FIX: Remove tracemalloc.stop() from memory leak tests

[wtn]

The CI fails intermittently on memory leak tests, such as here.

The failing memory leak tests called tracemalloc.stop() inside Python::attach. Under concurrent test execution this races with tracemalloc_raw_alloc in other threads, causing a SIGSEGV. Removing the calls fixes the crash without affecting the measurements.

Replication details

The faulting thread was case_1_csv_mbo. Its stack was:

traceback_new          ← null deref at 0x8
tracemalloc_add_trace
tracemalloc_raw_alloc
new_threadstate
PyGILState_Ensure
Python::attach
PyFileLike::write      ← encode.rs:83
BufWriter::flush_buf
csv::Writer::drop      ← Drop impl flushing on scope exit
...
test_from_test_data_file::case_1_csv_mbo

The CSV encoder's Drop triggered a Python::attach, which called PyGILState_Ensure → new_threadstate → tracemalloc_raw_alloc. That allocator hook tried to call traceback_new, which dereferenced a pointer at offset 8 from null, because tracemalloc.stop() in the memory leak test had already freed the internal traceback table.

I've replicated locally on macOS with the following looped command:

for i in $(seq 1 20); do
  echo "=== Run $i ===" && \
  PYO3_PYTHON=/opt/homebrew/opt/python@3.10/bin/python3.10 \
    /usr/sbin/taskpolicy -c maintenance cargo test --all-features -- --test-threads=3 \
    || { echo "FAILED on run $i"; break; }
done

Note: It triggers readily on older slow systems but is harder to trigger on newer fast systems. When I did trigger it on a current device, it was while running multiple instances simultaneously.

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist

• My code builds locally with no new warnings (scripts/build.sh)
• My code follows the style guidelines (scripts/lint.sh and scripts/format.sh)
• New and existing unit tests pass locally with my changes (scripts/test.sh)
• I have made corresponding changes to the documentation (CHANGELOG)
• I have added tests that prove my fix is effective or that my feature works (not applicable)

Declaration

I confirm this contribution is made under an Apache 2.0 license and that I have the authority
necessary to make this contribution on behalf of its copyright owner.