From a621ab8695937176527839f47a1e717340d404da Mon Sep 17 00:00:00 2001 From: Anna Henningsen Date: Wed, 20 Nov 2019 00:57:49 +0100 Subject: [PATCH] src,doc: add C++ internals documentation This aims to help explain some of the internal patterns and utilities that we use. It is by no means exhaustive, and suggestions for additions are welcome. Some of this is based on the existing work from #26929. Refs: https://github.com/nodejs/node/pull/26929 PR-URL: https://github.com/nodejs/node/pull/30552 Reviewed-By: Sam Roberts Reviewed-By: Richard Lau Reviewed-By: Jiawen Geng Reviewed-By: David Carlier Reviewed-By: Joyee Cheung Reviewed-By: Colin Ihrig Reviewed-By: Gireesh Punathil Reviewed-By: Denys Otrishko Reviewed-By: James M Snell Reviewed-By: Michael Dawson --- CPP_STYLE_GUIDE.md | 3 + doc/guides/contributing/pull-requests.md | 4 +- src/README.md | 896 +++++++++++++++++++++++ 3 files changed, 902 insertions(+), 1 deletion(-) create mode 100644 src/README.md diff --git a/CPP_STYLE_GUIDE.md b/CPP_STYLE_GUIDE.md index 8808405b2c6553..f29c8df3210caa 100644 --- a/CPP_STYLE_GUIDE.md +++ b/CPP_STYLE_GUIDE.md @@ -1,5 +1,8 @@ # C++ Style Guide +See also the [C++ codebase README](src/README.md) for C++ idioms in the Node.js +codebase not related to stylistic issues. + ## Table of Contents * [Guides and References](#guides-and-references) diff --git a/doc/guides/contributing/pull-requests.md b/doc/guides/contributing/pull-requests.md index f23c92fa024e63..31e3ba64757c12 100644 --- a/doc/guides/contributing/pull-requests.md +++ b/doc/guides/contributing/pull-requests.md @@ -121,7 +121,9 @@ in the API docs will also be checked when running `make lint` (or use `REPLACEME` for the version number in the documentation YAML. For contributing C++ code, you may want to look at the -[C++ Style Guide](../../../CPP_STYLE_GUIDE.md). +[C++ Style Guide](../../../CPP_STYLE_GUIDE.md), as well as the +[README of `src/`](../../../src/README.md) for an overview over Node.js +C++ internals. ### Step 4: Commit diff --git a/src/README.md b/src/README.md new file mode 100644 index 00000000000000..48cf99991552e2 --- /dev/null +++ b/src/README.md @@ -0,0 +1,896 @@ +# Node.js C++ codebase + +Hi! 👋 You’ve found the C++ code backing Node.js. This README aims to help you +get started working on it and document some idioms you may encounter while +doing so. + +## Coding style + +Node.js has a document detailing its [C++ coding style][] +that can be helpful as a reference for stylistic issues. + +## V8 API documentation + +A lot of the Node.js codebase is around what the underlying JavaScript engine, +V8, provides through its API for embedders. Knowledge of this API can also be +useful when working with native addons for Node.js written in C++, although for +new projects [N-API][] is typically the better alternative. + +V8 does not provide much public API documentation beyond what is +available in its C++ header files, most importantly `v8.h`, which can be +accessed online in the following locations: + +* On GitHub: [`v8.h` in Node.js master][] +* On GitHub: [`v8.h` in V8 master][] +* On the Chromium project’s Code Search application: [`v8.h` in Code Search][] + +V8 also provides an [introduction for V8 embedders][], +which can be useful for understanding some of the concepts it uses in its +embedder API. + +Important concepts when using V8 are the ones of [`Isolate`][]s and +[JavaScript value handles][]. + +## libuv API documentation + +The other major dependency of Node.js is [libuv][], providing +the [event loop][] and other operation system abstractions to Node.js. + +There is a [reference documentation for the libuv API][]. + +## Helpful concepts + +A number of concepts are involved in putting together Node.js on top of V8 and +libuv. This section aims to explain some of them and how they work together. + + +### `Isolate` + +The `v8::Isolate` class represents a single JavaScript engine instance, in +particular a set of JavaScript objects that can refer to each other +(the “heap”). + +The `v8::Isolate` is often passed to other V8 API functions, and provides some +APIs for managing the behaviour of the JavaScript engine or querying about its +current state or statistics such as memory usage. + +V8 APIs are not thread-safe unless explicitly specified. In a typical Node.js +application, the main thread and any `Worker` threads each have one `Isolate`, +and JavaScript objects from one `Isolate` cannot refer to objects from +another `Isolate`. + +Garbage collection, as well as other operations that affect the entire heap, +happen on a per-`Isolate` basis. + +Typical