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JamesClarke7283 · 2026-05-27T06:41:10Z

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.

Rust's std::thread::Builder defaults to a 2 MB stack when no size is set, which is too small for the call chains the Python stdlib runs on helper threads in debug builds (e.g. test.test_ssl's threaded server). CPython on glibc Linux relies on pthread's ~8 MB default instead. Apply 8 MB as the default in apply_thread_stack_size when the user has not explicitly called threading.stack_size(N). This matches CPython's effective default across builds while preserving the existing API: threading.stack_size() still returns 0 by default ("platform default"), and explicit user values still take precedence. Fixes RustPython#7941 Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

coderabbitai · 2026-05-27T06:41:23Z

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

📝 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

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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Satisfy docstring coverage check on the PR. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

The previous commit applied an 8 MB default unconditionally, which slowed down multiprocessing tests on release: many forked children each spawning many threads with oversized virtual stack mappings caused the flaky MP CI step to time out (60 min, vs ~9 min on main). Issue RustPython#7941 only manifested in debug builds — release builds were already fine on Rust's 2 MB std default. Restrict the 8 MB override to `#[cfg(debug_assertions)]` so release behavior is unchanged from before the fix, while the original SSL segfault on debug stays fixed. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

youknowone · 2026-05-28T03:52:09Z

please also add failing test before patch

JamesClarke7283 · 2026-06-03T17:18:04Z

Brings the branch up to date with main so CI job names match the required status checks (the windows snippets/cpython job was renamed by RustPython#8004 when its matrix skips were cleared). Also picks up RustPython#8018's _thread.rs stack-margin updates, which are orthogonal to the apply_thread_stack_size fix in this PR. Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

JamesClarke7283 and others added 3 commits May 27, 2026 09:47

Add doc comment to apply_thread_stack_size

e4011c1

Satisfy docstring coverage check on the PR. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Retrigger CI for flaky multiprocessing tests

2a5e734

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

JamesClarke7283 marked this pull request as draft June 3, 2026 13:59

JamesClarke7283 marked this pull request as ready for review June 3, 2026 17:02

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Fix stack overflow segfault on debug builds for Python threads#7986

Fix stack overflow segfault on debug builds for Python threads#7986
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JamesClarke7283 commented May 27, 2026 •

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JamesClarke7283 commented May 27, 2026 •

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