Support child_process in snapshot

[H4ad]

Version

21.1.0

Platform

Linux h4ad 5.15.0-89-generic #99~20.04.1-Ubuntu SMP Thu Nov 2 15:16:47 UTC 2023 x86_64 x86_64 x86_64 GNU/Linux

Subsystem

child_process

What steps will reproduce the bug?

The following code:

// save as file.js
const child_process = require("node:child_process");
child_process.execSync("echo 1", { stdio: "inherit" });

Run with: node --snapshot-blob snapshot.blob --build-snapshot file.js

How often does it reproduce? Is there a required condition?

Always

What is the expected behavior? Why is that the expected behavior?

The snapshot being created without core dumped.is

What do you see instead?

1
(node:136443) Warning: built-in module node:child_process is not yet supported in user snapshots
(Use `node --trace-warnings ...` to show where the warning was created)
Unknown external reference 0xf0e510.
<unresolved>
[1]    136443 trace trap (core dumped)  node --snapshot-blob snapshot.blob --build-snapshot file.js

Additional information

Found at https://github.com/mehulkar/bench-childprocess

[joyeecheung]

I think at the very least we can register some of the child process methods. Some additional work may be done to ensure that users don't leave any ProcessWrap lingering when the snapshot is taken too.

[H4ad]

@joyeecheung Is supposed for all unsupported modules to crash? I tried just require('cluster'), require('http') and I got some error/strange message.

(node:143236) Warning: built-in module cluster is not yet supported in user snapshots
(Use `node --trace-warnings ...` to show where the warning was created)
Unknown external reference 0xf0e510.
<unresolved>

(node:143589) Warning: built-in module http is not yet supported in user snapshots
(Use `node --trace-warnings ...` to show where the warning was created)
global handle not serialized: 0x43dc40a99f1: [[api object] 0] in OldSpace
 - map: 0x2a08a1ccfce9 <Map[40](HOLEY_ELEMENTS)> [FastProperties]
 - prototype: 0x25c423a8b9c1 <Object map = 0x3aa3b8ae20b9>
 - elements: 0x1c4597f00379 <FixedArray[0]> [HOLEY_ELEMENTS]
 - embedder fields: 2
 - properties: 0x043dc40ae701 <PropertyArray[3]>
 - All own properties (excluding elements): {
    0x25a483a72469: [String] in OldSpace: #methods: 0x25c423ab69e9 <JSArray[34]> (const data field 0), location: properties[0]
    0x2a08a1cc9519: [String] in OldSpace: #HTTPParser: 0x2a08a1cced31 <JSFunction HTTPParser (sfi = 0x2a08a1ccecf1)> (const data field 1), location: properties[1]
    0x2a08a1ccfcc9: [String] in OldSpace: #ConnectionsList: 0x2a08a1ccfc41 <JSFunction ConnectionsList (sfi = 0x2a08a1ccfc01)> (const data field 2), location: properties[2]
 }
 - embedder fields = {
    0, aligned pointer: 0x6c9b322
    0, aligned pointer: 0x6cea4b0
 }



#
# Fatal error in , line 0
# CheckGlobalAndEternalHandles failed
#
#
#
#FailureMessage Object: 0x7ffcb8661ae0
 1: 0xe5a541  [/home/h4ad/.asdf/installs/nodejs/21.1.0/bin/node]
 2: 0x22c7291 V8_Fatal(char const*, ...) [/home/h4ad/.asdf/installs/nodejs/21.1.0/bin/node]
 3: 0x169b14d v8::internal::SnapshotCreatorImpl::CreateBlob(v8::SnapshotCreator::FunctionCodeHandling, v8::base::Flags<v8::internal::Snapshot::SerializerFlag, int, int>) [/home/h4ad/.asdf/installs/nodejs/21.1.0/bin/node]
 4: 0xe99fd5 node::SnapshotBuilder::CreateSnapshot(node::SnapshotData*, node::CommonEnvironmentSetup*, unsigned char) [/home/h4ad/.asdf/installs/nodejs/21.1.0/bin/node]
 5: 0xe9a3c1 node::BuildSnapshotWithoutCodeCache(node::SnapshotData*, std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&, std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&, std::optional<std::basic_string_view<char, std::char_traits<char> > >) [/home/h4ad/.asdf/installs/nodejs/21.1.0/bin/node]
 6: 0xe9bbd5 node::SnapshotBuilder::Generate(node::SnapshotData*, std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&, std::vector<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::allocator<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > > > const&, std::optional<std::basic_string_view<char, std::char_traits<char> > >) [/home/h4ad/.asdf/installs/nodejs/21.1.0/bin/node]
 7: 0xd9a410 node::GenerateAndWriteSnapshotData(node::SnapshotData const**, node::InitializationResultImpl const*) [/home/h4ad/.asdf/installs/nodejs/21.1.0/bin/node]
 8: 0xd9a948 node::Start(int, char**) [/home/h4ad/.asdf/installs/nodejs/21.1.0/bin/node]
 9: 0x7f963d606083 __libc_start_main [/lib/x86_64-linux-gnu/libc.so.6]
10: 0xcf2c6e _start [/home/h4ad/.asdf/installs/nodejs/21.1.0/bin/node]
[1]    143589 trace trap (core dumped)  node --snapshot-blob snapshot.blob --build-snapshot empty-file.js

[joyeecheung]

Is supposed for all unsupported modules to crash?

Not necessarily, they are crashing because they either have leftover wrapper objects that are not properly handled for the snapshot (could be a leak, even), or unregistered external references. Being unsupported means we haven't checked that they work in snapshot or make them to work in the snapshot, so they can fail in different ways, or work in some cases.

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[joyeecheung]

The synchronous methods are supported since #50943 so the snippet in the OP works now and I am closing this. Asynchronous method support would be a different beast (I think as long as the method finishes and resources are cleaned up before snapshot is taken it should work, haven't verified it yet though).