ways of accessing the current `Isolate` in the Node.js code are: + +* Given a `FunctionCallbackInfo` for a [binding function][], + using `args.GetIsolate()`. +* Given a [`Context`][], using `context->GetIsolate()`. +* Given a [`Environment`][], using `env->isolate()`. + +### V8 JavaScript values + +V8 provides classes that mostly correspond to JavaScript types; for example, +`v8::Value` is a class representing any kind of JavaScript type, with +subclasses such as `v8::Number` (which in turn has subclasses like `v8::Int32`), +`v8::Boolean` or `v8::Object`. Most types are represented by subclasses +of `v8::Object`, e.g. `v8::Uint8Array` or `v8::Date`. + + +### Internal fields + +V8 provides the ability to store data in so-called “internal fields” inside +`v8::Object`s that were created as instances of C++-backed classes. The number +of fields needs to be defined when creating that class. + +Both JavaScript values and `void*` pointers may be stored in such fields. +In most native Node.js objects, the first internal field is used to store a +pointer to a [`BaseObject`][] subclass, which then contains all relevant +information associated with the JavaScript object. + +The most typical way of working internal fields are: + +* `obj->InternalFieldCount()` to look up the number of internal fields for an + object (`0` for regular JavaScript objects). +* `obj->GetInternalField(i)` to get a JavaScript value from an internal field. +* `obj->SetInternalField(i, v)` to store a JavaScript value in an + internal field. +* `obj->GetAlignedPointerFromInternalField(i)` to get a `void*` pointer from an + internal field. +* `obj->SetAlignedPointerInInternalField(i, p)` to store a `void*` pointer in an + internal field. + +[`Context`][]s provide the same feature under the name “embedder data”. + + +### JavaScript value handles + +All JavaScript values are accessed through the V8 API through so-called handles, +of which there are two types: [`Local`][]s and [`Global`][]s. + + +#### `Local` handles + +A `v8::Local` handle is a temporary pointer to a JavaScript object, where +“temporary” usually means that is no longer needed after the current function +is done executing. `Local` handles can only be allocated on the C++ stack. + +Most of the V8 API uses `Local` handles to work with JavaScript values or return +them from functions. + +Whenever a `Local` handle is created, a `v8::HandleScope` or +`v8::EscapableHandleScope` object must exist on the stack. The `Local` is then +added to that scope and deleted along with it. + +When inside a [binding function][], a `HandleScope` already exists outside of +it, so there is no need to explicitly create one. + +`EscapableHandleScope`s can be used to allow a single `Local` handle to be +passed to the outer scope. This is useful when a function returns a `Local`. + +The following JavaScript and C++ functions are mostly equivalent: + +```js +function getFoo(obj) { + return obj.foo; +} +``` + +```c++ +v8::Local GetFoo(v8::Local context, + v8::Local obj) { + v8::Isolate* isolate = context->GetIsolate(); + v8::EscapableHandleScope handle_scope(isolate); + + // The 'foo_string' handle cannot be returned from this function because + // it is not “escaped” with `.Escape()`. + v8::Local foo_string = + v8::String::NewFromUtf8(isolate, + "foo", + v8::NewStringType::kNormal).ToLocalChecked(); + + v8::Local return_value; + if (obj->Get(context, foo_string).ToLocal(&return_value)) { + return handle_scope.Escape(return_value); + } else { + // There was a JS exception! Handle it somehow. + return v8::Local(); + } +} +``` + +See [exception handling][] for more information about the usage of `.To()`, +`.ToLocalChecked()`, `v8::Maybe` and `v8::MaybeLocal` usage. + +##### Casting local handles + +If it is known that a `Local` refers to a more specific type, it can +be cast to that type using `.As<...>()`: + +```c++ +v8::Local some_value; +// CHECK() is a Node.js utilitity that works similar to assert(). +CHECK(some_value->IsUint8Array()); +v8::Local as_uint8 = some_value.As(); +``` + +Generally, using `val.As()` is only valid if `val->IsX()` is true, and +failing to follow that rule may lead to crashes. + +##### Detecting handle leaks + +If it is expected that no `Local` handles should be created within a given +scope unless explicitly within a `HandleScope`, a `SealHandleScope` can be used. + +For example, there is a `SealHandleScope` around the event loop, forcing +any functions that are called from the event loop and want to run or access +JavaScript code to create `HandleScope`s. + + +#### `Global` handles + +A `v8::Global` handle (sometimes also referred to by the name of its parent +class `Persistent`, although use of that is discouraged in Node.js) is a +reference to a JavaScript object that can remain active as long as the engine +instance is active. + +Global handles can be either strong or weak. Strong global handles are so-called +“GC roots”, meaning that they will keep the JavaScript object they refer to +alive even if no other objects refer to them. Weak global handles do not do +that, and instead optionally call a callback when the object they refer to +is garbage-collected. + +```c++ +v8::Global reference; + +void StoreReference(v8::Isolate* isolate, v8::Local obj) { + // Create a strong reference to `obj`. + reference.Reset(isolate, obj); +} + +// Must be called with a HandleScope around it. +v8::Local LoadReference(v8::Isolate* isolate) { + return reference.Get(isolate); +} +``` + +##### `Eternal` handles + +`v8::Eternal` handles are a special kind of handles similar to `v8::Global` +handles, with the exception that the values they point to are never +garbage-collected while the JavaScript Engine instance is alive, even if +the `v8::Eternal` itself is destroyed at some point. This type of handle +is rarely used. + + +### `Context` + +JavaScript allows multiple global objects and sets of built-in JavaScript +objects (like the `Object` or `Array` functions) to coexist inside the same +heap. Node.js exposes this ability through the [`vm` module][]. + +V8 refers to each of these global objects and their associated builtins as a +`Context`. + +Currently, in Node.js there is one main `Context` associated with an +[`Environment`][] instance, and most Node.js features will only work inside +that context. (The only exception at the time of writing are +[`MessagePort`][] objects.) This restriction is not inherent to the design of +Node.js, and a sufficiently committed person could restructure Node.js to +provide built-in modules inside of `vm.Context`s. + +Often, the `Context` is passed around for [exception handling][]. +Typical ways of accessing the current `Environment` in the Node.js code are: + +* Given an [`Isolate`][], using `isolate->GetCurrentContext()`. +* Given an [`Environment`][], using `env->context()` to get the `Environment`’s + main context. + + +### Event loop + +The main abstraction for an event loop inside Node.js is the `uv_loop_t` struct. +Typically, there is one event loop per thread. This includes not only the main +thread and Workers, but also helper threads that may occasionally be spawned +in the course of running a Node.js program. + +The current event loop can be accessed using `env->event_loop()` given an +[`Environment`][] instance. The restriction of using a single event loop +is not inherent to the design of Node.js, and a sufficiently committed person +could restructure Node.js to provide e.g. the ability to run parts of Node.js +inside an event loop separate from the active thread’s event loop. + + +### `Environment` + +Node.js instances are represented by the `Environment` class. + +Currently, every `Environment` class is associated with: + +* One [event loop][] +* One [`Isolate`][] +* One main [`Context`][] + +The `Environment` class contains a large number of different fields for +different Node.js modules, for example a libuv timer for `setTimeout()` or +the memory for a `Float64Array` that the `fs` module uses for storing data +returned from a `fs.stat()` call. + +It also provides [cleanup hooks][] and maintains a list of [`BaseObject`][] +instances. + +Typical ways of accessing the current `Environment` in the Node.js code are: + +* Given a `FunctionCallbackInfo` for a [binding function][], + using `Environment::GetCurrent(args)`. +* Given a [`BaseObject`][], using `env()` or `self->env()`. +* Given a [`Context`][], using `Environment::GetCurrent(context)`. + This requires that `context` has been associated with the `Environment` + instance, e.g. is the main `Context` for the `Environment` or one of its + `vm.Context`s. +* Given an [`Isolate`][], using `Environment::GetCurrent(isolate)`. This looks + up the current [`Context`][] and then uses that. + + +### `IsolateData` + +Every Node.js instance ([`Environment`][]) is associated with one `IsolateData` +instance that contains information about or associated with a given +[`Isolate`][]. + +#### String table + +`IsolateData` contains a list of strings that can be quickly accessed +inside Node.js code, e.g. given an `Environment` instance `env` the JavaScript +string “name” can be accessed through `env->name_string()` without