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

[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 90.32%. Comparing base (3553a34) to head (99b20d4).
⚠️ Report is 16 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63491      +/-   ##
==========================================
+ Coverage   90.13%   90.32%   +0.18%     
==========================================
  Files         718      730      +12     
  Lines      227914   234211    +6297     
  Branches    42811    43926    +1115     
==========================================
+ Hits       205435   211543    +6108     
- Misses      14248    14373     +125     
- Partials     8231     8295      +64     

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

... and 91 files with indirect coverage changes

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[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.