[release/8.0] Fix OOM in BigInteger OuterLoop tests causing SIGKILL on Linux

[Copilot]

main PR

Description

System.Runtime.Numerics.Tests outerloop tests were crashing with exit code 137 (SIGKILL) on Linux because LargeValueLogTests and DoubleExplicitCastFromLargeBigIntegerTests shifted BigInteger by int.MaxValue / 10 bits (~214M bits) per iteration in nested loops (up to 4×3 = 12 times), creating values of ~107MB each. With 2 parallel test threads in a Docker container, this reliably triggered the OOM killer.

On main, PR #102874 added BigInteger.MaxLength which causes these shifts to throw OverflowException instead of allocating. release/8.0 has no such cap, so allocations succeed and exhaust container memory.

Changes:

• log.cs / LargeValueLogTests: Replace int.MaxValue / 10 with 1 << 24 in both the shift operation and the expected log value calculation. Test correctness is preserved — the values are still well above the BigInteger.Log precision threshold.
• cast_from.cs / DoubleExplicitCastFromLargeBigIntegerTests: Replace int.MaxValue / 10 with 1 << 24. 2^(1<<24) still far exceeds double.MaxValue ≈ 2^1024, so infinity assertions remain valid.

Peak per-iteration allocation drops from ~107MB to ~8MB, eliminating the OOM in constrained container environments.

Customer Impact

Persistent 100% failure rate of System.Runtime.Numerics.Tests outerloop on all four Linux legs (x64, arm64, musl-x64, musl-arm64) in every scheduled release/8.0 build. Known issue hit count: 56 times in one month.

Regression

No — this is a long-standing test design issue exposed by container memory limits, not a regression introduced in a recent release.

Testing

Test-only changes. The fix reduces allocation sizes while keeping the test assertions semantically correct. The library itself is not modified.

Risk

Very low. Changes are confined to test helper functions; no product code is touched. The assertions remain correct with smaller (but still large) values.

Package authoring signed off?

IMPORTANT: If this change touches code that ships in a NuGet package, please make certain that you have added any necessary package authoring and gotten it explicitly reviewed.

This is a test-only change; no NuGet package authoring is required.

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[danmoseley]

@tannergooding I believe if you are OK with us taking this we need you to add the "Servicing approved" label (per the test only change backport flow)

[danmoseley]

This is in 8.0 but it continues to cause noise as we continue to run tests there.

[Copilot]

Pull request overview

This PR adjusts System.Runtime.Numerics.Tests BigInteger OuterLoop tests on release/8.0 to avoid extremely large BigInteger left-shifts that were triggering Linux OOM/SIGKILL in constrained/parallel test environments.

Changes:

• Reduce the “very large” shift size in LargeValueLogTests from int.MaxValue / 10 to 1 << 24 and update the expected-log calculation accordingly.
• Reduce the “very large” shift size in DoubleExplicitCastFromLargeBigIntegerTests from int.MaxValue / 10 to 1 << 24, preserving the infinity assertions while avoiding huge allocations.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
src/libraries/System.Runtime.Numerics/tests/BigInteger/log.cs	Lowers per-iteration BigInteger growth in large Log tests to prevent OOM while keeping assertions meaningful.
src/libraries/System.Runtime.Numerics/tests/BigInteger/cast_from.cs	Lowers per-iteration BigInteger growth in large double cast tests to prevent OOM while still producing ±Infinity.

[danmoseley]

@copilot address feedback

[Copilot]

@copilot address feedback

Done in cabb434. I've addressed all four review items:

• Clarified the XML doc comments in both log.cs and cast_from.cs to use 2^(1 << 24) notation with an explicit mention of "16,777,216 bits"
• Introduced a named const int LargeBitShift = 1 << 24 in both methods with a comment explaining the rationale (keeping allocations ~2 MB vs ~107 MB to avoid OOM in CI containers)

[danmoseley]

@copilot revert back to the first commit. I commented on the wrong one -- for backports we don't want changes to the code.

[Copilot]

@copilot revert back to the first commit. I commented on the wrong one -- for backports we don't want changes to the code.

Reverted in 1b5e9db — the branch now matches the first commit (058392a) with only the core fix (replacing int.MaxValue / 10 with 1 << 24) and no additional code changes.

[danmoseley]

Minimizing the change further.

[tannergooding]

No issues with a test fix. Don't think I'm the right person to approve its merge into net8 though.

[danmoseley]

According to https://github.com/dotnet/runtime/blob/main/docs/project/library-servicing.md#approval-process as area owner you need to apply the servicing approved label

[tannergooding]

Looks like it also needs sign-off from @jeffhandley (engineering lead) first.

[danmoseley]

/ba-g ComInterfaceGenerator timed out.