actually +creating a new JavaScript string. + +### Platform + +Every process that uses V8 has a `v8::Platform` instance that provides some +functionalities to V8, most importantly the ability to schedule work on +background threads. + +Node.js provides a `NodePlatform` class that implements the `v8::Platform` +interface and uses libuv for providing background threading abilities. + +The platform can be accessed through `isolate_data->platform()` given an +[`IsolateData`][] instance, although that only works when: + +* The current Node.js instance was not started by an embedder; or +* The current Node.js instance was started by an embedder whose `v8::Platform` + implementation also implement’s the `node::MultiIsolatePlatform` interface + and who passed this to Node.js. + + +### Binding functions + +C++ functions exposed to JS follow a specific signature. The following example +is from `node_util.cc`: + +```c++ +void ArrayBufferViewHasBuffer(const FunctionCallbackInfo& args) { + CHECK(args[0]->IsArrayBufferView()); + args.GetReturnValue().Set(args[0].As()->HasBuffer()); +} +``` + +(Namespaces are usually omitted through the use of `using` statements in the +Node.js source code.) + +`args[n]` is a `Local` that represents the n-th argument passed to the +function. `args.This()` is the `this` value inside this function call. +`args.Holder()` is equivalent to `args.This()` in all use cases inside of +Node.js. + +`args.GetReturnValue()` is a placeholder for the return value of the function, +and provides a `.Set()` method that can be called with a boolean, integer, +floating-point number or a `Local` to set the return value. + +Node.js provides various helpers for building JS classes in C++ and/or attaching +C++ functions to the exports of a built-in module: + +```c++ +void Initialize(Local target, + Local unused, + Local context, + void* priv) { + Environment* env = Environment::GetCurrent(context); + + env->SetMethod(target, "getaddrinfo", GetAddrInfo); + env->SetMethod(target, "getnameinfo", GetNameInfo); + + // 'SetMethodNoSideEffect' means that debuggers can safely execute this + // function for e.g. previews. + env->SetMethodNoSideEffect(target, "canonicalizeIP", CanonicalizeIP); + + // ... more code ... + + // Building the `ChannelWrap` class for JS: + Local channel_wrap = + env->NewFunctionTemplate(ChannelWrap::New); + // Allow for 1 internal field, see `BaseObject` for details on this: + channel_wrap->InstanceTemplate()->SetInternalFieldCount(1); + channel_wrap->Inherit(AsyncWrap::GetConstructorTemplate(env)); + + // Set various methods on the class (i.e. on the prototype): + env->SetProtoMethod(channel_wrap, "queryAny", Query); + env->SetProtoMethod(channel_wrap, "queryA", Query); + // ... + env->SetProtoMethod(channel_wrap, "querySoa", Query); + env->SetProtoMethod(channel_wrap, "getHostByAddr", Query); + + env->SetProtoMethodNoSideEffect(channel_wrap, "getServers", GetServers); + + Local channel_wrap_string = + FIXED_ONE_BYTE_STRING(env->isolate(), "ChannelWrap"); + channel_wrap->SetClassName(channel_wrap_string); + target->Set(env->context(), channel_wrap_string, + channel_wrap->GetFunction(context).ToLocalChecked()).Check(); +} + +// Run the `Initialize` function when loading this module through +// `internalBinding('cares_wrap')` in Node.js’s built-in JavaScript code: +NODE_MODULE_CONTEXT_AWARE_INTERNAL(cares_wrap, Initialize) +``` + + +### Exception handling + +The V8 engine provides multiple features to work with JavaScript exceptions, +as C++ exceptions are disabled inside of Node.js: + +#### Maybe types + +V8 provides the `v8::Maybe` and `v8::MaybeLocal` types, typically used +as return values from API functions that can run JavaScript code and therefore +can throw exceptions. + +Conceptually, the idea is that every `v8::Maybe` is either empty (checked +through `.IsNothing()`) or holds a value of type `T` (checked through +`.IsJust()`). If the `Maybe` is empty, then a JavaScript exception is pending. +A typical way of accessing the value is using the `.To()` function, which +returns a boolean indicating success of the operation (i.e. the `Maybe` not +being empty) and taking a pointer to a `T` to store the value if there is one. + +##### Checked conversion + +`maybe.Check()` can be used to assert that the maybe is not empty, i.e. crash +the process otherwise. `maybe.FromJust()` (aka `maybe.ToChecked()`) can be used +to access the value and crash the process if it is not set. + +This should only be performed if it is actually sure that the operation has +not failed. A lot of Node.js’s source code does **not** follow this rule, and +can be brought to crash through this. + +##### MaybeLocal + +`v8::MaybeLocal` is a variant of `v8::Maybe` that is either empty or +holds a value of type `Local`. It has methods that perform the same +operations as the methods of `v8::Maybe`, but with different names: + +| `Maybe` | `MaybeLocal` | +| ---------------------- | ------------------------------- | +| `maybe.IsNothing()` | `maybe_local.IsEmpty()` | +| `maybe.IsJust()` | – | +| `maybe.To(&value)` | `maybe_local.ToLocal(&local)` | +| `maybe.ToChecked()` | `maybe_local.ToLocalChecked()` | +| `maybe.FromJust()` | `maybe_local.ToLocalChecked()` | +| `maybe.Check()` | – | +| `v8::Nothing()` | `v8::MaybeLocal()` | +| `v8::Just(value)` | `v8::MaybeLocal(value)` | + +##### Handling empty `Maybe`s + +Usually, the best approach to encountering an empty `Maybe` is to just return +from the current function as soon as possible, and let execution in JavaScript +land resume. If the empty `Maybe` is encountered inside a nested function, +is may be a good idea to use a `Maybe` or `MaybeLocal` for the return type +of that function and pass information about pending JavaScript exceptions along +that way. + +Generally, when an empty `Maybe` is encountered, it is not valid to attempt +to perform further calls to APIs that return `Maybe`s. + +A typical pattern for dealing with APIs that return `Maybe` and `MaybeLocal` is +using `.ToLocal()` and `.To()` and returning early in case there is an error: + +```c++ +// This could also return a v8::MaybeLocal, for example. +v8::Maybe SumNumbers(v8::Local context, + v8::Local array_of_integers) { + v8::Isolate* isolate = context->GetIsolate(); + v8::HandleScope handle_scope(isolate); + + double sum = 0; + + for (uint32_t i = 0; i < array_of_integers->Length(); i++) { + v8::Local entry; + if (array_of_integers->Get(context, i).ToLocal(&entry)) { + // Oops, we might have hit a getter that throws an exception! + // It’s better to not continue return an empty (“nothing”) Maybe. + return v8::Nothing(); + } + + if (!entry->IsNumber()) { + // Let’s just skip any non-numbers. It would also be reasonable to throw + // an exception here, e.g. using the error system in src/node_errors.h, + // and then to return an empty Maybe again. + continue; + } + + // This cast is valid, because we’ve made sure it’s really a number. + v8::Local entry_as_number = entry.As(); + + sum += entry_as_number->Value(); + } + + return v8::Just(sum); +} + +// Function that is exposed to JS: +void SumNumbers(const v8::FunctionCallbackInfo& args) { + // This will crash if the first argument is not an array. Let’s assume we + // have performed type checking in a JavaScript wrapper function. + CHECK(args[0]->IsArray()); + + double sum; + if (!SumNumbers(args.GetIsolate()->GetCurrentContext(), + args[0].As()).To(&sum)) { + // Nothing to do, we can just return directly to JavaScript. + return; + } + + args.GetReturnValue().Set(sum); +} +``` + +#### TryCatch + +If there is a need to catch JavaScript exceptions in C++, V8 provides the +`v8::TryCatch` type for doing so, which we wrap into our own +`node::errors::TryCatchScope` in Node.js. The latter has the additional feature +of providing the ability to shut down the program in the typical Node.js way +(printing the exception + stack trace) if an exception is caught. + + +### libuv handles and requests + +Two central concepts when working with libuv are handles and requests. + +Handles are subclasses of the `uv_handle_t` “class”, and generally refer to +long-lived objects that can emit events multiple times, such as network sockets +or file system watchers. + +In Node.js, handles are often managed through a [`HandleWrap`][] subclass. + +Requests are one-time asynchronous function calls on the event loop, such as +file system requests or network write operations, that either succeed or fail. + +In Node.js, requests are often managed through a [`ReqWrap`][] subclass. + +### Environment cleanup + +When a Node.js [`Environment`][] is destroyed, it generally needs to clean up +any resources owned by it, e.g. memory or libuv requests/handles. + + +#### Cleanup hooks + +Cleanup hooks are provided that run before the [`Environment`][] +is destroyed. They can be added and removed through by using +`env->AddCleanupHook(callback, hint);` and +`env->RemoveCleanupHook(callback, hint);`, where callback takes a `void* hint` +argument. + +Inside these cleanup hooks, new asynchronous operations *may* be started on the +event loop, although ideally that is avoided as much as possible. + +Every [`BaseObject`][] has its own