[Toybox] [PATCH] Implement `file -`.

[enh]

On Sat, May 5, 2018 at 10:53 AM, Rob Landley <rob at landley.net> wrote:
> On 05/04/2018 12:53 PM, enh wrote:
>> okay, here's an alternative patch that only does the temporary file
>> dance for regular files...
>
> Aren't regular files already seekable?
>
> Do pipes show up as regular files for the originally unseekable case (which I'm
> guessing is something like "cat a.out | file -" because it needs to follow
> pointers to data placed arbitrarily later in the file to find things like the
> dynamic loader string... except it's not doing that currently.
>
> And if "zcat blah.gz | file -" isn't ISREG() then we still have non-seekable
> input to deal with, so we're not simplifying anything by being able to assume we
> can always lseek() our input...  except grep doesn't find the string "seek" in
> toys/*/file.c...?
>
> I'm confused.

well, strictly i think only the ELF case actually needs this
currently. we could fix the logic for the non-ELF cases (see below)
and only copy to a temporary file for the ELF case. (alternatively, we
could always read into memory and unify the cases that way --- from
strace that seems to be what FSF file does?)

>> [PATCH] Implement `file -`.
>>
>> Previously we'd just always bogusly report "empty".
>
> That's not what I was seeing before this patch.
>
>   $ zcat ../filesystems.tar.gz | file -
>   /dev/stdin: POSIX tar archive (GNU)
>   $ zcat ../filesystems.tar.gz | ./file -
>   -: POSIX tar archive (GNU)
>
> Even with input from a pipe, we're detecting stuff?
>
> Could you clarify what problem this patch is trying to solve? I don't understand.

i don't think you're accidentally running FSF file there, or you're
accidentally running toybox file _with_ my patch.

here's a clean toybox checkout:

$ ./toybox file - < README
-: empty
$ ./toybox file - < toybox
-: empty
$ ./toybox file - < tests/files/tar/tar.tgz
-: empty
$ cat tests/files/tar/tar.tgz | ./toybox file -
-: empty
$ zcat tests/files/tar/tar.tgz | ./toybox file -
-: empty
$


> Thanks,
>
> Rob