fix: deduplicate Directory nodes on re-collection to preserve fixture identity (#14635)

[RonnyPfannschmidt]

Summary

Fixes #14635.

When Session re-collects a parent Directory (due to handle_dupes=False for file CLI args), fresh child nodes were created. Since fixture registration uses node identity for matching (_matchfactories checks fixturedef.node in parent_nodes), the new Directory children didn't match fixtures registered with the original instances.

This caused fixture closure computation to fail for tests collected after unrelated paths, because the conftest fixtures were registered with a now-orphaned Directory node instance.

Root cause: _collect_one_node with handle_dupes=False calls collect() on the parent Directory, which creates brand-new child Directory/Package instances. The _collection_cache is then updated with these new children. Later collection paths that look up cached children find different node objects than the ones conftest fixtures were registered with.

Fix: After re-collection, replace any freshly-created Directory children with the previously-seen instances for the same path. Module/File nodes are still recreated to preserve --keep-duplicates semantics (which needs fresh test items).

Test plan

• Added regression test test_fixture_closure_order_independence_with_parametrize that reproduces the exact scenario from Error collecting tests on 9.1.1 - fixtures not found when a parent directory appears multiple times in a argument list  #14635
• Existing TestOverlappingCollectionArguments::test_complex_combined_handling (which exercises --keep-duplicates with overlapping file/dir args) passes
• --keep-duplicates file.py file.py still collects the file twice
• All fixture and conftest tests pass (274 tests)
• All collection tests pass

Made with Cursor

[Copilot]

Pull request overview

Fixes a regression where collecting multiple CLI paths (mixing directories and files) could cause a parent directory to be re-collected and its child Directory nodes to be replaced with fresh instances, breaking fixture visibility because fixture registration relies on collector node identity.

Changes:

• Reconciles re-collected Directory children with previously-seen instances when handle_dupes=False, preserving fixture identity while still recreating file/module nodes for --keep-duplicates semantics.
• Adds a regression test covering order-dependent fixture-closure/parametrize validation failures triggered by mixed dir/file collection arguments.
• Adds a changelog entry describing the fixed behavior.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/_pytest/main.py	Reuses cached Directory children after re-collection to keep fixture registration tied to stable node identities.
testing/test_conftest.py	Adds a regression test that fails on order-dependent collection when directory nodes are recreated.
changelog/14635.bugfix.rst	Documents the fixture-resolution regression and its order-dependent symptom.

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[patchback]

Backport to 9.1.x: 💚 backport PR created

✅ Backport PR branch: patchback/backports/9.1.x/a933cf5de4d64192d1d588a851ca4e76eec71efc/pr-14645

Backported as #14968

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