cleanup hook that deletes it. For +[`ReqWrap`][] and [`HandleWrap`][] instances, cleanup of the associated libuv +objects is performed automatically, i.e. handles are closed and requests +are cancelled if possible. + +#### Closing libuv handles + +If a libuv handle is not managed through a [`HandleWrap`][] instance, +it needs to be closed explicitly. Do not use `uv_close()` for that, but rather +`env->CloseHandle()`, which works the same way but keeps track of the number +of handles that are still closing. + +#### Closing libuv requests + +There is no way to abort libuv requests in general. If a libuv request is not +managed through a [`ReqWrap`][] instance, the +`env->IncreaseWaitingRequestCounter()` and +`env->DecreaseWaitingRequestCounter()` functions need to be used to keep track +of the number of active libuv requests. + +#### Calling into JavaScript + +Calling into JavaScript is not allowed during cleanup. Worker threads explicitly +forbid this during their shutdown sequence, but the main thread does not for +backwards compatibility reasons. + +When calling into JavaScript without using [`MakeCallback()`][], check the +`env->can_call_into_js()` flag and do not proceed if it is set to `false`. + +## Classes associated with JavaScript objects + +### `MemoryRetainer` + +A large number of classes in the Node.js C++ codebase refer to other objects. +The `MemoryRetainer` class is a helper for annotating C++ classes with +information that can be used by the heap snapshot builder in V8, so that +memory retained by C++ can be tracked in V8 heap snapshots captured in +Node.js applications. + +Inheriting from the `MemoryRetainer` class enables objects (both from JavaScript +and C++) to refer to instances of that class, and in turn enables that class +to point to other objects as well, including native C++ types +such as `std::string` and track their memory usage. + +This can be useful for debugging memory leaks. + +The [`memory_retainer.h`][] header file explains how to use this class. + + +### `BaseObject` + +A frequently recurring situation is that a JavaScript object and a C++ object +need to be tied together. `BaseObject` is the main abstraction for that in +Node.js, and most classes that are associated with JavaScript objects are +subclasses of it. It is defined in [`base_object.h`][]. + +Every `BaseObject` is associated with one [`Environment`][] and one +`v8::Object`. The `v8::Object` needs to have at least one [internal field][] +that is used for storing the pointer to the C++ object. In order to ensure this, +the V8 `SetInternalFieldCount()` function is usually used when setting up the +class from C++. + +The JavaScript object can be accessed as a `v8::Local` by using +`self->object()`, given a `BaseObject` named `self`. + +Accessing a `BaseObject` from a `v8::Local` (frequently that is +`args.This()` or `args.Holder()` in a [binding function][]) can be done using +the `Unwrap(obj)` function, where `T` is a subclass of `BaseObject`. +A helper for this is the `ASSIGN_OR_RETURN_UNWRAP` macro that returns from the +current function if unwrapping fails (typically that means that the `BaseObject` +has been deleted earlier). + +```c++ +void Http2Session::Request(const FunctionCallbackInfo& args) { + Http2Session* session; + ASSIGN_OR_RETURN_UNWRAP(&session, args.Holder()); + Environment* env = session->env(); + Local context = env->context(); + Isolate* isolate = env->isolate(); + + // ... + // The actual function body, which can now use the `session` object. + // ... +} +``` + +#### Lifetime management + +The `BaseObject` class comes with a set of features that allow managing the +lifetime of its instances, either associating it with the lifetime of the +corresponding JavaScript object or untying the two. + +The `BaseObject::MakeWeak()` method turns the underlying [`Global`][] handle +into a weak one, and makes it so that the `BaseObject::OnGCCollect()` virtual +method is called when the JavaScript object is garbage collected. By default, +that methods deletes the `BaseObject` instance. + +`BaseObject::ClearWeak()` undoes this effect. + +It generally makes sense to call `MakeWeak()` in the constructor of a +`BaseObject` subclass, unless that subclass is referred to by e.g. the event +loop, as is the case for the [`HandleWrap`][] and [`ReqWrap`][] classes. + +In addition, there are two kinds of smart pointers that can be used to refer +to `BaseObject`s. + +`BaseObjectWeakPtr` is similar to `std::weak_ptr`, but holds on to +an object of a `BaseObject` subclass `T` and integrates with the lifetime +management of the