Bug 2024449 - Add importer for streamed profile format (JSON Lines)

[fqueze]

Add support for importing the "streamed profile" format, a JSON Lines format where each line is a JSON object: a meta line, a thread declaration line, then marker lines. This format is produced by tools like resourcemonitor.py for streaming resource-usage profiles (with the patches in bug 2024449).

The importer passes through meta and thread objects as-is and respects the input's preprocessedProfileVersion, so the standard profile upgraders handle all format migrations. stringArray placement is version-aware: per-thread for versions < 56, in profile.shared for versions >= 56.

Also adds early detection in _extractJsonFromArrayBuffer so that JSON Lines content served as application/json reaches the string-based format detection instead of failing JSON.parse.

Example profiles on the timed out xpcshell jobs of this try push.

[mstange]

Also adds early detection in _extractJsonFromArrayBuffer so that JSON Lines content served as application/json reaches the string-based format detection instead of failing JSON.parse.

😧

[mstange]

I insist that if it's not valid JSON, it should not be served as application/json. I don't want to maintain workarounds for this.

[codecov]

Codecov Report

❌ Patch coverage is 92.30769% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 85.46%. Comparing base (8ac20f5) to head (bcf0a07).
⚠️ Report is 26 commits behind head on main.

Files with missing lines	Patch %	Lines
src/profile-logic/import/streamed-profile.ts	93.47%	3 Missing ⚠️
src/actions/receive-profile.ts	75.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5901      +/-   ##
==========================================
+ Coverage   85.44%   85.46%   +0.02%     
==========================================
  Files         321      322       +1     
  Lines       32061    32132      +71     
  Branches     8753     8858     +105     
==========================================
+ Hits        27393    27463      +70     
- Misses       4236     4237       +1     
  Partials      432      432              

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[fqueze]

Also adds early detection in _extractJsonFromArrayBuffer so that JSON Lines content served as application/json reaches the string-based format detection instead of failing JSON.parse.

😧

Sorry, this part of the commit message was misleading, the check is not for application/json but for /\.json/ which matches both json and jsonl.

I don't want to maintain workarounds for this.

To me it's not a workaround, just a simple if check:

if (text.startsWith('{"type":"meta"')) {
  return text;
}

I don't see how that could grow into more workarounds in the future.

The part where I'm the least confident for the future is about how we'll handle profile format changes for the parts that aren't part of what's streamed as lines. The profiler.shared object specifically is difficult to stream.