Compression encoder tests fail on mobile platforms with DestinationTooSmall

[github-actions]

Reasoning

Three compression encoder/decoder tests are failing consistently across all mobile platforms (iOS, tvOS, macCatalyst, Android) in the runtime-extra-platforms pipeline:

• DeflateEncoderDecoderTests.RoundTrip_AllWindowLogs
• ZLibEncoderDecoderTests.RoundTrip_AllWindowLogs
• GZipEncoderDecoderTests.RoundTrip_AllWindowLogs

All three fail with the same assertion error: the encoder's Compress operation returns OperationStatus.DestinationTooSmall instead of OperationStatus.Done when testing with various window log sizes. This suggests that GetMaxCompressedLength() is returning an insufficient buffer size for certain window sizes on mobile platforms, or there is a platform-specific issue in the native compression library (zlib/brotli) integration.

The test code allocates a destination buffer using GetMaxCompressedLength(input.Length) and expects all data to compress in a single Compress call. The failure indicates the buffer is too small despite using the API's recommended size calculation.

This appears to be a product bug rather than a test incompatibility — the encoder API contract is not being honored on mobile platforms. The issue affects all compression formats (Deflate, ZLib, GZip) and all mobile architectures (arm64, x64, arm, x86).

Impact on platforms

Failing on all mobile platforms and architectures tested in build 1399212 (2026-04-28):

• maccatalyst-x64 / OSX.15.Amd64.Open / exit code 1
• maccatalyst-arm64 / OSX.15.Arm64.Open / exit code 1
• iossimulator-x64 / OSX.15.Amd64.Open / exit code 1
• iossimulator-arm64 / OSX.15.Arm64.Open / exit code 1
• tvos-arm64 / osx.15.amd64.appletv.open / exit code 1
• android-arm64 / Windows.11.Amd64.Android.Open / exit code 1
• android-arm / Windows.11.Amd64.Android.Open / exit code 1
• android-x64 / Windows.11.Amd64.Android.Open / exit code 1

Also confirmed failing in build 1397424 (2026-04-27).

Errors log

From maccatalyst-x64 Helix work item System.IO.Compression.Tests (console log):

Failed tests:
1) 	[FAIL] System.IO.Compression.DeflateEncoderDecoderTests.RoundTrip_AllWindowLogs
   Test name: System.IO.Compression.DeflateEncoderDecoderTests.RoundTrip_AllWindowLogs
   Assembly:  [System.IO.Compression.Tests, Version=11.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51]
   Exception messages: Assert.Equal() Failure: Values differ
Expected: Done
Actual:   DestinationTooSmall
   Exception stack traces:    at System.IO.Compression.EncoderDecoderTestBase.RoundTrip_AllWindowLogs()
   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
   Execution time: 0.0011576

2) 	[FAIL] System.IO.Compression.ZLibEncoderDecoderTests.RoundTrip_AllWindowLogs
   Test name: System.IO.Compression.ZLibEncoderDecoderTests.RoundTrip_AllWindowLogs
   Assembly:  [System.IO.Compression.Tests, Version=11.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51]
   Exception messages: Assert.Equal() Failure: Values differ
Expected: Done
Actual:   DestinationTooSmall
   Exception stack traces:    at System.IO.Compression.EncoderDecoderTestBase.RoundTrip_AllWindowLogs()
   at System.Reflection.MethodBaseInvoker.InterpretedInvoke_Method(Object obj, IntPtr* args)
   at System.Reflection.MethodBaseInvoker.InvokeWithNoArgs(Object obj, BindingFlags invokeAttr)
   Execution time: 0.0002248

3) 	[FAIL] System.IO.Compression.GZipEncoderDecoderTests.RoundTrip_AllWindowLogs
   Test name: System.IO.Compression.GZipEncoderDecoderTests.RoundTrip_AllWindowLogs
   Assembly:  [System.IO.Compression.Tests, Version=11.0.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51]
   Exception messages: Assert.Equal() Failure: Values differ
Expected: Done
Actual:   DestinationTooSmall

First build it occurred

First observed (within the scanned window of ~20 builds) in build 1399212, which finished on 2026-04-28T07:31:50Z (commit 2ac6372). Also confirmed in build 1397424 (2026-04-27, commit 395fea5).

Note: This window represents the last ~8 days of builds. The true origin may be earlier if the test or underlying code was added/changed before 2026-04-20.

Occurrences in scanned window: Present in all mobile library test jobs across both builds checked (100% failure rate for these tests on mobile platforms).

Recommended action

1. Area owner: @dotnet/area-system-io-compression should investigate why GetMaxCompressedLength returns an insufficient buffer size on mobile platforms.

2. Possible root causes:

   • Platform-specific difference in zlib's compressBound calculation
   • Mobile-specific compiler or packing issue affecting struct sizes or buffer calculations
   • Recent change to the encoder/decoder implementation that broke mobile platform assumptions

3. Investigation steps:

   • Run the test locally on a mobile simulator with verbose logging to see which window log value triggers the failure
   • Compare GetMaxCompressedLength output between desktop and mobile for the same input size
   • Check if this is related to native zlib version differences on mobile vs desktop
   • Review recent commits to src/libraries/System.IO.Compression and related native code

4. Short-term workaround: If the investigation will take time, consider temporarily skipping these three tests on mobile platforms with [ActiveIssue] to unblock the pipeline, but the underlying product bug must be fixed — users relying on these APIs on mobile would hit the same buffer sizing issue.

Note

🔒 Integrity filter blocked 3 items

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

• #125825 search_issues: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
• #56828 search_issues: has lower integrity than agent requires. The agent cannot read data with integrity below "approved".
• #29927 search_issues: 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 · ● 7.6M · ◷

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Tagging subscribers to 'os-maccatalyst': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
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Tagging subscribers to 'os-ios': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
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Tagging subscribers to 'arch-android': @vitek-karas, @simonrozsival, @steveisok, @akoeplinger
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Tagging subscribers to 'os-tvos': @vitek-karas, @kotlarmilos, @steveisok, @akoeplinger
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Tagging subscribers to this area: @karelz, @dotnet/area-system-io-compression
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[github-actions]

Build 1403076 occurrence (2026-04-30)

The compression encoder test failures (DestinationTooSmall) are still present in build 1403076 which completed on 2026-04-30T10:36:57Z.

Confirmed failing on:

• android-arm (Helix job a15256d6) - 3 failures
• android-arm64
• android-x64
• android-x86
• maccatalyst-arm64
• maccatalyst-x64
• iossimulator-x64
• iossimulator-arm64
• tvos-arm64

Same three tests failing: DeflateEncoderDecoderTests.RoundTrip_AllWindowLogs, ZLibEncoderDecoderTests.RoundTrip_AllWindowLogs, GZipEncoderDecoderTests.RoundTrip_AllWindowLogs

Generated by Mobile Platform Failure Scanner · ● 3.1M · ◷