[teunbrand]

Temporary draft PR to debug a SIGSEGV in the Build and Test workflow on the arrows branch (#PR). Runs tests single-threaded to identify the crashing test. Will be closed.

[teunbrand]

Closing: the segfault reproduces on unmodified main (run 42655753480), so this is a runner environment issue, not branch-specific.

[teunbrand]

Posting summary of investigation by agent here:

SIGSEGV in ggsql CI test suite after ubuntu-latest rolled to 26.04

Symptom. cargo test --lib --bins on ubuntu-latest dies mid-suite with SIGSEGV (signal 11, invalid memory reference). No test is named — the whole ggsql test binary crashes. Always during the writer::hephaestus tests, i.e. the ones that render plots through hephaestus. First seen 2026-11-09; the same workflow was green 2026-11-04.

Environment change is the trigger. The crash reproduces on unmodified main (run 42655753480). Last green run used image ubuntu-24.04; failing runs use ubuntu-26.04 — ubuntu-latest rolled in between. Nothing in ggsql or hephaestus changed.

It's concurrency-dependent. Deterministic across reruns in parallel mode (3/3 crashed, at different points in the suite — expected, since the last "ok" isn't the crasher under parallel scheduling). With --test-threads=1 the entire ~2177-test suite passes cleanly, twice, including the full hephaestus writer tests. So: parallel-only, Linux-only, text-rendering-tests-only. All tests pass in parallel on macOS.

Suspect: fontconfig via the parley/fontique text stack. Hephaestus itself contains no unsafe (checked 0.4.1 sources). The native code on the crashing path is the text stack: parley → fontique → yeslogic-fontconfig-sys (dlopen mode, loads libfontconfig.so.1 at runtime) → system fontconfig. Fontconfig has a history of thread-safety problems, and ubuntu-26.04 ships a much newer libfontconfig than 24.04. This is inference from the crash's shape, not yet confirmed with a backtrace.

Not yet done: a native backtrace (run the test binary under gdb -batch -ex run -ex "thread apply all bt" on the runner) to name the crashing library and thread. That would confirm or refute fontconfig. Possible stopgap meanwhile: pin CI to ubuntu-24.04.

Repro on a 26.04 machine/container should just be: cargo test --lib writer:: in the ggsql repo (parallel, default features).