perf(html-report): stream report JSON through pooled chunks and overlap sidecar serialization

[thomhurst]

Summary

The HTML reporter and JSON sidecar are on by default, so every run pays for them. Profiling a 10,000-test suite (the Bare shape from meziantou's framework benchmark) put the reporter at ~318 MB allocated and ~800 ms of work after the session ends. That work is inside the process wall clock the benchmark measures.

Where the cost went:

• Sidecar serialization: ReportDataJson.SerializeToBytes wrote into an ArrayBufferWriter that grew by doubling and copying (~68 MB of byte[]). It then flushed into a MemoryStream that also grows, and finally called ToArray(), which copied the ~20 MB payload once more.
• HTML payload: HtmlReportGenerator followed the same growth pattern. It then decoded the whole JSON into a ~22 MB UTF-16 string, only to re-encode it for gzip + base64.
• Span collection: span collection used one ConcurrentQueue per test plus LINQ ToArray over tags, events and links.

Changes:

• SegmentedBufferWriter: a new append-only IBufferWriter<byte> backed by pooled 256 KB chunks that are never resized or copied. Both JSON outputs write into it.
  • The sidecar is streamed to disk chunk by chunk.
  • The HTML payload is gzipped straight from the chunks, so the UTF-16 string and the re-encoding are gone.
  • byte[] versions of the write methods remain for existing callers and tests.
• Sidecar in parallel: the sidecar is serialized on another core while the HTML is generated and written. It is reused only when the GitHub integration returned no artifact URL; otherwise it is re-serialized with the URL, as before. If HTML generation fails or produces nothing, the early buffer is released.
• Gzip level: on .NET 9 and later Optimal is about 2× faster than SmallestSize on this payload and about 3% smaller, so it is used there. .NET 8 keeps SmallestSize, where it is 8% smaller. The choice is made at runtime.
• ActivityCollector: each trace's spans go into a small locked list. Tags, events and links are read without LINQ.
• TestExecutor: caches the assembly name used for the test-case span instead of calling Assembly.GetName() per test.

Report content is unchanged:

• Round-tripping a baseline sidecar gives byte-identical JSON.
• The HTML's embedded payload is byte-identical once decoded, and the rest of the page is identical. The raw HTML bytes differ on .NET 9+ only because of the gzip level.

Benchmark

10,000 passing tests, net10.0, default reporters. Medians of 9 interleaved runs, measured with a ProcessExit hook so the post-session reporter work is included:

	main	this PR
Total allocated at exit	461 MB	289 MB
Time spent after session end	813 ms	583 ms
Process uptime at exit	2,036 ms	1,775 ms
Sidecar JSON size	20.7 MB	20.7 MB
HTML size	1.94 MB	1.88 MB

Measured alone:

• HTML generation: 92 MB → 12 MB allocated.
• Sidecar serialization: 105 MB → 19 MB allocated.

Tests

• New SegmentedBufferWriterTests (6 tests).
• TUnit.UnitTests: report aggregation tests and HtmlReporterTruncateOutputTests pass.
• TUnit.Engine.Tests: HtmlReporterTests (43), HtmlReporterConfigurationTests (22), HtmlReportCliTests, DefaultHtmlReportCliTests and ReportingSettingsTests pass.
• TUnit.OpenTelemetry.Tests (42) and TUnit.TestProject.HtmlReportDefaults pass.
• Engine builds on netstandard2.0, net8.0, net9.0 and net10.0; TUnit.Reporting.Tool builds.

Serializing in parallel keeps both buffers alive at the same time, about 11 MB more than serializing one after the other. That is still far below the baseline.

Summary by CodeRabbit

• Performance

  • Improved HTML and JSON report generation for large test runs by reducing memory usage and avoiding unnecessary data copying.
  • Report files and sidecar data are now written more efficiently, including while HTML reports are being generated.

• Bug Fixes

  • Preserved the arrival order of collected tracing spans for more consistent reporting.
  • Improved compression behavior across supported runtime versions.

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Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-22T20:01:23.158984Z	3958793	PR opened

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📝 Walkthrough

Walkthrough

The change adds pooled segmented buffers for report serialization and streaming output. It overlaps sidecar serialization with HTML generation. It also updates trace span storage and caches assembly name lookups.

Changes

Segmented reporting output

Layer / File(s)	Summary
Segmented buffer implementation src/TUnit.Engine/Reporters/Aggregation/SegmentedBufferWriter.cs, src/TUnit.Reporting.Tool/TUnit.Reporting.Tool.csproj, tests/TUnit.UnitTests/SegmentedBufferWriterTests.cs	SegmentedBufferWriter stores data in pooled chunks, streams chunks to a Stream, converts data to an array, and returns chunks on disposal. Tests cover ordering, JSON output, empty output, and invalid Advance values.
Segmented report serialization and writing src/TUnit.Engine/Reporters/Aggregation/ReportDataJson.cs, src/TUnit.Engine/Reporters/Html/HtmlReportGenerator.cs, src/TUnit.Engine/Reporters/Aggregation/AtomicFile.cs, src/TUnit.Engine/Reporters/Aggregation/ReportAggregator.cs	Report JSON and HTML compression use segmented buffers. Atomic file and sidecar writers stream the buffer without creating one contiguous payload.
Concurrent sidecar serialization src/TUnit.Engine/Reporters/Html/HtmlReporter.cs	Sidecar serialization starts while HTML output is generated. The code reuses or disposes buffers based on artifact upload results and cleans up abandoned tasks and buffers.