former. When the `BaseObject` no longer exists, e.g. when +it was garbage collected, accessing it through `weak_ptr.get()` will return +`nullptr`. + +`BaseObjectPtr` is similar to `std::shared_ptr`, but also holds on to +objects of a `BaseObject` subclass `T`. While there are `BaseObjectPtr`s +pointing to a given object, the `BaseObject` will always maintain a strong +reference to its associated JavaScript object. This can be useful when one +`BaseObject` refers to another `BaseObject` and wants to make sure it stays +alive during the lifetime of that reference. + +A `BaseObject` can be “detached” throught the `BaseObject::Detach()` method. +In this case, it will be deleted once the last `BaseObjectPtr` referring to +it is destroyed. There must be at least one such pointer when `Detach()` is +called. This can be useful when one `BaseObject` fully owns another +`BaseObject`. + + +### `AsyncWrap` + +`AsyncWrap` is a subclass of `BaseObject` that additionally provides tracking +functions for asynchronous calls. It is commonly used for classes whose methods +make calls into JavaScript without any JavaScript stack below, i.e. more or less +directly from the event loop. It is defined in [`async_wrap.h`][]. + +Every `AsyncWrap` subclass has a “provider type”. A list of provider types is +maintained in `src/async_wrap.h`. + +Every `AsyncWrap` instance is associated with two numbers, the “async id” +and the “async trigger id”. The “async id” is generally unique per `AsyncWrap` +instance, and only changes when the object is re-used in some way. + +See the [`async_hooks` module][] documentation for more information about how +this information is provided to async tracking tools. + + +#### `MakeCallback` + +The `AsyncWrap` class has a set of methods called `MakeCallback()`, with the +intention of the naming being that it is used to “make calls back into +JavaScript” from the event loop, rather than making callbacks in some way. +(As the naming has made its way into Node.js’s public API, it’s not worth +the breakage of fixing it). + +`MakeCallback()` generally calls a method on the JavaScript object associated +with the current `AsyncWrap`, and informs async tracking code about these calls +as well as takes care of running the `process.nextTick()` and `Promise` task +queues once it returns. + +Before calling `MakeCallback()`, it is typically necessary to enter both a +`HandleScope` and a `Context::Scope`. + +```c++ +void StatWatcher::Callback(uv_fs_poll_t* handle, + int status, + const uv_stat_t* prev, + const uv_stat_t* curr) { + // Get the StatWatcher instance associated with this call from libuv, + // StatWatcher is a subclass of AsyncWrap. + StatWatcher* wrap = ContainerOf(&StatWatcher::watcher_, handle); + Environment* env = wrap->env(); + HandleScope handle_scope(env->isolate()); + Context::Scope context_scope(env->context()); + + // Transform 'prev' and 'curr' into an array: + Local arr = ...; + + Local argv[] = { Integer::New(env->isolate(), status), arr }; + wrap->MakeCallback(env->onchange_string(), arraysize(argv), argv); +} +``` + +See [Callback scopes][] for more information. + + +### `HandleWrap` + +`HandleWrap` is a subclass of `AsyncWrap` specifically designed to make working +with [libuv handles][] easier. It provides the `.ref()`, `.unref()` and +`.hasRef()` methods as well as `.close()` to enable easier lifetime management +from JavaScript. It is defined in [`handle_wrap.h`][]. + +`HandleWrap` instances are [cleaned up][cleanup hooks] automatically when the +current Node.js [`Environment`][] is destroyed, e.g. when a Worker thread stops. + +`HandleWrap` also provides facilities for diagnostic tooling to get an +overview over libuv handles managed by Node.js. + + +### `ReqWrap` + +`ReqWrap` is a subclass of `AsyncWrap` specifically designed to make working +with [libuv requests][] easier. It is defined in [`req_wrap.h`][]. + +In particular, its `Dispatch()` method is designed to avoid the need to keep +track of the current count of active libuv requests. + +`ReqWrap` also provides facilities for diagnostic tooling to get an +overview over libuv handles managed by Node.js. + + +### Callback scopes + +The public `CallbackScope` and the internally used `InternalCallbackScope` +classes provide the same facilities as [`MakeCallback()`][], namely: + +* Emitting the `'before'` event for async tracking when entering the scope +* Setting the current async IDs to the ones passed to the constructor +* Emitting the `'after'` event for async tracking when leaving the scope +* Running the `process.nextTick()` queue +* Running