Fix REPL crash when V8 emits warnings during preview mode (Issue #63473)

[DivyanshuX9]

Fix: Handle V8 Warnings in DisallowJavascriptExecutionScope

---

The Problem

When the Node.js REPL runs with preview mode enabled, certain code (such as
asm.js modules) causes V8 to emit deprecation warnings inside a
DisallowJavascriptExecutionScope : a V8 context where JavaScript execution
is temporarily forbidden.

Node's warning handler (PerIsolateMessageListener) was unconditionally
calling ProcessEmitWarningGeneric, which invokes process.emit() -> a JS
call which is inside this forbidden scope, causing a fatal crash:

Fatal error in , line 0

Invoke in DisallowJavascriptExecutionScope

---

What Changed

Modified PerIsolateMessageListener in src/node_errors.cc to check
can_call_into_js() before attempting to emit warnings.

When JavaScript cannot be called (i.e. we are inside a forbidden scope),
the warning is deferred via env->SetImmediate() until after the scope
exits at which point it is safe to call JavaScript again.

Files changed:

• src/node_errors.cc -> added can_call_into_js() guard with deferred emission
• test/parallel/test-repl-asm-warning-no-crash.js : new regression test

---

What Is Preserved

The fix does not degrade warning quality in any case:

Property	Status
Full (node:PID) [DEPXXXX] DeprecationWarning: format	Preserved
--redirect-warnings file routing	Preserved
All existing warning behaviour in normal (non-forbidden) scopes	Unchanged
No new allocations or memory leak risk	Lambda captures string by value

---

Approach

Instead of falling back to raw fprintf(stderr, ...) (which loses structured
formatting and --redirect-warnings support), this fix uses env->SetImmediate()
to queue the warning for the next safe event loop iteration.

This is an established Node.js pattern for deferring work that requires
JavaScript execution. The lambda captures the warning string by value,
ensuring it remains valid until the callback fires.

// Before (crashes inside DisallowJavascriptExecutionScope)
ProcessEmitWarningGeneric(env, message, ...);

// After (defers safely when JS is forbidden)
if (!env->can_call_into_js()) {
  std::string deferred_msg = message;
  env->SetImmediate([deferred_msg](Environment* env) {
    ProcessEmitWarningGeneric(env, deferred_msg, ...);
  });
  return;
}
ProcessEmitWarningGeneric(env, message, ...);

---

How to Test

Run the regression test in isolation:

node test/parallel/test-repl-asm-warning-no-crash.js

Or as part of the test suite:

npm test -- test/parallel/test-repl-asm-warning-no-crash.js

---

Regression Test

test/parallel/test-repl-asm-warning-no-crash.js reproduces the issue by:

1. Starting a Node.js REPL with usePreview: true (triggers the inspector path)
2. Sending the asm.js reproduction case from V8 warnings crash process within DisallowJavascriptExecutionScope #63473
3. Asserting the process exits cleanly with exit code 0
4. Asserting no FATAL ERROR appears in stderr

---

Checklist

• Regression test added
• No breaking changes to existing behaviour or public API
• --redirect-warnings routing preserved
• No changes outside the two files listed above
• Commit message follows Node.js format
• No trailing whitespace or formatting changes to unrelated code

Fixes: #63473

[DivyanshuX9]

@Renegade334 could you please review this fix?

[Renegade334]

Both this and #63486 seem to have borrowed from each other, but the core concept seems to have originated here, so I'm going to aim to get this one merged.

You need to fix your commit authorship information, and add a Signed-off-by: trailer as per

node/doc/contributing/pull-requests.md

Lines 202 to 208 in 985b608

	6. Your commit must contain the `Signed-off-by` line with your name and email
	address as an acknowledgement that you agree to the [Developer Certificate of Origin][].
	Bot generated commits are exempt from this requirement. If a commit has
	multiple authors, the `Signed-off-by` line should be added for each author;
	and at least one should match the author information in the commit metadata.
	This rule does not apply to dependency updates (e.g. cherry-picks), release
	commits, or backport commits.

[DivyanshuX9]

Removed the obsolete REPL asm warning regression test and updated node_errors.cc so deferred warnings capture warning directly in SetImmediate instead of copying to warning_str. Also added the Signed-off-by trailer and pushed the rewritten branch history.

@Renegade334 please check again

[Renegade334]

Thanks for the changes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.61%. Comparing base (3553a34) to head (6a9412f).
⚠️ Report is 88 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63491      +/-   ##
==========================================
+ Coverage   90.13%   90.61%   +0.47%     
==========================================
  Files         718      730      +12     
  Lines      227914   236123    +8209     
  Branches    42811    46356    +3545     
==========================================
+ Hits       205435   213957    +8522     
+ Misses      14248    13932     -316     
- Partials     8231     8234       +3     

Files with missing lines	Coverage Δ
src/node_errors.cc	64.00% <100.00%> (-0.18%)	⬇️

... and 129 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[DivyanshuX9]

should i make my PR message from
"Fixes: #62473"

to

"#63473"

to fix the one failing test , do i have to get review again or it will let it slide???

edit: its not letting me give the full link :( what to do

[Renegade334]

Please don't ping collaborators unnecessarily.

The easiest thing to do is to remove the Fixes: trailer from your commit entirely. It will be added by the tooling based on the PR description.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73730/

[Renegade334]

Please don't make unnecessary commits once the PR is in its final stages, it invalidates the existing CI.

In addition, merge commits aren't compatible with the tooling (https://github.com/nodejs/node/blob/main/doc/contributing/pull-requests.md#step-5-rebase). I have reverted to the branch state prior to the merge.

The CI was blocking on nodejs/build#4345 which has only just been resolved.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/73827/

[nodejs-github-bot]

Commit Queue failed

- Loading data for nodejs/node/pull/63491
✔  Done loading data for nodejs/node/pull/63491
----------------------------------- PR info ------------------------------------
Title      Fix REPL crash when V8 emits warnings during preview mode (Issue #63473) (#63491)
   ⚠  Could not retrieve the email or name of the PR author's from user's GitHub profile!
Branch     DivyanshuX9:fix/issue-63473-repl-asm-warning -> nodejs:main
Labels     c++, author ready, needs-ci
Commits    1
 - errors: handle V8 warnings in DisallowJavascriptExecutionScope
Committers 1
 - Divyanshu Sharma <Divyanshu88999@gmail.com>
PR-URL: https://github.com/nodejs/node/pull/63491
Fixes: https://github.com/nodejs/node/issues/63473
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/63491
Fixes: https://github.com/nodejs/node/issues/63473
Reviewed-By: René <contact.9a5d6388@renegade334.me.uk>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
--------------------------------------------------------------------------------
   ⚠  Commits were pushed since the last approving review:
   ⚠  - errors: handle V8 warnings in DisallowJavascriptExecutionScope
   ℹ  This PR was created on Fri, 22 May 2026 17:18:29 GMT
   ✔  Approvals: 3
   ✔  - René (@Renegade334): https://github.com/nodejs/node/pull/63491#pullrequestreview-4354330688
   ✔  - James M Snell (@jasnell) (TSC): https://github.com/nodejs/node/pull/63491#pullrequestreview-4351460626
   ✔  - Anna Henningsen (@addaleax): https://github.com/nodejs/node/pull/63491#pullrequestreview-4362601541
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   ℹ  Last Full PR CI on 2026-05-29T15:07:05Z: https://ci.nodejs.org/job/node-test-pull-request/73827/
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[nodejs-github-bot]

Landed in 31965d6