Tests in a file with set_up or tear_down cost five process forks each (28.3ms vs 8.5ms per test) #1345

Summary

A test in a file that defines set_up or tear_down costs 3.3x a test in a file that does not. Each hook mints a mktemp scratch file for its output, and the ownership marker that file leaves behind makes the runner rm -rf the test's temp files at exit. Five process forks per test, none of them needed.

.claude/rules/perf-fork-budget.md says "the per-test path is genuinely fork-free". That is true, but only for a hookless file, which is what its census fixture uses. That is why this never showed up.

Measured

200 trivial tests (assert_same 1 1), best of 3, macOS arm64, bash 3.2.57:

PATH-shim census over the same two fixtures, using the method in perf-fork-budget.md:

Exactly five binary forks per hooked test: 2 mktemp, 2 rm -f, 1 rm -rf. Hooks cost +19.7 ms per test.

This suite has 1052 tests in files defining set_up or tear_down, out of ~2491. So roughly 20 seconds of a sequential run.

Current behavior

bashunit::runner::execute_test_hook, src/runner/hooks.sh:196-265:

hook_output_file=$(bashunit::temp_file "${hook_name}_output")   # :215
...
rm -f "$hook_output_file"                                       # :258

bashunit::temp_file (src/api/globals.sh:38-51) forks $MKTEMP, and also writes $BASHUNIT_TEMP_DIR/${BASHUNIT_CURRENT_TEST_ID}_.mark (src/api/globals.sh:25-36). The hook's rm -f removes the output file but not that marker. So bashunit::cleanup_testcase_temp_files (src/api/globals.sh:70-100), reached from the EXIT trap at src/runner/hooks.sh:3-40, sees the marker and runs rm -rf.

A test that creates no temp file of its own still gets a marker, purely because a hook ran.

Same shape once per file in execute_file_hook (src/runner/hooks.sh:82-145): mktemp at :93, rm -f at :141.

Expected behavior

No process fork per hook. The hook still needs a file, because it runs in the current shell on purpose (see the comment at src/runner/hooks.sh:206-212: variables set before a failure have to survive), so only the file's name and lifecycle should change.

Name it arithmetically inside the run directory instead of going through temp_file. The > redirect at :239-241 truncates, so no rm is needed at all. The precedent is already in the tree: src/runner/result.sh:28-62 names the per-test .result file by an ordinal the dispatcher assigns before forking (#851), because a Bash 3 fork inherits both $$ and the RANDOM state and cannot mint a unique token.

Two details for whoever picks this up:

• Key on the ordinal, not BASHUNIT_CURRENT_TEST_ID. bashunit::helper::generate_id (src/helper/encoding.sh:44-65) produces ${fn}_$$, which repeats across data-provider rows.
• _test_ordinal is bumped only on the --parallel branches today (src/runner/exec.sh:169-172 and :216-219), so a sequential run leaves it at 0. It is also a local of call_test_functions, so it restarts per file, and needs combining with the folded file key the way parallel_suite_dir_to_slot does (src/runner/result.sh:36-41).

The RED test belongs in tests/acceptance/bashunit_run_forks_test.sh, with a fixture that defines set_up and tear_down, asserting the mktemp and rm counts. perf-fork-budget.md needs the "fork-free" paragraph corrected to say hookless.