Fix stack overflow segfault on debug builds for Python threads

[JamesClarke7283]

Summary

• Spawned Python threads were getting Rust's std::thread::Builder default stack of 2 MB, which is too small for the call chains the Python stdlib runs on helper threads in debug builds (where Rust stack frames are substantially larger). CPython on glibc Linux relies on pthread's ~8 MB default instead — a 4× difference that explains why this hits us but not CPython.
• Apply 8 MB in apply_thread_stack_size whenever the user hasn't explicitly set a value via threading.stack_size(N). This matches CPython's effective default while keeping the Python API contract exact: threading.stack_size() still returns 0 ("platform default"), and explicit overrides still take precedence.
• Single-file change in crates/vm/src/stdlib/_thread.rs.

Closes #7941

Test plan

• cargo build succeeds.
• Original reproducer passes on a debug build: cargo run -- -m test.test_ssl ThreadedTests.test_socketserver → Ran 1 test in 21.59s, OK (was segfaulting on main).
• Python API contract preserved: threading.stack_size() returns 0 by default; setting to 1 MiB then reading back returns 1048576; previous value reported correctly.
• Basic threaded workload (5 threads doing work + join) still completes normally.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Made thread stack sizing more predictable in debug builds by using a consistent fallback when no size is configured.
  • Applied configured stack sizes to newly spawned threads.
  • Release builds continue using the platform default when no stack size is set.

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• crates/vm/src/stdlib/_thread.rs

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📝 Walkthrough

Walkthrough

When vm.state.stacksize is non-zero, apply_thread_stack_size applies it. When it is zero, debug builds use an 8 MiB stack and release builds use Rust’s default. A platform-specific regression test verifies the debug fallback.

Changes

Thread Stack Sizing

Layer / File(s)	Summary
Stack-size selection logic crates/vm/src/stdlib/_thread.rs	Adds the debug-only DEFAULT_THREAD_STACK_SIZE constant. apply_thread_stack_size handles configured, debug, and release stack-size cases.
Debug stack-size regression test crates/vm/src/stdlib/_thread.rs	Adds Linux and macOS helpers that measure the spawned thread stack size. The test checks that debug builds provide at least 8 MiB.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: youknowone

Merge Risk: ⚪ Minimal · up to a5e5b

The debug default stack fallback is implemented while explicit Python stack-size settings retain precedence. No actionable merge risk remains.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 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 change: preventing stack-overflow segmentation faults for Python threads in debug builds.
Linked Issues check	✅ Passed	Issue #7941 requires Python threaded tests to avoid debug-build stack overflows and segmentation faults. The PR applies an 8 MiB stack when no explicit stack size is configured, while preserving expli…
Out of Scope Changes check	✅ Passed	The changes are limited to crates/vm/src/stdlib/_thread.rs. They implement the debug-build thread-stack fix for #7941 and add its regression test. The changes do not add unrelated behavior.
Docstring Coverage	✅ Passed	Docstring coverage is 83.33% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 6 functions across 1 files.

✨ Finishing Touches 💡 1

⚔️ Resolve merge conflicts 💡

• Resolve merge conflict in branch fix-7941-thread-stack-overflow

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[youknowone]

please also add failing test before patch

[JamesClarke7283]

please also add failing test before patch

Which test? the original one from the original issue that now passes or something different.
Please clarify.

[youknowone]

please add a test, which failed before the patch but success after the patch. If adding patch is not possible, please share discussion why it is impossible

[JamesClarke7283]

please add a test, which failed before the patch but success after the patch. If adding patch is not possible, please share discussion why it is impossible

Done