Pre-check json.dumps indent string size against resource limits

[dsp-ant]

The integer indent= argument was converted directly into a repeated space string on the native heap with no resource pre-check, so a large value would allocate well past the configured memory budget. Validate the requested count against the resource tracker before materializing the indent string, raising MemoryError instead. Other .repeat() call sites already carry the same guard.

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Summary by cubic

Pre-checks json.dumps(indent=...) against resource limits to prevent unbounded indent string allocation. Large values now raise MemoryError before allocating, matching guards used at other .repeat() sites.

• Bug Fixes
  • Validate indent size via check_repeat_size before building the space string.
  • Added tests for rejecting huge indents and allowing small indents within limits.

^{Written for commit 582ea1b. Summary will update on new commits.}

[codspeed-hq]

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 15 skipped benchmarks¹

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_{Comparing dsp-ant:fix/json-dumps-indent-resource-check (582ea1b) with main (ac5b383)}

Footnotes

1. 15 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. ↩

[cubic-dev-ai]

No issues found across 2 files

[codecov]

Codecov Report

❌ Patch coverage is 62.50000% with 3 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
crates/monty/src/modules/json/dump.rs	62.50%	3 Missing ⚠️

📢 Thoughts on this report? Let us know!

[davidhewitt]

This doesn't seem like the right defence to me; the indent can be used multiple times over when dumping so a single check doesn't appear to make sense.

I have been playing around with a StringBuilder type which checks against memory limits as the string is formed, I think the correct defence for the JSON module in general is probably to switch to that construction rather than apply a defence at this point?

[davidhewitt]

I pushed the StringBuilder experiment to #448, I will see how it looks here.

[samuelcolvin]

@davidhewitt is this now fixed?

[davidhewitt]

I have a branch (not pushed yet) which uses StringBuilder in json.dumps. The challenge is that the ResourceTracker has to be slightly decoupled from the Heap so that the builder can have a reference to the tracker while the VM is busy traversing heap values. Most likely will finish this week after next.