diagnostics_channel: add BoundedChannel and scopes

[Qard]

This adds BoundedChannel, adds a using syntax equivalent to runStores, and modifies the internals of TracingChannel to use these to avoid closures in several places.

Why BoundedChannel?

Reviewers asked why a new class is needed instead of just using the existing channel() or tracingChannel() APIs. Here's the motivation:

• vs channel(): A bare Channel has no built-in concept of start/end lifecycle events or store binding for async context propagation. BoundedChannel combines two channels with a withScope() / run() API that ties start → store entry → user code → end → store exit into a single unit, which a raw channel can't express.

• vs tracingChannel(): TracingChannel is designed for tracing operations with async continuations (it has 5 events: start, end, asyncStart, asyncEnd, error). BoundedChannel is a simpler, synchronous-only primitive with just start and end. It is also used internally by TracingChannel to implement its call window and continuation window, making the code cleaner and removing several closure allocations.

The name "Bounded" reflects that the scope is bounded by the using block — start fires when the scope begins, end fires when it is disposed.

Depends on #61674

cc @nodejs/diagnostics

[nodejs-github-bot]

Review requested:

• @nodejs/web-infra

[github-actions]

Failed to start CI

   ⚠  No approving reviews found
   ✘  Refusing to run CI on potentially unsafe PR

https://github.com/nodejs/node/actions/runs/21678728231

[RafaelGSS]

Could you please expand on PR description on why this is needed? We have channels, tracingChannels and now windowChannel, it would be great to know for which kind of situations we need each one of them.

[RafaelGSS]

Suggested change

	> Stability: 1 - Experimental
	> Stability: 1 - Experimental

Perhaps 1.1 Active Development instead?

[Qard]

The particular change is inherited from #61674. Do you want that made there? What about the new APIs in diagnostics_channel? Same status?

[Qard]

WindowChannel is mainly just to enable using syntax scopes, but is exposed as there are possible use cases where one may want to emit events around a block/scope without the greater complexity of TracingChannel.

It's also replacing most of the internals of TracingChannel to make it more coherent since there seemed to be a lot of misunderstanding of how it worked. Now TracingChannel is just a WindowChannel around start + end, a WindowChannel around asyncStart + asyncEnd, and an error channel.

[codecov]

Codecov Report

❌ Patch coverage is 98.42271% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.69%. Comparing base (06a8240) to head (bf59a1f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/diagnostics_channel.js	98.17%	5 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #61680    +/-   ##
========================================
  Coverage   89.68%   89.69%            
========================================
  Files         676      677     +1     
  Lines      206578   206784   +206     
  Branches    39555    39588    +33     
========================================
+ Hits       185267   185470   +203     
- Misses      13446    13448     +2     
- Partials     7865     7866     +1     

Files with missing lines	Coverage Δ
...nternal/async_local_storage/async_context_frame.js	100.00% <100.00%> (ø)
lib/internal/async_local_storage/async_hooks.js	98.03% <100.00%> (+0.08%)	⬆️
lib/internal/async_local_storage/run_scope.js	100.00% <100.00%> (ø)
lib/diagnostics_channel.js	98.88% <98.17%> (+0.38%)	⬆️

... and 29 files with indirect coverage changes

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[legendecas]

This name window channel is confusing. It's not related to the specific window API in JavaScript. It reads to me that it is a version of tracing channel that enables using support. I feel terms like scope could be a more intuitive name.

[Qard]

I feel "scope" is an equally problematic name in that it makes many think of lexical scope which is typically associated with closures, which is exactly what this is explicitly trying to not be. Naming is hard. 🤷🏻

I'm happy to pick a different name if you have a better idea for one, but I'm not convinced "scope" is much better. I had also considered "block" but that also has weird associations with "blocking" which is again misleading. 😐

[Qard]

I went with BoundedChannel. Hopefully that is clear and unique enough naming.