Re: [PATCH 1/3] t/lib-httpd: fix apply-one-time-script race under concurrent requests

>
> "Michael Montalbo via GitGitGadget" <gitgitgadget@gmail.com> writes:
> >
> > +#
> > +# Apache can run this CGI for concurrent requests (for example a partial fetch
> > +# that lazily fetches a missing object while the first response is still in
> > +# flight), so the helper claims the marker atomically with a rename, and only
> > +# once it has decided to modify the response. A request that loses the race
> > +# finds the marker already gone and serves its response unchanged; no request
> > +# is left emitting an empty body, which the server would report as HTTP 500.
> > +# Scratch files are per-request ($$) so concurrent requests do not clobber each
> > +# other.
> > +
> > +test -f one-time-script || exec "$GIT_EXEC_PATH/git-http-backend"
> >
> > -     "$GIT_EXEC_PATH/git-http-backend" >out
> > -     ./one-time-script out >out_modified
> > +LC_ALL=C
> > +export LC_ALL
>
> The original was somehow inconsistent in that it forced C locale
> only when one-time-script munged the output, and otherwise the
> backend was run in the original locale.  I am not sure if that
> matters very much.
>

>
> Ah, we assume running one-time-script itself multiple times is safe
> and does not cause issues.  Our objective is to avoid returning
> modified output twice.  So while the first instance of us
> successfully renames one-time-script to one-time-script.$$ and emits
> the modified result, even if the second instance raced and managed
> to run the script again, it will fail to rename with "mv", and
> discard the modified output, and instead show the unmodified output
> generated by the backend.
>
> OK.  It is a bit tricky.  It may help future readers if we said
> something about this in the proposed log message (i.e., we consider
> that it is perfectly fine to run one-time-script more than once; we
> only want to avoid letting the second invocation's output used).
>