flaky: cJSON / lua corpus runners race on a shared /tmp input path under parallel testing

• Type: test-harness flakiness. Track C (owns test/cjson/**, test/lua/**).
• Found: 2026-07-08, while resolving regression-cfront-stmt-expr-25c1dded.

Problem

Both corpus runners read a single hardcoded input path and rely on the Makefile copying each fixture there before running:

• test/cjson/runner.c → #define PXX_CJSON_INPUT "/tmp/pxx_cjson_input.json"
• test/lua/runner.c → #define PXX_LUA_SCRIPT "/tmp/pxx_lua_input.lua" (comment: "C argv is not wired yet, so the Makefile copies each case to that fixed path")

The test-cjson / test-lua Makefile loops do cp $case /tmp/pxx_..._input; run. Serially this is fine. Under parallel test execution (borg full-tier), two cases share the one path — case B overwrites the input while case A's runner is mid-read, so A serializes B's document. Manifested as a false wrong-output RED: test-cjson#00 reported scalars.json emitting strings.json content (see the parent ticket). Not a compiler bug — a shared-mutable-file race in the harness.

Fix options

1. Wire C argv (main(argc, argv)) in the runners and have the Makefile pass the fixture path directly — no shared temp at all. Preferred; also removes the cp.
2. If argv stays unavailable: make each invocation's temp path unique (mktemp, or embed $$/case-name), and pass it via an env var the runner reads with crtl getenv.
3. Or serialize these two suites (mark them non-parallel in testmgr). Cheapest, but leaves the latent footgun for any future fixed-/tmp-path runner.

Gate

make test-cjson + make test-lua green, and the two suites pass when run under tools/testmgr.py parallel full-tier without input-file cross-contamination.

Log

• 2026-07-08 — resolved, commit dbdc0c2c.