58ac6551ff
Part of the tools/snapshot/README.md is out of date since we have made more progress on the snapshot integration, so update it accordingly with some details about the various snapshots in the snapshot blob. Also update the section in src/README.md about external reference registration for clarification. PR-URL: https://github.com/nodejs/node/pull/56505 Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
98 lines
4.8 KiB
Markdown
98 lines
4.8 KiB
Markdown
# Node.js startup snapshot builder
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This is the V8 startup snapshot builder of Node.js. Not to be confused with
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V8's own snapshot builder, which builds a snapshot containing JavaScript
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builtins, this builds a snapshot containing Node.js builtins that can be
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deserialized on top of V8's own startup snapshot. When Node.js is launched,
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instead of executing code to bootstrap, it can deserialize the context from
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an embedded snapshot, which readily contains the result of the bootstrap, so
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that Node.js can start up faster.
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The built-in snapshot consists of the following snapshots:
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## Isolate snapshot
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Which is used whenever a `v8::Isolate` is created using the data returned by
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`node::SnapshotBuilder::GetEmbeddedSnapshotData`.
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## Context snapshots
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Which are used when a `v8::Context` is created from a `v8::Isolate` deserialized
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from the snapshot. There are four context snapshots in the snapshot blob.
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1. The default context snapshot, used for contexts created by
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`v8::Context::New()`, it only contains V8 built-ins, matching the
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layout of the isolate snapshot.
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1. The vm context snapshot, which can be deserialized using
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`v8::Context::FromSnapshot(isolate, node::SnapshotData::kNodeVMContextIndex, ...)`.
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It captures initializations specific to vm contexts done by
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`node::contextify::ContextifyContext::CreateV8Context()`.
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1. The base context snapshot, which can be deserialized using
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`v8::Context::FromSnapshot(isolate, node::SnapshotData::kNodeBaseContextIndex, ...)`.
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It currently captures initializations done by `node::NewContext()`
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but is intended to include more as a basis for worker thread
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contexts.
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1. The main context snapshot, which can be deserialized using
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`v8::Context::FromSnapshot(isolate, node::SnapshotData::kNodeMainContextIndex, ...)`.
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This is the snapshot for the main context on the main thread, and
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captures initializations done by `node::CommonEnvironmentSetup::CreateForSnapshotting()`,
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most notably `node::CreateEnvironment()`, which runs the following scripts via
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`node::Realm::RunBootstrapping()` for the main context as a principal realm,
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so that at runtime, these scripts do not need to be run. Instead only the context
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initialized by them is deserialized at runtime.
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1. `internal/bootstrap/realm`
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2. `internal/bootstrap/node`
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3. `internal/bootstrap/web/exposed-wildcard`
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4. `internal/bootstrap/web/exposed-window-or-worker`
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5. `internal/bootstrap/switches/is_main_thread`
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6. `internal/bootstrap/switches/does_own_process_state`
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For more information about these contexts, see the comment in `src/node_context_data.h`.
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## How it's built and used
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The snapshot builder is built with the `node_mksnapshot` target in `node.gyp`
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when `node_use_node_snapshot` is set to true, which is currently done by
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default.
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In the default build of the Node.js executable, to embed a V8 startup snapshot
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into the Node.js executable, `libnode` is first built with these unresolved
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symbols:
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- `node::SnapshotBuilder::GetEmbeddedSnapshotData`
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Then the `node_mksnapshot` executable is built with C++ files in this
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directory, as well as `src/node_snapshot_stub.cc` which defines the unresolved
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symbols.
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`node_mksnapshot` is run to generate a C++ file
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`<(SHARED_INTERMEDIATE_DIR)/node_snapshot.cc` that is similar to
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`src/node_snapshot_stub.cc` in structure, but contains the snapshot data
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written as static char array literals. Then `libnode` is built with
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`node_snapshot.cc` to produce the final Node.js executable with the snapshot
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data embedded.
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For debugging, Node.js can be built without Node.js's own snapshot if
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`--without-node-snapshot` is passed to `configure`. A Node.js executable
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with Node.js snapshot embedded can also be launched without deserializing
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from it if the command line argument `--no-node-snapshot` is passed.
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### When `node_mksnapshot` crashes..
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Due to this two-phase building process, sometimes when there is an issue
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in the code, the build may crash early at executing `node_mksnapshot` instead of crashing
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at executing the final executable `node`. If the crash can be reproduced when running
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the `node` executable built with `--without-node-snapshot`, it means the crash likely
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has nothing to do with snapshots, and only shows up in `node_mksnapshot` because it's
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the first Node.js executable being run.
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If the crash comes from a `mksnapshot` executable (notice that it doesn't have the `node_`
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prefix), that comes from V8's own snapshot building process, not the one in Node.js, and the
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fix likely needs to be in V8 or the build configurations of V8.
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If it `node_mksnapshot` crashes with an error message containing
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something like `Unknown external reference 0x107769200`, see
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[Registering binding functions used in bootstrap][] on how to fix it.
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[Registering binding functions used in bootstrap]: ../../src/README.md#registering-binding-functions-used-in-bootstrap
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