Re: jar files won't open in emacs 27.1 on debian 11.6 "bullseye"

[Eli Zaretskii <eliz@gnu.org>]

> From: Steinar Bang <sb@dod.no>
> Date: Sun, 02 Apr 2023 08:15:27 +0200
> 
> Platform: amd64
>           debian 11.6 "bullseye"
>           GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.24, cairo version 1.16.0) of 2023-02-23, modified by Debian
>           unzip 6.0-26+deb11u1
>           libbz2 1.0.8-4
> 
> I tried opening some jar files yesterday. I expected them to open in
> unzip mode and show the "directory listing" of the jar file contents
> (jar files are zip files).
> 
> But of these files:
>   -rw-r--r--  1 sb sb  52055 Apr  1 20:41 modelstore.backend-1.0.0-SNAPSHOT.jar
>   -rw-r--r--  1 sb sb 455179 Apr  1 20:41 modelstore.backend-1.0.0-SNAPSHOT-javadoc.jar
>   -rw-r--r--  1 sb sb  28639 Apr  1 20:41 modelstore.backend-1.0.0-SNAPSHOT-sources.jar
> 
> The modelstore.backend-1.0.0-SNAPSHOT.jar and the
> modelstore.backend-1.0.0-SNAPSHOT-sources.jar file opened showing just
> the following contents:
> M Filemode      Length  Date         Time      File
> - ----------  --------  -----------  --------  --------
> - ----------  --------  -----------  --------  --------
>                      0                         0 files
> 
> The modelstore.backend-1.0.0-SNAPSHOT-javadoc.jar files just gives the
> following error message, and doesn't open:
> File mode specification error: (args-out-of-range modelstore.backend-1.0.0-SNAPSHOT-javadoc.jar 445526 445530)

I cannot reproduce this with a random .jar file I have here and with
Emacs 27.1.  So I guess this is something specific to that particular
jar file.

> Any idea of how to debug this?

Step with Edebug through arc-mode.el and see why it signals the error?
It is possible that the jar file you tried uses some extensions of the
ZIP format which were not yet supported in Emacs 27, so perhaps try
with a later version of Emacs.

> Does the zip support of emacs 27.1 come in the form of running unzip in
> a sub-process? Or does it use a linked in library? (libbz2...?)

Neither.  Just visiting a zip file doesn't require any of that, at
least not by default (I have no idea whether Ubuntu made any changes
in the upstream sources or in site-wide customizations).