Mutation testing: diff against a configured branch, cover only the changed files

[ondrejmirtes]

Mutation Testing has been dying with "The runner has received a shutdown signal" on most pull requests that change a source file. The cause was two problems layered on top of each other.

1. Wrong diff base since 2026-09-02

The "Determine default branch" step took the base from git remote show origin. When 2.3.x became the default branch (Open 2.3.x, 2026-09-02 ~12:30 UTC), every PR against 2.2.x was diffed against 2.3.x. The last passing 2.2.x run was at 13:06 UTC. From 15:01 UTC they die, and the #5982 run at 15:24 generated 309 mutations.

The base is now configured in the workflow itself (MUTATION_TESTING_BASE_BRANCH), per branch, like the push branch. The "Open X.Y.x" commit flips it.

2. Infection reads all of the coverage, whatever the diff

For every file in the coverage index.xml, Infection loads that file's coverage XML just to learn its source path (SourceFileInfoProvider::provideFileInfo()), even with --git-diff-lines. The XML-coverage report of src/ is 3.6 GB (MutatingScope.php.xml alone is 321 MB, about 1.7 GB as a DOM). It grows with tests × covered lines: the 2.3.x coverage artifact grew 27% in September alone. Infection's memory sits in libxml, outside PHP's accounting: it reports Memory: 0.31GB while the process RSS reaches 12–14 GB. On the 16 GB runner that ends with the VM being shut down. Small 2.3.x PRs still fit until about 2026-09-23. Since then it's a coin toss: the const branch passed on 8.3 and died on 8.4 at the same commit.

Coverage is now collected only for the files Infection will mutate. A new small job lists them, using the same --git-diff-base + AM filter Infection applies. A generated phpunit-mutation-testing.xml restricts <source> to those files. When nothing in src/ or build/PHPStan/Build changed, the mutation jobs are skipped instead of spending ~10 minutes discovering that.

Measured locally (PHP 8.4, pcov) on a change to two ->yes() lines in IntersectionType.php:

	full src/ coverage	changed files only
coverage XML	3.6 GB	30 MB
coverage run	164 s	82 s
Infection peak RSS	12.1 GB	0.43 GB
Infection result	4 generated, 4 killed	4 generated, 4 killed

Verified on CI

With a temporary commit touching two ->yes() lines in IntersectionType.php (since dropped), both Mutation Testing jobs passed:

• The changed-files job listed exactly src/Type/IntersectionType.php.
• Coverage ran for that file only (~4 min).
• Infection generated 4 mutants and killed all 4 (MSI 100%), using 0.13 GB.

run 37118192701

Merge-up

On the merge-up into 2.3.x, MUTATION_TESTING_BASE_BRANCH must become "2.3.x" there, the same way the push branch differs between the branches.

🤖 Generated with Claude Code

https://claude.ai/code/session_01JsHiTmsN24NMF7zvEhwbip

[phpstan-bot]

This pull request has been marked as ready for review.