Re: [PATCH] difftool: always honor "command not found" exit code

> At the moment difftool's "trust exit code" logic always suppresses the
> exit status of the diff utility we invoke.  This is useful because we
> don't want to exit just because diff returned "1" because the files
> differ, but it's confusing if the shell returns an error because the
> selected diff utility is not found.
> 
> POSIX specifies 127 as the exit status for "command not found" and 126
> for "command found but is not executable" [1] and at least bash and dash
> follow this specification, while diff utilities generally use "1" for
> the exit status we want to ignore.
> 
> Handle 126 and 127 as special values, assuming that they always mean
> that the command could not be executed.
> 
> [1] http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_08_02
> 
> Signed-off-by: John Keeping <john@keeping.me.uk>

> ---
> On Sat, Aug 13, 2016 at 11:36:39AM +0100, John Keeping wrote:
> > It would be nice if there was a way to differentiate between complete
> > failure and just the diff tool exiting with a non-zero return status
> > because the files differ, but I'm not sure whether we can do that
> > reliably.  POSIX uses 127 and 126 as errors that mean the command didn't
> > run [1] so it may be sensible to to treat those as special values.
> 
> Something like this perhaps?  I think this is probably safe, but it's
> always possible that some diff utility does use 126 or 127 as a "normal"
> exit status.  I'm not sure what we can do about that oaaaather than add a
> "really, really don't trust the exit status" option!