fix(lib): move DecimalsTooLarge guard into LibFtsoCurrentPriceUsd (#79)

[thedavidmeister]

Summary

• Moves the DecimalsTooLarge guard from LibOpFtsoCurrentPriceUsd.run() into LibFtsoCurrentPriceUsd.ftsoCurrentPriceUsd(), enforcing the bound at the library trust boundary for every caller.
• Narrows the return type from (uint256, uint256) to (uint256, uint8) — Solidity now enforces the decimals invariant at every call site.
• Removes the now-redundant check and DecimalsTooLarge import from the op layer.

Closes #79

Test plan

• All existing tests pass unchanged: the DecimalsTooLarge revert (tested in testRunDecimalOverflow and testRunDecimalsBoundary) is still raised with the same selector and args, just sourced from the library rather than the op.
• No new tests needed — the behavior is identical; the structural change is that the guard fires earlier (in the library) rather than later (in the op).

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes

  • Improved USD price fetching by validating the fetched FTSO decimals before converting prices.
  • Prevents incorrect conversions when the returned decimal precision is outside the supported range.
  • Streamlined current USD price conversion to use a smaller, safer decimal representation.

• Tests

  • Updated stale-price test coverage to keep decimal values within the supported range.

[coderabbitai]

Important

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Walkthrough

LibFtsoCurrentPriceUsd now enforces the DecimalsTooLarge bound and returns uint8 decimals. LibOpFtsoCurrentPriceUsd removes its duplicate check and import, and the stale-price test constrains fuzzed decimals to the uint8 range.

Changes

Decimals guard relocation

Layer / File(s)	Summary
Library: add guard and narrow return type to uint8 src/lib/price/LibFtsoCurrentPriceUsd.sol	Imports DecimalsTooLarge, adds a decimals > type(uint8).max revert, and returns decimals as uint8. The function signature changes to return (uint256, uint8).
Op: remove redundant check and import src/lib/op/LibOpFtsoCurrentPriceUsd.sol, test/src/lib/op/LibOpFtsoCurrentPriceUsd.t.sol	Removes the op-side DecimalsTooLarge import and overflow check, destructures the library return as uint8, and bounds the stale-price fuzz input to the uint8 range.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly describes the main change: moving the DecimalsTooLarge guard into LibFtsoCurrentPriceUsd.
Linked Issues check	✅ Passed	The PR implements the preferred fix by enforcing the decimals bound inside LibFtsoCurrentPriceUsd and removing the caller-side guard.
Out of Scope Changes check	✅ Passed	The visible code changes stay focused on the FTSO decimals guard and related test adjustment, with no unrelated additions.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/issue-79-decimals-guard-in-library

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