Title

coverage combine with relative_files=true collapses nested directories with duplicate names, causing data loss

Description

Summary

When using relative_files=true, coverage combine generates overly broad path mapping rules that collapse files from nested directories sharing the same name. For example, both features/templates.py and pkg/features/templates.py map to features/templates.py, causing coverage data loss.

Expected vs Actual

Expected: Both files tracked separately

• features/templates.py → features/templates.py
• pkg/features/templates.py → pkg/features/templates.py

Actual: Nested file collapses into root-level file

• features/templates.py → features/templates.py ✓
• pkg/features/templates.py → features/templates.py ❌ (data lost)

Reproducer

Minimal reproduction: https://github.com/dephiros/pytest-coverage-path-bug

# Clone and setup
git clone https://github.com/dephiros/pytest-coverage-path-bug.git
cd pytest-coverage-path-bug
uv sync

# Run workflow
uv run coverage erase
COVERAGE_DEBUG=pathmap uv run coverage run --parallel-mode -m pytest
COVERAGE_DEBUG=pathmap uv run coverage combine
uv run coverage report

Result: Report shows 44 statements instead of 48. pkg/features/templates.py is missing.

Without combine (for comparison):

uv run coverage erase
uv run coverage run -m pytest  # No --parallel-mode
uv run coverage report

Result: Report correctly shows 48 statements with all files.

Debug Output

coverage combine shows the collision:

Generating rule: '*/features' -> 'features/' using regex '^(.*[\\\\/])?features[\\\\/]'
Matched path 'features/templates.py' to rule '*/features' -> 'features/', producing 'features/templates.py'
Matched path 'pkg/features/templates.py' to rule '*/features' -> 'features/', producing 'features/templates.py'

The */features rule matches any path ending with /features/, causing the collision.

Versions

• Python: 3.11.10
• coverage.py: 7.11.0
• pytest: 8.4.2
• pytest-cov: 7.0.0

Config

[tool.coverage.run]
relative_files = true
branch = true
source = ["."]

Repository Structure

.
├── features/
│   └── templates.py          # 4 statements
├── pkg/
│   └── features/
│       └── templates.py      # 4 statements - GETS COLLAPSED
└── pyproject.toml

Related Issues

• Path mapping collapses nested directories with identical names pytest-dev/pytest-cov#724 - Initially filed against pytest-cov, but investigation revealed the bug is in coverage combine itself