From: Patrick Steinhardt Date: Tue, 04 Aug 2026 08:03:19 GMT Subject: Re: [PATCH v2 3/3] t/README: document writing concurrency-safe helpers Message-ID: In-Reply-To: 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