExit() const {
return IsWeakOrDetached() ||
!HandleWrap::HasRef(this) ||
!uv_is_active(GetHandle());
}


void HandleWrap::MarkAsInitialized() {
env()->handle_wrap_queue()->PushBack(this);
state_ = kInitialized;
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1 change: 1 addition & 0 deletions src/handle_wrap.h
Original file line number Diff line number Diff line change
Expand Up @@ -85,6 +85,7 @@ class HandleWrap : public AsyncWrap {
AsyncWrap::ProviderType provider);
virtual void OnClose() {}
void OnGCCollect() final;
bool IsNotIndicativeOfMemoryLeakAtExit() const override;

void MarkAsInitialized();
void MarkAsUninitialized();
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4 changes: 4 additions & 0 deletions src/inspector_js_api.cc
Original file line number Diff line number Diff line change
Expand Up @@ -155,6 +155,10 @@ class JSBindingsConnection : public AsyncWrap {
SET_MEMORY_INFO_NAME(JSBindingsConnection)
SET_SELF_SIZE(JSBindingsConnection)

bool IsNotIndicativeOfMemoryLeakAtExit() const override {
return true; // Binding connections emit events on their own.
}

private:
std::unique_ptr<InspectorSession> session_;
Global<Function> callback_;
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6 changes: 6 additions & 0 deletions src/module_wrap.h
Original file line number Diff line number Diff line change
Expand Up @@ -60,6 +60,12 @@ class ModuleWrap : public BaseObject {
SET_MEMORY_INFO_NAME(ModuleWrap)
SET_SELF_SIZE(ModuleWrap)

bool IsNotIndicativeOfMemoryLeakAtExit() const override {
// XXX: The garbage collection rules for ModuleWrap are *super* unclear.
// Do these objects ever get GC'd? Are we just okay with leaking them?
return true;
}

private:
ModuleWrap(Environment* env,
v8::Local<v8::Object> object,
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2 changes: 2 additions & 0 deletions src/node_contextify.h
Original file line number Diff line number Diff line change
Expand Up @@ -174,6 +174,8 @@ class CompiledFnEntry final : public BaseObject {
v8::Local<v8::ScriptOrModule> script);
~CompiledFnEntry();

bool IsNotIndicativeOfMemoryLeakAtExit() const override { return true; }

private:
uint32_t id_;
v8::Global<v8::ScriptOrModule> script_;
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9 changes: 9 additions & 0 deletions src/node_http_parser.cc
Original file line number Diff line number Diff line change
Expand Up @@ -880,6 +880,15 @@ class Parser : public AsyncWrap, public StreamListener {
return HPE_PAUSED;
}


bool IsNotIndicativeOfMemoryLeakAtExit() const override {
// HTTP parsers are able to emit events without any GC root referring
// to them, because they receive events directly from the underlying
// libuv resource.
return true;
}


llhttp_t parser_;
StringPtr fields_[kMaxHeaderFieldsCount]; // header fields
StringPtr values_[kMaxHeaderFieldsCount]; // header values
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1 change: 1 addition & 0 deletions src/node_main_instance.cc
Original file line number Diff line number Diff line change
Expand Up @@ -147,6 +147,7 @@ int NodeMainInstance::Run() {
}

env->set_trace_sync_io(false);
if (!env->is_stopping()) env->VerifyNoStrongBaseObjects();
exit_code = EmitExit(env.get());
}

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4 changes: 4 additions & 0 deletions src/node_options.cc
Original file line number Diff line number Diff line change
Expand Up @@ -483,6 +483,10 @@ EnvironmentOptionsParser::EnvironmentOptionsParser() {
"exit code 1 unless 'unhandledRejection' hook is set).",
&EnvironmentOptions::unhandled_rejections,
kAllowedInEnvironment);
AddOption("--verify-base-objects",
"", /* undocumented, only for debugging */
&EnvironmentOptions::verify_base_objects,
kAllowedInEnvironment);

AddOption("--check",
"syntax check script without executing",
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6 changes: 6 additions & 0 deletions src/node_options.h
Original file line number Diff line number Diff line change
Expand Up @@ -151,6 +151,12 @@ class EnvironmentOptions : public Options {
bool trace_warnings = false;
std::string unhandled_rejections;
std::string userland_loader;
bool verify_base_objects =
#ifdef DEBUG
true;
#else
false;
#endif // DEBUG

bool syntax_check_only = false;
bool has_eval_string = false;
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10 changes: 9 additions & 1 deletion src/node_worker.cc
Original file line number Diff line number Diff line change
Expand Up @@ -366,8 +366,10 @@ void Worker::Run() {
{
int exit_code;
bool stopped = is_stopped();
if (!stopped)
if (!stopped) {
env_->VerifyNoStrongBaseObjects();
exit_code = EmitExit(env_.get());
}
Mutex::ScopedLock lock(mutex_);
if (exit_code_ == 0 && !stopped)
exit_code_ = exit_code;
Expand Down Expand Up @@ -718,6 +720,12 @@ void Worker::MemoryInfo(MemoryTracker* tracker) const {
tracker->TrackField("parent_port", parent_port_);
}

bool Worker::IsNotIndicativeOfMemoryLeakAtExit() const {
// Worker objects always stay alive as long as the child thread, regardless
// of whether they are being referenced in the parent thread.
return true;
}

class WorkerHeapSnapshotTaker : public AsyncWrap {
public:
WorkerHeapSnapshotTaker(Environment* env, Local<Object> obj)
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1 change: 1 addition & 0 deletions src/node_worker.h
Original file line number Diff line number Diff line change
Expand Up @@ -50,6 +50,7 @@ class Worker : public AsyncWrap {
void MemoryInfo(MemoryTracker* tracker) const override;
SET_MEMORY_INFO_NAME(Worker)
SET_SELF_SIZE(Worker)
bool IsNotIndicativeOfMemoryLeakAtExit() const override;

bool is_stopped() const;

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8 changes: 8 additions & 0 deletions src/stream_base.h
Original file line number Diff line number Diff line change
Expand Up @@ -423,6 +423,10 @@ class SimpleShutdownWrap : public ShutdownWrap, public OtherBase {
SET_NO_MEMORY_INFO()
SET_MEMORY_INFO_NAME(SimpleShutdownWrap)
SET_SELF_SIZE(SimpleShutdownWrap)

bool IsNotIndicativeOfMemoryLeakAtExit() const override {
return OtherBase::IsNotIndicativeOfMemoryLeakAtExit();
}
};

template <typename OtherBase>
Expand All @@ -436,6 +440,10 @@ class SimpleWriteWrap : public WriteWrap, public OtherBase {
SET_NO_MEMORY_INFO()
SET_MEMORY_INFO_NAME(SimpleWriteWrap)
SET_SELF_SIZE(SimpleWriteWrap)

bool IsNotIndicativeOfMemoryLeakAtExit() const override {
return OtherBase::IsNotIndicativeOfMemoryLeakAtExit();
}
};

} // namespace node
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Original file line number Diff line number Diff line change
Expand Up @@ -88,6 +88,7 @@ assert(undocumented.delete('--experimental-report'));
assert(undocumented.delete('--experimental-worker'));
assert(undocumented.delete('--no-node-snapshot'));
assert(undocumented.delete('--loader'));
assert(undocumented.delete('--verify-base-objects'));

assert.strictEqual(undocumented.size, 0,
'The following options are not documented as allowed in ' +
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