bug#18370: insert-file-contents: forbids also beg, end for non-regular (special) files

[Ivan Shmakov]

Package:  emacs
Severity: minor

        As currently implemented, insert-file-contents disallows not
        only (as per (elisp.info) Reading from Files) the case of either
        ‘visit’ or ‘replace’ arguments being non-nil, but also when
        non-nil are either ‘beg’ or ‘end’ (or both):

  3506    /* This code will need to be changed in order to work on named
  3507       pipes, and it's probably just not worth it.  So we should at
  3508       least signal an error.  */
  3509    if (!S_ISREG (st.st_mode))
  3510      {
  3511        not_regular = 1;
⋯
  3516        if (! NILP (replace) || ! NILP (beg) || ! NILP (end))
  3517          xsignal2 (Qfile_error,
  3518                    build_string ("not a regular file"), orig_filename);
  3519      }

        This, however, precludes the use of insert-file-contents not
        only on named pipes, but also on /dev/cdrom, /dev/random, and
        the like, – for (and especially in the case of the latter) the
        ‘end’ argument gets rather essential here, as otherwise the
        function is likely to read much more than the caller will be
        able to handle at any single time.

        Instead, I’d suggest that ‘end’ is always allowed, and ‘beg’ is
        allowed when the file in question is /seekable/, – which, ISTR,
        is possible to check beforehand (lseek (fd, 0, SEEK_CUR) < 0?)

        From a glance over the code, this new behavior wouldn’t be all
        that hard to implement (some not_regular checks will have to be
        replaced with the ones against beg_offset, end_offset, etc.),
        but from what I read, – it’s already going to be more than I can
        test right now.  So no .diff this time, alas.

-- 
FSF associate member #7257  http://boycottsystemd.org/  … 3013 B6A0 230E 334A