7.15.1 regression: coverage json uses ~5-6x more memory than 7.15.0 #2229

coveragepy-json-memory-repro.zip

Describe the bug

Coverage.py 7.15.1 introduces a large memory-usage regression when generating a JSON report from a sizeable branch-coverage database.

Using the attached synthetic reproducer with the same source tree, configuration, and .coverage database:

This is approximately a 6x increase in peak RSS between 7.15.0 and 7.15.1.

This was originally causing a CI runner on our project to hang due to it being OOM (we observed a 5x increase there.

The excessive memory usage occurs during the coverage json command.

To Reproduce

1. What version of Python are you using?

   CPython 3.10.17.

2. What version of coverage.py shows the problem?

   Coverage.py 7.15.1 shows the regression. Coverage.py 7.15.0 does not.

   Relevant output from coverage debug sys:

   coverage_version: 7.15.1
   core: -none-
   CTracer: available
   configs_read: pyproject.toml
   python: 3.10.17
   platform: macOS-26.5.1-arm64-arm-64bit
   implementation: CPython
   

3. What versions of what packages do you have installed?

   The reproducer only requires Coverage.py and its TOML extra:

   coverage==7.15.1
   tomli==2.0.1
   

4. What code shows the problem?

   The attached coveragepy-json-memory-repro.zip is a self-contained, synthetic reproducer. It generates:

   4,114 synthetic Python modules
   744,634 recorded arcs
   a 26 MiB coverage database
   a 27 MiB JSON report
   

   It contains no application or third-party source code.

5. What commands should we run to reproduce the problem?

   mkdir coveragepy-json-memory-repro
   unzip coveragepy-json-memory-repro.zip -d coveragepy-json-memory-repro
   cd coveragepy-json-memory-repro
   
   python3.10 -m venv .venv
   source .venv/bin/activate
   
   python -m pip install 'coverage[toml]==7.15.0'
   python make_reproducer.py
   COVERAGE_FILE=.coverage \
     /usr/bin/time -l python -m coverage json -o coverage-7.15.0.json
   
   python -m pip install --force-reinstall 'coverage[toml]==7.15.1'
   COVERAGE_FILE=.coverage \
     /usr/bin/time -l python -m coverage json -o coverage-7.15.1.json

   On Linux, use /usr/bin/time -v instead of /usr/bin/time -l.

   The observed maximum resident set sizes on macOS ARM64 were:

   coverage 7.15.0:   206,995,456 bytes
   coverage 7.15.1: 1,239,580,672 bytes
   

Expected behavior

coverage json in 7.15.1 should use memory comparable to 7.15.0 for the same source tree and coverage database. The 6x increase can cause the command to fail on memory-constrained CI runners where 7.15.0 completes successfully.

Additional context