bug#63832: fix failed inflation of .el.gz archives due to passing empty buffer to inflate()

[Eli Zaretskii]

> From: Amritpal Singh <icy.amrit@gmail.com>
> Date: Fri, 2 Jun 2023 12:21:07 +0530
> 
> Compile emacs with the system's gzip program set to `pigz`.
> Run emacs and then `M-x eww RET`
> 
> Expected behavior:
> Enter URL prompt in mini-buffer
> 
> Actual behavior:
> hashing failed '/usr/share/emacs/30.0.50/lisp/gnus/gnus.el.gz'
> 
> Report:
> The bug has been reproduced on emacs version 29.0.91 and HEAD which
> seems to be at 30.0.50.
> Later, a copy of the aforementioned file was saved somewhere else and
> the program was uninstalled. Then emacs was recompiled with system's
> gzip program set to GNU gzip and the initial steps were repeated and
> the expected behavior was the result.
> This lead to believing either that there's a bug with how zlib's
> `inflate()` handles archives or emacs code was having an issue with
> archives files.
> 
> The hashes for gz archives generated with different programs were as follows
> > md5sum gnus-gzip.el.gz
> edb3d0ffba7f19ff1d4ec3f889609e8a  gnus-gzip.el.gz
> > md5sum gnus.el.gz
> 985deaaec6a5845ac8d6bd9648957b50  gnus.el.gz
> 
> And when uncompressing these archives, the resulting file was the same
> and the hash for the files was the same (omitted for brevity).
> 
> Now after logging some code in $EMACS_REPO/src/decompress.c, it was
> learned that in the pigz specific case, `inflate()` was returning
> Z_BUF_ERROR(-5) which is an indicator for zstream's either `avail_in`
> or `avail_out` fields are 0.
> 
> Observing the code in `$EMACS_REPO/src/decompress.c`
> L154:
>     } while (!stream.avail_out);
> only checks stream.avail_out and not stream.avail_in which also might
> have been set to 0. A special case here can be constructed where
> `avail_in` is 0, and the code keeps looping even though our input
> buffer is empty and thus causing a Z_BUF_ERROR. Placing a simple check
> for it fixes the bug in pigz's gz archives case and does not cause any
> issue with gzip archives.
> 
> A patch with a simple fix is attached below

Thanks, installed on the master branch, and closing the bug.