bootstrap: update comments in bootstrap/node.js

lib/internal/bootstrap/node.js

@@ -1,39 +1,55 @@
 // Hello, and welcome to hacking node.js!
 //
-// This file is invoked by `node::RunBootstrapping()` in `src/node.cc`, and is
-// responsible for setting up node.js core before executing main scripts
-// under `lib/internal/main/`.
+// This file is invoked by `Realm::BootstrapNode()` in `src/node_realm.cc`,
+// and is responsible for setting up node.js core before main scripts
+// under `lib/internal/main/` are executed.
 //
-// This file is expected not to perform any asynchronous operations itself
+// By default, Node.js binaries come with an embedded V8 startup snapshot
+// that is generated at build-time with a `node_mksnapshot` executable.
+// The snapshot generation code can be found in `SnapshotBuilder::Generate()`
+// from `src/node_snapshotable.cc`
+// This snapshot captures the V8 heap initialized by scripts under
+// `lib/internal/bootstrap/`, including this file. When initializing the main
+// thread, Node.js deserializes the heap from the snapshot, instead of actually
+// running this script and others in `lib/internal/bootstrap/`. To disable this
+// behavior, pass `--no-node-snapshot` when starting the process so that
+// Node.js actually runs this script to initialize the heap.
+//
+// This script is expected not to perform any asynchronous operations itself
 // when being executed - those should be done in either
-// `lib/internal/process/pre_execution.js` or in main scripts. The majority
-// of the code here focuses on setting up the global proxy and the process
-// object in a synchronous manner.
-// As special caution is given to the performance of the startup process,
-// many dependencies are invoked lazily.
+// `lib/internal/process/pre_execution.js` or in main scripts. It should not
+// query any run-time states (e.g. command line arguments, environment
+// variables) when being executed - functions in this script that are invoked
+// at a later time can, however, query those states lazily.
+// The majority of the code here focuses on setting up the global object and
+// the process object in a synchronous, environment-independent manner.
 //
 // Scripts run before this file:
-// - `lib/internal/per_context/primordials.js`: to save copies of JavaScript
+// - `lib/internal/per_context/primordials.js`: this saves copies of JavaScript
 //   builtins that won't be affected by user land monkey-patching for internal
 //   modules to use.
-// - `lib/internal/bootstrap/loaders.js`: to setup internal binding and
+// - `lib/internal/per_context/domexception.js`: implementation of the
+//   `DOMException` class.
+// - `lib/internal/per_context/messageport.js`: JS-side components of the
+//   `MessagePort` implementation.
+// - `lib/internal/bootstrap/loaders.js`: this sets up internal binding and
 //   module loaders, including `process.binding()`, `process._linkedBinding()`,
 //   `internalBinding()` and `BuiltinModule`.
 //
-// This file is run to bootstrap both the main thread and the worker threads.
-// After this file is run, certain properties are setup according to the
-// configuration of the Node.js instance using the files in
+// The initialization done in this script is included in both the main thread
+// and the worker threads. After this, further initialization is done based
+// on the configuration of the Node.js instance by executing the scripts in
 // `lib/internal/bootstrap/switches/`.
 //
 // Then, depending on how the Node.js instance is launched, one of the main
 // scripts in `lib/internal/main` will be selected by C++ to start the actual
 // execution. They may run additional setups exported by
-// `lib/internal/process/pre_execution.js` depending on the runtime states.
+// `lib/internal/process/pre_execution.js` depending on the run-time states.
 
 'use strict';
 
 // This file is compiled as if it's wrapped in a function with arguments
-// passed by node::RunBootstrapping()
+// passed by BuiltinLoader::CompileAndCall().
 /* global process, require, internalBinding, primordials */
 
 setupPrepareStackTrace();