An ExUnit formatter that writes a machine-readable per-test report.

ExUnit has no such output, and mix verify.change's quarantine ceiling needs one: the count of tests excluded by the :quarantine tag is the number the ceiling bounds, and stdout scraping cannot distinguish that exclusion from the several others this suite already applies (:playwright, :integration_caddy, :integration_cli).

Why it lives in ex_sandbox, and why in lib/

A consumer with several applications needs one formatter every one of their suites can load, and the only module that qualifies is one they all already depend on. This library is that module for its own consumers, which is the whole of the reason — nothing here is about sandboxing, and nothing here names any particular application.

⚠️ lib/, and it was test/support/ until 1.0.1. Inside an umbrella that worked, because test/support is on elixirc_paths under MIX_ENV=test and the compiled module landed in _build/test/lib/ex_sandbox/ebin where every sibling app's mix test could load it. A published package has no such arrangement: test/ is not in package/0's files:, so 1.0.0 listed this module as Public in priv/boundary.md and shipped a release that does not contain it. The consumer's own --formatter ExSandbox.LoopFormatter then fails to load a module its dependency promised.

Why TSV rather than JSON

ex_sandbox depends on :telemetry and nothing else -- no JSON encoder is available here, and adding one to the dependency root of the umbrella to format a test report is not a trade worth making. The consumer is a bash 3.2 runner with awk, which reads TSV natively.

The last line is the point

The report ends with a summary row whose final field is COMPLETE. A run that crashes, is killed, or fills the disk leaves a report without it.

This matters more than it looks. A gate that reads a truncated report sees fewer failures and fewer quarantined tests than actually exist, and passes -- reporting green because it was handed less evidence, which is the exact failure docker/census-gate.sh was written to close and the one this repository has been bitten by three times. mix verify.change therefore treats a report without a trailing COMPLETE as a gate failure, never as an empty result.

Enabling

Set LOOP_REPORT to the destination path. Unset, the formatter records nothing and costs nothing, so it is safe to leave configured.

LOOP_REPORT=.loop/runs/report.tsv mix test --formatter ExSandbox.LoopFormatter

Rows are appended, because an umbrella mix test runs each app in its own ExUnit process and this formatter is initialised once per app. The consumer sums across every summary row and requires the file's last row to be one.