[PTruscott]

Description

Fixes #6203.

DIALECT_PATTERN matches comments between tokens with /\*.*?\*/. Under re.DOTALL that lazy match can stretch past its own */ to a later one, so a run of n block comments has about 2ⁿ⁻¹ ways to match. When a MODEL or AUDIT block names no dialect, which is the documented default when model_defaults.dialect is set, the search fails. It then tries every one of those ways at every ( or , it reaches, so loading a file with a few dozen block comments hangs with no error.

Block comments are now matched balanced: each */ closes exactly one /*, so a run of comments has exactly one match, and a failed search stays linear in the comments it passes. SQLGlot reads block comments as nested, so the pattern matches nesting up to three levels deep, built from the flat pattern in a short loop. The current pattern handles nested comments only because it can stretch to a later */, which is the behaviour that causes the hang. Nesting deeper than three levels before the dialect key now falls back to the default dialect.

Test Plan

• Added three cases to test_dialect_pattern:

  • a dialect key after a run of 40 block comments still matches;
  • the same run with no dialect key returns no match;
  • a dialect key after a three-level nested comment matches.

  With the current pattern, the second case was still running after 60 seconds. With this change the test takes under a second.

• Compared the old and new patterns over 363 SQL files: every .sql file under examples/ and tests/, plus a production project's models and audits. They return the same dialect for every file the old pattern finishes on. The one file it doesn't finish on, a real model with no dialect key, matches nothing in under a millisecond with the new pattern.

• Edge cases agree with the old pattern: nested comments up to three levels, a dialect key between two comments, a comment that only mentions dialect, and an unterminated comment. Inputs that fill the 10,000-character window with commas and comments finish in about a millisecond.

• Every stage of make fast-test passes locally, apart from six MSSQL/Fabric tests in tests/core/test_connection_config.py. Those can't load pyodbc here because this machine has no system unixodbc library.

Checklist

• I have run make style and fixed any issues
• I have added tests for my changes (if applicable)
• All existing tests pass (make fast-test), apart from the six pyodbc tests noted above
• My commits are signed off (git commit -s) per the DCO

Summary by CodeRabbit

• Bug Fixes
  • Dialect detection now handles nested block comments up to three levels and long sequences of consecutive comments before a dialect setting. When comments are present without a dialect setting, detection correctly returns no match. These improvements make dialect recognition more reliable in SQL files containing extensive or nested comments.

[coderabbitai]

Walkthrough

The dialect search pattern now matches nested block comments up to three levels deep. Tests cover dialect detection after long runs of comments and nested comments, and no dialect match when the comments are followed by cron.

Changes: Dialect comment matching

Layer / File(s)	Summary
Block-comment matching and tests sqlmesh/core/dialect.py, tests/core/test_model.py	WS_OR_COMMENT now uses a block-comment pattern that supports nesting up to three levels. Tests cover dialect detection after 40 consecutive comments and nested comments, and no match when the comments are followed by cron.

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Bug fix · Severity of issue fixed: Medium

Suggested reviewers: treysp

Merge Risk: Minimal · up to 34a0b

The change appears ready to merge after normal checks; no actionable dialect-detection risk remains identified.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentat…	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title names the main change: it fixes DIALECT_PATTERN so it no longer backtracks over runs of block comments. The description and objectives describe the same change, which replaces the lazy block…
Description check		The description covers the required template sections. It has a Description section that explains the cause (lazy /\.?\*/ matching under re.DOTALL) and the fix (balanced matching with nesting up to …
Linked Issues check		Issue #6203 requires bounded dialect detection for block-comment runs, support for nested comments up to three levels, and tests for the affected cases. The PR replaces the stretching lazy comment pat…
Out of Scope Changes check		The changes are limited to sqlmesh/core/dialect.py and tests/core/test_model.py. The production change fixes DIALECT_PATTERN comment matching. The tests verify the linked issue behavior. No unre…

Full details: Title check

Explanation

The title names the main change: it fixes DIALECT_PATTERN so it no longer backtracks over runs of block comments. The description and objectives describe the same change, which replaces the lazy block-comment match with balanced matching.

Full details: Description check

Explanation

The description covers the required template sections. It has a Description section that explains the cause (lazy /*.*?*/ matching under re.DOTALL) and the fix (balanced matching with nesting up to three levels). It has a Test Plan that lists the added test cases, the old-versus-new comparison over 363 SQL files, and the make fast-test results, including six pyodbc tests that could not run locally. All four checklist items are marked. The description does not include a link to CONTRIBUTING.md or DCO, but those are template comments and not required content.

Full details: Linked Issues check

Explanation

Issue #6203 requires bounded dialect detection for block-comment runs, support for nested comments up to three levels, and tests for the affected cases. The PR replaces the stretching lazy comment pattern with balanced BLOCK_COMMENT matching and builds two nested levels from the base pattern, for three supported levels. The added test_dialect_pattern cases cover 40 comments with and without dialect, plus three-level nesting. The PR evidence reports preserved dialect results for completed old-pattern cases and prompt completion for the failing case.

Full details: Out of Scope Changes check

Explanation

The changes are limited to sqlmesh/core/dialect.py and tests/core/test_model.py. The production change fixes DIALECT_PATTERN comment matching. The tests verify the linked issue behavior. No unrelated change is demonstrated.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff with cyclomatic complexity of at least 2. Documentation-only edits and deletions within surviving functions are included; unchanged diff context and fully deleted functions are excluded. Analyzed 1 functions across 2 files.

Resolution

Write docstrings for the functions missing them to satisfy the coverage threshold.

Finishing Touches

Generate unit tests (beta)

• Create a new PR

• Autofix · Keep fixing CodeRabbit findings and required CI, and resolving merge conflicts

[coderabbitai]

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Failed pre-merge checks:
- Docstring Coverage:
Write docstrings for the functions missing them to satisfy the coverage
threshold.

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Commits

Reviewing files that changed from the base of the PR and between 7975a8d and 34a0b96.

Files selected for processing: 2

• sqlmesh/core/dialect.py
• tests/core/test_model.py

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