Re: [PATCH 2/2] Allow passing pipes for input pipes to diff --no-index

> Hi Taylor,
>
> On Fri, 18 Sep 2020 at 10:36, Taylor Blau <me@ttaylorr.com> wrote:
> > On Fri, Sep 18, 2020 at 07:32:56AM -0400, Thomas Guyot-Sionnest wrote:
> > > A very handy way to pass data to applications is to use the <() process
> > > substitution syntax in bash variants. It allow comparing files streamed
> > > from a remote server or doing on-the-fly stream processing to alter the
> > > diff. These are usually implemented as a symlink that points to a bogus
> > > name (ex "pipe:[209326419]") but opens as a pipe.
> >
> > This is true in bash, but sh does not support process substitution with
> > <().
>
> Bash, ksh, zsh and likely any more moden shell. Other programming
> languages also setup such pipes. It's much cleaner than creating temp
> files and cleaning them up and in some cases faster too (I've ran
> diff's like this over GB's of test data, it's very handy to remove
> known patterns that would cause needless diffs).

> Now you mention it, maybe I could do that stat first, rule this out
> from the beginning... less work for the general case.
>
> *untested*:
>
>     if (!lstat(name, &st)) {
>         if (!S_ISLNK(st.st_mode))
>             return(0);
>         if (!stat(name, &st)) {
>             if (!S_ISFIFO(st.st_mode))
>                 return(0);

>
>             /* We have a symlink that points to a pipe. If it's resolved
>              * target doesn't really exist we can safely assume it's a
>              * special file and use it */
>             struct strbuf sb = STRBUF_INIT;
>             strbuf_realpath(&sb, name, 0);
>             /* We're abusing strbuf_realpath here, it may append
>              * pipe:[NNNNNNNNN] to an abs path */
>             if (stat(sb.buf, &st))
>                 return(1); /* stat failed, special one */

>         }
>     }
>     return(0);
>
> TL;DR - the conditions we need:
>
> - lstat(name) == 0  // name exists
> - islink(lstat(name))  // name is a symlink
> - stat(name) == 0  // target of name is reachable
> - isfifo(stat(name))  // Target of name is a fifo
> - stat(realpath(readlink(name))) != 0  // Although we can reach it,
> name's destination doesn't actually exist.
>
> BTW is st/sb too confusing ? I took examples elsewhere in the code, I
> can rename them if it's easier to read.

> > > +test_expect_success 'diff --no-index finds diff in piped subshells' '
> > > +     (
> > > +             set -- <(cat /dev/null) <(cat /dev/null)
>
> Precautions/portability. The file names are somewhat dynamic (at least
> the fd part...) this is to be sure I capture the names of the pipes
> that will be used (assuming the fd's will be reallocated in the same
> order which I think is fairly safe). An alternative is to sed "actual"
> to remove known variables, but then I hope it would be reliable (and
> can I use sed -r?). IIRC earlier versions of bash or on some systems a
> temp file could be used for these - although it defeats the purpose
> it's not a reason to fail....

> I cannot develop this on other systems but I tested the pipe names on
> Windows and Sunos, and also using ksh and zsh on Linux (zsh uses /proc
> directly, kss uses lower fd's which means it can easily clash with
> scripts if you don't use named fd's, but not our problem....)
>
> Thanks,
>
> Thomas