doc: improve internal documentation on built-in snapshot · nodejs/node@b0b77d7

an embedded snapshot, which readily contains the result of the bootstrap, so

that Node.js can start up faster.

Currently only the main context of the main Node.js instance supports snapshot

deserialization, and the snapshot does not yet cover the entire bootstrap

process. Work is being done to expand the support.

The built-in snapshot consists of the following snapshots:

## Isolate snapshot

Which is used whenever a `v8::Isolate` is created using the data returned by

`node::SnapshotBuilder::GetEmbeddedSnapshotData`.

## Context snapshots

Which are used when a `v8::Context` is created from a `v8::Isolate` deserialized

from the snapshot. There are four context snapshots in the snapshot blob.

1. The default context snapshot, used for contexts created by

`v8::Context::New()`, it only contains V8 built-ins, matching the

layout of the isolate snapshot.

1. The vm context snapshot, which can be deserialized using

`v8::Context::FromSnapshot(isolate, node::SnapshotData::kNodeVMContextIndex, ...)`.

It captures initializations specific to vm contexts done by

`node::contextify::ContextifyContext::CreateV8Context()`.

1. The base context snapshot, which can be deserialized using

`v8::Context::FromSnapshot(isolate, node::SnapshotData::kNodeBaseContextIndex, ...)`.

It currently captures initializations done by `node::NewContext()`

but is intended to include more as a basis for worker thread

contexts.

1. The main context snapshot, which can be deserialized using

`v8::Context::FromSnapshot(isolate, node::SnapshotData::kNodeMainContextIndex, ...)`.

This is the snapshot for the main context on the main thread, and

captures initializations done by `node::CommonEnvironmentSetup::CreateForSnapshotting()`,

most notably `node::CreateEnvironment()`, which runs the following scripts via

`node::Realm::RunBootstrapping()` for the main context as a principal realm,

so that at runtime, these scripts do not need to be run. Instead only the context

initialized by them is deserialized at runtime.

1. `internal/bootstrap/realm`

2. `internal/bootstrap/node`

3. `internal/bootstrap/web/exposed-wildcard`

4. `internal/bootstrap/web/exposed-window-or-worker`

5. `internal/bootstrap/switches/is_main_thread`

6. `internal/bootstrap/switches/does_own_process_state`

For more information about these contexts, see the comment in `src/node_context_data.h`.

## How it's built and used

`--without-node-snapshot` is passed to `configure`. A Node.js executable

with Node.js snapshot embedded can also be launched without deserializing

from it if the command line argument `--no-node-snapshot` is passed.

### When `node_mksnapshot` crashes..

Due to this two-phase building process, sometimes when there is an issue

in the code, the build may crash early at executing `node_mksnapshot` instead of crashing

at executing the final executable `node`. If the crash can be reproduced when running

the `node` executable built with `--without-node-snapshot`, it means the crash likely

has nothing to do with snapshots, and only shows up in `node_mksnapshot` because it's

the first Node.js executable being run.

If the crash comes from a `mksnapshot` executable (notice that it doesn't have the `node_`

prefix), that comes from V8's own snapshot building process, not the one in Node.js, and the

fix likely needs to be in V8 or the build configurations of V8.

If it `node_mksnapshot` crashes with an error message containing

something like `Unknown external reference 0x107769200`, see

[Registering binding functions used in bootstrap][] on how to fix it.

[Registering binding functions used in bootstrap]: ../../src/README.md#registering-binding-functions-used-in-bootstrap