debugger: wait for target startup

[panva]

Important

This ought to deflake parallel/test-debugger-profile and parallel/test-debugger-exceptions.

The inspector can accept a connection before an --inspect-brk target enters its frontend wait. Runtime.runIfWaitingForDebugger can then be handled too early, allowing the target to subsequently block forever.

Wait for NodeRuntime.waitingForDebugger before initializing and releasing launched targets. Race the handshake against disconnects and apply it to both interactive and probe startup.

Refs: #64116
Assisted-by: codex:gpt-5.6-sol

[panva]

The startup-readiness handshake and its initial integration were adapted from @Archkon's work in #64304. This version narrows and hardens that approach by covering disconnects during the full handshake, preserving probe-mode structured errors, and adding focused failure and cleanup tests.

[codecov]

Codecov Report

❌ Patch coverage is 97.01493% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.33%. Comparing base (404b0cf) to head (18ce20f).
⚠️ Report is 85 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/internal/debugger/inspect_probe.js	83.33%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #65194      +/-   ##
==========================================
+ Coverage   90.32%   90.33%   +0.01%     
==========================================
  Files         760      751       -9     
  Lines      248525   250395    +1870     
  Branches    46894    47339     +445     
==========================================
+ Hits       224488   226206    +1718     
- Misses      15469    15553      +84     
- Partials     8568     8636      +68     

Files with missing lines	Coverage Δ
lib/internal/debugger/inspect_helpers.js	97.92% <100.00%> (+0.51%)	⬆️
lib/internal/debugger/inspect_repl.js	91.34% <100.00%> (+0.03%)	⬆️
lib/internal/debugger/inspect_probe.js	83.15% <83.33%> (+1.10%)	⬆️

... and 122 files with indirect coverage changes

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[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75746/

[inoway46]

I ran parallel/test-debugger-exceptions 1,000 times with -j16 on macOS 15:

• this PR (821e267): 1,000/1,000 passed
• parent commit (404b0cf): 25/1,000 failed, all timing out while waiting for the initial break

The A/B result suggests this likely fixes the flake, and the code looks good to me. My only non-blocking nit is the lack of a probe-mode disconnect test while waitForDebugger() is pending. LGTM.

[nodejs-github-bot]

CI: https://ci.nodejs.org/job/node-test-pull-request/75860/

[inoway46]

Thanks for adding the probe disconnect test!
I also ran a broader A/B stress test across the debugger tests on macOS 15, repeating each test 100 times.

• this PR (821e267): 7,900/7,900 passed
• parent commit (404b0cf): 34/7,800 failed

This suggests that the startup handshake may deflake debugger tests beyond parallel/test-debugger-profile and parallel/test-debugger-exceptions called out in the description.

Additional validation:

stress test for parallel/test-debugger-profile 1,000 times with -j16 on macOS 15:

• this PR (821e267): 1,000/1,000 passed
• parent commit (404b0cf): 2/1,000 failed, both timing out while waiting for the initial break