Fix --fail-under with SystemExit raising from pytest or script

[ownik]

pytest or python script could raise SystemExit and code block with --fail-under never ran before.

[Copilot]

Pull request overview

This PR fixes the --fail-under flag when running pytest as a module with slipcover. Previously, pytest would raise a SystemExit exception through runpy.run_module, preventing the --fail-under code from executing. The fix catches the SystemExit exception and preserves the exit code to return it after the coverage check completes.

Changes:

• Added exception handling to catch SystemExit from runpy.run_module() and preserve the exit code
• Modified return logic to return the captured exit code instead of always returning 0
• Added test cases to verify --fail-under works correctly when running pytest as a module

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/slipcover/main.py	Added try-except block to catch SystemExit from pytest and preserve its exit code for proper propagation
tests/test_coverage.py	Added test cases verifying --fail-under works with pytest module execution

Comments suppressed due to low confidence (1)

src/slipcover/main.py:302

• The script execution path (lines 277-302) doesn't catch SystemExit exceptions like the module execution path does (lines 308-311). This inconsistency means scripts that call sys.exit() would still terminate slipcover prematurely, preventing --fail-under checks. Consider wrapping the exec() call with similar exception handling for consistency.

            exec(code, script_globals)

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