Avoid stack overflow in nested asyncio task chains

[jay3cx]

The waiter-error path re-entered VM::handle_exception recursively, adding one native Rust stack frame for each nested asyncio.gather waiter.

This changes waiter error propagation to an iterative loop while preserving the existing exception handling and ownership behavior.

A regression test covers a 4,000-level nested asyncio.gather chain and verifies that the original ValueError remains catchable.

Tests

• cargo test -p monty --test asyncio
• cargo test -p monty

Fixes #649

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Summary by cubic

Prevents stack overflow when exceptions propagate through deep asyncio.gather waiter chains by switching from recursive to iterative handling. Preserves exception behavior; adds a regression test.

• Bug Fixes
  • Made handle_exception_with_value iterative via a loop over handle_exception_step.
  • Added ExceptionHandlingResult with Caught, Unhandled, and PropagateToWaiter to control propagation and handle waiter handoff without growing the stack.
  • Added a regression test for a 4,000-level nested asyncio.gather; verifies the original ValueError is catchable.

^{Written for commit 4f92024. Summary will update on new commits.}

[codecov]

Codecov Report

❌ Patch coverage is 92.85714% with 1 line in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/monty/src/bytecode/vm/exceptions.rs	92.85%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!

[cubic-dev-ai]

No issues found across 2 files

_{Re-trigger cubic}

[codspeed-hq]

Merging this PR will not alter performance

✅ 34 untouched benchmarks
⏩ 16 skipped benchmarks¹

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_{Comparing jay3cx:codex/fix-async-exception-stack-overflow (4f92024) with main (ca9cca0)}

Footnotes

1. 16 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[rewitt94]

Thanks for contributing! This looks good to me!

[jay3cx]

Done

[jay3cx]

@rewitt94 Review comments are in 4f92024. Mind taking another look when you have a minute?

[rewitt94]

Thanks @jay3cx! I will forward this to @davidhewitt