Cap CollectString/CollectStreams so print loops can't OOM the host

[Shaurya-Sethi]

Summary

• Cap host-side CollectString / CollectStreams buffers at 10 MiB by default (max_bytes=None to opt out), since ResourceLimits.max_memory only tracks the Monty heap and does not protect parent-process collectors (PrintWriter::CollectString and PrintWriter::CollectStreams probably need to have a size check #464).
• Enforce the same check in Rust PrintWriter and Python PrintTarget::write_event (pool host path); exceeding raises MemoryError with the familiar memory limit exceeded: … message.
• Add regression tests and document the divergence in limitations/print.md.

Test plan

• cargo test -p monty --test print_collect_limits --test print_writer
• cargo test -p monty --test repl call_function_captures_print -- --exact
• make format-rs / make lint-rs
• uv run pytest crates/monty-python/tests/test_print.py::test_collect_string_max_bytes_raises (and existing collector tests)
• make lint-py (with CONDA_PREFIX unset if needed)

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Summary by cubic

Add a 10 MiB default cap to host-side print collectors (CollectString, CollectStreams) so print loops can’t OOM the host. Python CollectStreams charges a fixed per-entry overhead and tracks bytes in O(1); use max_bytes=None to opt out (issue #464).

• New Features

  • Default cap: DEFAULT_MAX_PRINT_COLLECT_BYTES (10 MiB). Exceeding raises MemoryError with “memory limit exceeded: {used} bytes > {limit} bytes”. Not covered by ResourceLimits.max_memory.
  • Rust: added check_print_collect_limit; integrated caps into PrintWriter::{CollectString, CollectStreams} (now carry Option<usize>); helpers PrintWriter::collect_string and PrintWriter::collect_streams; exported DEFAULT_MAX_PRINT_COLLECT_BYTES. Tests now cover helper constructors and newline-push limit paths.
  • Python: CollectString(max_bytes=...) and CollectStreams(max_bytes=...) (default 10 MiB), enforced in PrintTarget::write_event. CollectStreams charges a 64-byte per-entry overhead with a running collected_bytes counter. pyi/docs updated; regression tests added.

• Migration

  • If you constructed enum variants directly, update to PrintWriter::collect_string(&mut buf) or PrintWriter::collect_streams(&mut vec) (or pass an explicit Option<usize> to the variants).
  • To disable the cap: Python CollectString(max_bytes=None) / CollectStreams(max_bytes=None); Rust pass None to the corresponding variant.

^{Written for commit 9b649cb. Summary will update on new commits.}

[cubic-dev-ai]

All reported issues were addressed across 9 files

<sub>Reply with feedback, questions, or to request a fix.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[codspeed-hq]

Merging this PR will not alter performance

✅ 26 untouched benchmarks
⏩ 16 skipped benchmarks¹

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_{Comparing Shaurya-Sethi:fix/print-collect-cap-464 (9b649cb) with main (c455a30)}

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. ↩