Compression RoundTrip_AllWindowLogs tests failing on Android after PR #127394

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Description

Three compression encoder/decoder tests started failing on Android (and likely all mobile platforms) after PR #127394 was merged:

1. System.IO.Compression.DeflateEncoderDecoderTests.RoundTrip_AllWindowLogs
2. System.IO.Compression.ZLibEncoderDecoderTests.RoundTrip_AllWindowLogs
3. System.IO.Compression.GZipEncoderDecoderTests.RoundTrip_AllWindowLogs

All three fail with the same assertion:

Assert.Equal() Failure: Values differ
Expected: Done
Actual:   DestinationTooSmall
   at System.IO.Compression.EncoderDecoderTestBase.RoundTrip_AllWindowLogs()
````

## Suspected Root Cause

PR #127394 replaced `MemoryMarshal.Read<T>` / `MemoryMarshal.Write<T>` calls with `BitConverter.ToInt32` / `BitConverter.GetBytes` equivalents throughout the codebase. While the PR didn't directly modify compression code, it did modify `BitConverterPolyfills.cs`, which may be used by the compression encoders/decoders on platforms where the native BitConverter methods aren't available.

The failure suggests that buffer size calculations or endianness handling may have changed subtly, causing the encoder to report `DestinationTooSmall` when it should complete successfully.

## Failure Details

**Build:** https://dev.azure.com/dnceng-public/public/_build/results?buildId=1396313

**Affected Jobs:** All Android Mono jobs (arm, arm64, x64, x86), likely iOS/tvOS/MacCatalyst as well

**Sample Helix Work Item:**
- Job ID: `93091357-214b-436d-a5c4-a31ee456a876`
- Work Item: `System.IO.Compression.Tests`
- Console log: https://helix.dot.net/api/2019-06-17/jobs/93091357-214b-436d-a5c4-a31ee456a876/workitems/System.IO.Compression.Tests/console

**Log excerpt:**
````
04-24 21:04:03.088  DOTNET  : [FAIL] System.IO.Compression.DeflateEncoderDecoderTests.RoundTrip_AllWindowLogs
04-24 21:04:03.088  DOTNET  : Assert.Equal() Failure: Values differ
04-24 21:04:03.088  DOTNET  : Expected: Done
04-24 21:04:03.088  DOTNET  : Actual:   DestinationTooSmall

Investigation Needed

1. Compare behavior of MemoryMarshal.Read<int> vs BitConverter.ToInt32 on Android/mobile platforms
2. Check if BitConverterPolyfills.cs is being used on these platforms
3. Verify endianness handling in the polyfill vs the original MemoryMarshal calls
4. Review all compression-related code paths that might indirectly use BitConverter

Related

• Regression PR: Replace MemoryMarshal generic Read/Write with BitConverter equivalents #127394
• Affects: Android, likely iOS/tvOS/MacCatalyst/WASI

Note

🔒 Integrity filter blocked 1 item

The following item were blocked because they don't meet the GitHub integrity level.

• #126805 search_pull_requests: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".

To allow these resources, lower min-integrity in your GitHub frontmatter:

tools:
  github:
    min-integrity: approved  # merged | approved | unapproved | none

Generated by Mobile Platform Failure Scanner · ● 4.1M · ◷

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Tagging subscribers to 'arch-android': @vitek-karas, @simonrozsival, @steveisok, @akoeplinger
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Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
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Tagging subscribers to 'os-tvos': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
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Tagging subscribers to 'os-maccatalyst': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
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Latest Occurrence

The compression failures continue to reproduce in build #1396313 (2026-04-25).

Platform	Runtime	Job ID	Status
android-arm64	Mono	93091357-214b-436d-a5c4-a31ee456a876	❌ 3 tests failed
android-arm	Mono	(job ID not extracted)	❌ Same failures
android-x64	Mono	(job ID not extracted)	❌ Same failures
android-x86	Mono	(job ID not extracted)	❌ Same failures
iossimulator-arm64	Mono	(job ID not extracted)	❌ Likely same
iossimulator-x64	Mono	(job ID not extracted)	❌ Likely same
tvos-arm64	Mono	(job ID not extracted)	❌ Likely same
maccatalyst-arm64	Mono	(job ID not extracted)	❌ Likely same
maccatalyst-x64	Mono	(job ID not extracted)	❌ Likely same

Helix console log: https://helix.dot.net/api/2019-06-17/jobs/93091357-214b-436d-a5c4-a31ee456a876/workitems/System.IO.Compression.Tests/console

The failure is consistent and reproducible across all mobile platforms and Mono configurations.

---

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Update from build 1397424 (2026-04-27)

The compression test failures reported in this issue are still present in the latest runtime-extra-platforms build:

Build: https://dev.azure.com/dnceng-public/public/_build/results?buildId=1397424

Affected platforms (confirmed):

• iossimulator-arm64 CoreCLR
• All Android variants (arm, arm64, x64, x86) Mono/CoreCLR
• Likely tvos, maccatalyst (not individually verified but pattern consistent)

Helix job example:
Job ID: 7da59944-242f-4296-8a21-1a36fe1d0562
Work item: System.IO.Compression.Tests

Failing tests:

• System.IO.Compression.DeflateEncoderDecoderTests.RoundTrip_AllWindowLogs
• System.IO.Compression.ZLibEncoderDecoderTests.RoundTrip_AllWindowLogs
• System.IO.Compression.GZipEncoderDecoderTests.RoundTrip_AllWindowLogs

All three still report:

Assert.Equal() Failure: Values differ
Expected: Done
Actual:   DestinationTooSmall

The issue persists 2 days after initial report, confirming this is a product regression requiring a code fix (not a transient infrastructure issue).

Note

🔒 Integrity filter blocked 3 items

The following items were blocked because they don't meet the GitHub integrity level.

• #126023 search_pull_requests: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
• #124461 search_pull_requests: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
• #108513 search_pull_requests: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".

To allow these resources, lower min-integrity in your GitHub frontmatter:

tools:
  github:
    min-integrity: approved  # merged | approved | unapproved | none

Generated by Mobile Platform Failure Scanner · ● 4.2M · ◷