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: when no stack size is configured, debug builds now use a consistent explicit fallback to avoid platform-dependent variance. Configured stack sizes are applied to newly spawned threads. Release builds continue to use the platform default when no stack size is set.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

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📥 Commits

Reviewing files that changed from the base of the PR and between c8b134e and 41b4f73.

📒 Files selected for processing (1)

• crates/vm/src/stdlib/_thread.rs

💤 Files with no reviewable changes (1)

• crates/vm/src/stdlib/_thread.rs

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📝 Walkthrough

Walkthrough

When starting helper threads, apply_thread_stack_size uses vm.state.stacksize if non-zero; if zero, debug builds set a debug-only DEFAULT_THREAD_STACK_SIZE (8 MiB), while non-debug builds do not modify the thread::Builder (Rust std default remains).

Changes

Thread Stack Sizing

Layer / File(s)	Summary
Stack-size constant and selection logic crates/vm/src/stdlib/_thread.rs	Adds DEFAULT_THREAD_STACK_SIZE (8 MiB, debug-only). apply_thread_stack_size applies vm.state.stacksize when non-zero; when zero it sets the builder to the default in debug builds and leaves the builder unchanged in non-debug builds.

Sequence Diagram(s)

sequenceDiagram
  participant Caller
  participant apply_thread_stack_size
  participant thread_Builder
  Caller->>apply_thread_stack_size: pass vm.state.stacksize
  alt vm.state.stacksize != 0
    apply_thread_stack_size->>thread_Builder: stack_size(configured)
  else vm.state.stacksize == 0 (debug)
    apply_thread_stack_size->>thread_Builder: stack_size(DEFAULT_THREAD_STACK_SIZE)
  else vm.state.stacksize == 0 (release)
    apply_thread_stack_size-->>thread_Builder: no stack_size call (std default)
  end

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Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

Possibly related PRs

• RustPython/RustPython#8018: Modifies _thread.stack_size validation/setting; closely related to how vm.state.stacksize is handled and used.

Suggested reviewers

• ShaharNaveh
• youknowone

Poem

🐰 I nudged the stack to eight MiB bright,
In debug I set its cozy height,
When users say zero, I choose with care,
Release leaves defaults, debug makes it fair,
Threads hop steady through the night.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 inconclusive)

Check name	Status	Explanation	Resolution
Out of Scope Changes check	❓ Inconclusive	The PR objectives indicate a test should be added per reviewer request, but the raw_summary shows only stdlib/_thread.rs modifications with no test additions visible.	Clarify whether the test mentioned in comments (to reproduce the segfault before the fix) has been added, as the summary does not show test file changes despite the comment thread suggesting one was requested.

✅ 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 directly and accurately describes the main change: fixing stack overflow segfaults on debug builds for Python threads by introducing a default 8 MiB stack size.
Linked Issues check	✅ Passed	The code changes fully address issue #7941 by implementing an 8 MiB default thread stack size in debug builds when no explicit value is set, preventing stack overflow segfaults while preserving API semantics.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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✨ Finishing Touches

🧪 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.