microtasks, in particular `Promise` callbacks and async/await + functions + +Usually, using `AsyncWrap::MakeCallback()` or using the constructor taking +an `AsyncWrap*` argument (i.e. used as +`InternalCallbackScope callback_scope(this);`) suffices inside of Node.js’s +C++ codebase. + +## C++ utilities + +Node.js uses a few custom C++ utilities, mostly defined in [`util.h`][]. + +### Memory allocation + +Node.js provides `Malloc()`, `Realloc()` and `Calloc()` functions that work +like their C stdlib counterparts, but crash if memory cannot be allocated. +(As V8 does not handle out-of-memory situations gracefully, it does not make +sense for Node.js to attempt to do so in all cases.) + +The `UncheckedMalloc()`, `UncheckedRealloc()` and `UncheckedCalloc()` functions +return `nullptr` in these cases (or when `size == 0`). + +#### Optional stack-based memory allocation + +The `MaybeStackBuffer` class provides a way to allocate memory on the stack +if it is smaller than a given limit, and falls back to allocating it on the +heap if it is larger. This can be useful for performantly allocating temporary +data if it is typically expected to be small (e.g. file paths). + +The `Utf8Value`, `TwoByteValue` (i.e. UTF-16 value) and `BufferValue` +(`Utf8Value` but copy data from a `Buffer` is that is passed) helpers +inherit from this class and allow accessing the characters in a JavaScript +string this way. + +```c++ +static void Chdir(const FunctionCallbackInfo& args) { + Environment* env = Environment::GetCurrent(args); + // ... + CHECK(args[0]->IsString()); + Utf8Value path(env->isolate(), args[0]); + int err = uv_chdir(*path); + if (err) { + // ... error handling ... + } +} +``` + +### Assertions + +Node.js provides a few macros that behave similar to `assert()`: + +* `CHECK(expression)` aborts the process with a stack trace + if `expression` is false. +* `CHECK_EQ(a, b)` checks for `a == b` +* `CHECK_GE(a, b)` checks for `a >= b` +* `CHECK_GT(a, b)` checks for `a > b` +* `CHECK_LE(a, b)` checks for `a <= b` +* `CHECK_LT(a, b)` checks for `a < b` +* `CHECK_NE(a, b)` checks for `a != b` +* `CHECK_NULL(val)` checks for `a == nullptr` +* `CHECK_NOT_NULL(val)` checks for `a != nullptr` +* `CHECK_IMPLIES(a, b)` checks that `b` is true if `a` is true. +* `UNREACHABLE([message])` aborts the process if it is reached. + +`CHECK`s are always enabled. For checks that should only run in debug mode, use +`DCHECK()`, `DCHECK_EQ()`, etc. + +### Scope-based cleanup + +The `OnScopeLeave()` function can be used to run a piece of code when leaving +the current C++ scope. + +```c++ +static void GetUserInfo(const FunctionCallbackInfo& args) { + Environment* env = Environment::GetCurrent(args); + uv_passwd_t pwd; + // ... + + const int err = uv_os_get_passwd(&pwd); + + if (err) { + // ... error handling, return early ... + } + + auto free_passwd = OnScopeLeave([&]() { uv_os_free_passwd(&pwd); }); + + // ... + // Turn `pwd` into a JavaScript object now; whenever we return from this + // function, `uv_os_free_passwd()` will be called. + // ... +} +``` + +[`BaseObject`]: #baseobject +[`Context`]: #context +[`Environment`]: #environment +[`Global`]: #global-handles +[`HandleWrap`]: #handlewrap +[`IsolateData`]: #isolate-data +[`Isolate`]: #isolate +[`Local`]: #local-handles +[`MakeCallback()`]: #makecallback +[`MessagePort`]: https://nodejs.org/api/worker_threads.html#worker_threads_class_messageport +[`ReqWrap`]: #reqwrap +[`async_hooks` module]: https://nodejs.org/api/async_hooks.html +[`async_wrap.h`]: async_wrap.h +[`base_object.h`]: base_object.h +[`handle_wrap.h`]: handle_wrap.h +[`memory_retainer.h`]: memory_retainer.h +[`req_wrap.h`]: req_wrap.h +[`util.h`]: util.h +[`v8.h` in Code Search]: https://cs.chromium.org/chromium/src/v8/include/v8.h +[`v8.h` in Node.js master]: https://github.com/nodejs/node/blob/master/deps/v8/include/v8.h +[`v8.h` in V8 master]: https://github.com/v8/v8/blob/master/include/v8.h +[`vm` module]: https://nodejs.org/api/vm.html +[C++ coding style]: ../CPP_STYLE_GUIDE.md +[Callback scopes]: #callback-scopes +[JavaScript value handles]: #js-handles +[N-API]: https://nodejs.org/api/n-api.html +[binding function]: #binding-functions +[cleanup hooks]: #cleanup-hooks +[event loop]: #event-loop +[exception handling]: #exception-handling +[internal field]: #internal-field +[introduction for V8 embedders]: https://v8.dev/docs/embed +[libuv handles]: #libuv-handles-and-requests +[libuv requests]: #libuv-handles-and-requests +[libuv]: https://libuv.org/ +[reference documentation for the libuv API]: http://docs.libuv.org/en/v1.x/