Trace collection updates

Layer / File(s)	Summary
Ordered span storage and extraction src/TUnit.Engine/Reporters/Html/ActivityCollector.cs	Per-trace spans use lock-protected SpanBucket instances. Tag, event, and link arrays use two-pass enumeration and exact sizing.
Assembly name caching src/TUnit.Engine/TestExecutor.cs	Test tracing obtains assembly names through a cached lookup.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~30 minutes

Change: Refactor

Sequence Diagram(s)

sequenceDiagram
  participant HtmlReporter
  participant HtmlReportGenerator
  participant SegmentedBufferWriter
  participant AtomicFile
  HtmlReporter->>SegmentedBufferWriter: Serialize report data
  HtmlReporter->>HtmlReportGenerator: Generate HTML
  HtmlReportGenerator->>SegmentedBufferWriter: Compress segmented JSON
  HtmlReporter->>AtomicFile: Write sidecar buffer
  AtomicFile->>SegmentedBufferWriter: Stream chunks to file

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Merge Risk: 🟡 Moderate · up to 39587

A failed atomic replacement can cause a completed test suite to disappear from aggregation. Remove the in-place fallback before merging.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 21.95% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 41 functions across 9 files. (1 skipped: …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main changes: pooled chunked report JSON streaming and overlapping sidecar serialization during HTML report generation.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

Full details: Docstring Coverage

Explanation

Docstring coverage is 21.95% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 41 functions across 9 files. (1 skipped: 1 unsupported.)

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[github-actions]

Code review

Reviewed the report-generation allocation/perf work (SegmentedBufferWriter, the sidecar/HTML overlap in HtmlReporter, ActivityCollector's span storage, and the TestExecutor assembly-name cache) against the diff and current pr-head state.

No high-confidence bugs found. The trickiest part of this PR — overlapping sidecar serialization with HTML generation while both read the same mutable ReportData — holds up under scrutiny:

• sidecarTask is kicked off before reportData.ArtifactUrl is ever set, and the code always awaits it (either on the happy path or in the finally) before reportData.ArtifactUrl = artifactUrl runs. That ordering rules out the read/write race the comment claims is avoided.
• Buffer ownership/disposal is consistent across every path I traced: early-return (empty HTML), the exception path, the 'artifact URL known → discard early buffer, re-serialize' path, and the normal path all dispose exactly once, with no double-dispose or leak.
• SegmentedBufferWriter itself (chunk rollover, pooled-array return timing, Advance bounds checks) matches its own tests, including the Utf8JsonWriter round-trip test against a contiguous buffer.
• ActivityCollector's switch from ConcurrentQueue to a locked List<SpanData> per trace preserves correctness (reads take a lock or snapshot); the added contention is the documented, deliberate trade-off for the memory win.
• Verified Activity.EnumerateTagObjects/EnumerateEvents/EnumerateLinks exist in the net8.0 reference assembly, so the net8.0 build under #if NET isn't at risk of a missing-API compile error.

One non-blocking observation, not raised as an issue: SegmentedBufferWriter.Dispose returns chunks to ArrayPool<byte>.Shared without clearing them, so a report's JSON (which can include test output/exception text) can linger in a pooled array until some unrelated Rent overwrites it. This matches typical ArrayPool usage elsewhere in the codebase and isn't a functional bug, so I'm not flagging it as one — just worth knowing if report content is ever considered sensitive.

No compile checks or test runs were performed in this review pass (sandboxed environment); this is a static read of the diff and surrounding code, cross-checked against the reference assemblies where the correctness question depended on it.

[coderabbitai]

Actionable comments posted: 1

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Inline comments:
In `@src/TUnit.Engine/Reporters/Aggregation/AtomicFile.cs`:
- Line 31: Update the fallback after TrySwap in the AtomicFile write flow to
throw an IOException instead of calling WriteBuffer on the destination path.
Preserve the temporary-file write and successful TrySwap behavior, and ensure a
failed atomic replacement does not publish the sidecar in place.

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ℹ️ Review info

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Plan: Advanced

Run ID: 283388d2-df17-482e-9c8c-8fb7aba73fff

📥 Commits

Reviewing files that changed from the base of the PR and between df25c1f and 3958793.

📒 Files selected for processing (10)

• src/TUnit.Engine/Reporters/Aggregation/AtomicFile.cs
• src/TUnit.Engine/Reporters/Aggregation/ReportAggregator.cs
• src/TUnit.Engine/Reporters/Aggregation/ReportDataJson.cs
• src/TUnit.Engine/Reporters/Aggregation/SegmentedBufferWriter.cs
• src/TUnit.Engine/Reporters/Html/ActivityCollector.cs
• src/TUnit.Engine/Reporters/Html/HtmlReportGenerator.cs
• src/TUnit.Engine/Reporters/Html/HtmlReporter.cs
• src/TUnit.Engine/TestExecutor.cs
• src/TUnit.Reporting.Tool/TUnit.Reporting.Tool.csproj
• tests/TUnit.UnitTests/SegmentedBufferWriterTests.cs

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