[PATCH 0/3] t/lib-httpd: make CGI test helpers concurrency-safe

The httpd tests share a handful of CGI helper scripts under t/lib-httpd. Two of them keep state between requests in the shared HTTPD_ROOT_PATH on the assumption that the web server hands them one request at a time. It does not: Apache serves requests concurrently, and a single Git operation can open more than one request to the same endpoint at once. A partial fetch that receives a REF_DELTA against a missing promisor object lazily fetches that base while the first response is still being served.

Under that overlap apply-one-time-script.sh loses: two requests both pass
its "test -f one-time-script" check, one removes the marker, and the other
fails to exec it and emits an empty body, which the server answers as HTTP
500. In the field this is an occasional failure[1] of:

t5616.47 tolerate server sending REF_DELTA against missing promisor objects

on the macOS CI runners, with:

fatal: ... The requested URL returned error: 500 fatal: could not fetch from promisor remote

I could not reproduce it against a live server (the window is tiny and timing-dependent), but the macOS CI error log names the exact failure, and the new test reproduces the helper's shell error.

http-429.sh keeps its "already returned 429 once" state with the same non-atomic test-and-set. Its retry flow is mostly sequential so it seems less likely to fail, but it is the same latent race.

Each fix is local: claim/consume the one-shot marker with an atomic rename, and elect the first request with an atomic mkdir, rather than a "test -f" followed by a separate remove or touch.

 * Patch 1 fixes apply-one-time-script.sh (the actual flake) and adds t5567,
   which drives the helper directly with no web server so the overlap can be
   forced deterministically.
 * Patch 2 makes http-429.sh atomic.
 * Patch 3 documents the atomic idioms generally in t/README (they are not
   specific to CGI or HTTP), citing Git's own lockfile machinery and
   make_symlink(), with a pointer from the lib-httpd list.

[1] https://github.com/gitgitgadget/git/actions/runs/28756172690/job/85263916762?pr=2169

Michael Montalbo (3):
  t/lib-httpd: fix apply-one-time-script race under concurrent requests
  t/lib-httpd: make http-429 first-request check atomic
  t/README: document writing concurrency-safe helpers

 t/README                             | 32 ++++++++++
 t/lib-httpd.sh                       |  3 +
 t/lib-httpd/apply-one-time-script.sh | 38 +++++++----
 t/lib-httpd/http-429.sh              | 21 +++---
 t/meson.build                        |  1 +
 t/t5567-one-time-script.sh           | 96 ++++++++++++++++++++++++++++
 6 files changed, 166 insertions(+), 25 deletions(-)
 create mode 100755 t/t5567-one-time-script.sh

base-commit: e9019fcafe0040228b8631c30f97ae1adb61bcdc
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2171%2Fmmontalbo%2Fmm%2Flib-httpd-cgi-safe-proto-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2171/mmontalbo/mm/lib-httpd-cgi-safe-proto-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2171