Re: [PATCH v2 3/3] t/README: document writing concurrency-safe helpers

[Patrick Steinhardt <ps@pks.im>]

On Fri, Jul 10, 2026 at 05:30:57PM +0000, Michael Montalbo via GitGitGadget wrote:
> diff --git a/t/README b/t/README
> index 085921be4b..a9d425f392 100644
> --- a/t/README
> +++ b/t/README
> @@ -854,6 +854,38 @@ from the test harness library.  At the end of the script, call
>  'test_done'.
>  
>  
> +Writing concurrency-safe helpers
> +--------------------------------

Nit: this paragraph is quite specific to lib-httpd, so it would make
sense to mention it in the header here. E.g.

    Writing concurrency-safe lib-httpd helpers

> +Some test code runs concurrently: a test may background work with '&',
> +and the helper scripts installed for the web server (in t/lib-httpd) are
> +run once per request, so the same script can execute for several
> +requests at once.  Such code cannot assume it has exclusive access to a
> +file.
> +
> +When exactly one of several concurrent processes needs to "win" a
> +decision, a single atomic filesystem operation can make it, rather than
> +a check followed by a separate action.  A "test -f X" then "touch X"
> +(or "rm X") races: two processes can both pass the check before either
> +acts.  Two atomic operations avoid this:
> +
> + - "mkdir dir", which fails if the directory already exists, so that
> +   exactly one caller wins, electing a first or only request (see
> +   t/lib-httpd/http-429.sh).
> +
> + - "mv src dst" (rename), which fails if the source is gone, so that
> +   exactly one caller consumes it, claiming a planted one-shot marker
> +   (see t/lib-httpd/apply-one-time-script.sh).

A simple "rm" (without "-f") should work as well, right?

> +A "$$" suffix on per-request scratch files keeps concurrent invocations
> +from clobbering each other's fixed-name files.

Nit: it might be a bit easier to read if we explicitly mention PIDs
instead of assuming that every reader immediately knows that "$$" will
expand to the PID. E.g.:

    Appending a PID to the per-request scratch filenames keeps...

Thanks!

Patrick