Bug#951651: lsb_release does not check for existence of "apt-cache"

[Daniel Bryant]

Package: lsb-release
Version: 11.1.0

There is a bug with lsb_release in that it does not check for the existence
of apt-cache before it tries to execute it. If apt-cache is not present on
the system this will result in the following error:

[  111s] Traceback (most recent call last):
[  111s]   File "/usr/bin/lsb_release", line 95, in <module>
[  111s]     main()
[  111s]   File "/usr/bin/lsb_release", line 59, in main
[  111s]     distinfo = lsb_release.get_distro_information()
[  111s]   File "/usr/lib/python3/dist-packages/lsb_release.py", line 356,
in get_distro_information
[  111s]     distinfo = guess_debian_release()
[  111s]   File "/usr/lib/python3/dist-packages/lsb_release.py", line 290,
in guess_debian_release
[  111s]     rinfo = guess_release_from_apt()
[  111s]   File "/usr/lib/python3/dist-packages/lsb_release.py", line 195,
in guess_release_from_apt
[  111s]     releases = parse_apt_policy()
[  111s]   File "/usr/lib/python3/dist-packages/lsb_release.py", line 178,
in parse_apt_policy
[  111s]     close_fds=True).communicate()[0].decode('utf-8')
[  111s]   File "/usr/lib/python3.7/subprocess.py", line 800, in __init__
[  111s]     restore_signals, start_new_session)
[  111s]   File "/usr/lib/python3.7/subprocess.py", line 1551, in
_execute_child
[  111s]     raise child_exception_type(errno_num, err_msg, err_filename)
[  111s] FileNotFoundError: [Errno 2] No such file or directory:
'apt-cache': 'apt-cache'
[  112s] Traceback (most recent call last):
[  112s]   File "/usr/bin/lsb_release", line 95, in <module>
[  112s]     main()
[  112s]   File "/usr/bin/lsb_release", line 59, in main
[  112s]     distinfo = lsb_release.get_distro_information()
[  112s]   File "/usr/lib/python3/dist-packages/lsb_release.py", line 356,
in get_distro_information
[  112s]     distinfo = guess_debian_release()
[  112s]   File "/usr/lib/python3/dist-packages/lsb_release.py", line 290,
in guess_debian_release
[  112s]     rinfo = guess_release_from_apt()
[  112s]   File "/usr/lib/python3/dist-packages/lsb_release.py", line 195,
in guess_release_from_apt
[  112s]     releases = parse_apt_policy()
[  112s]   File "/usr/lib/python3/dist-packages/lsb_release.py", line 178,
in parse_apt_policy
[  112s]     close_fds=True).communicate()[0].decode('utf-8')
[  112s]   File "/usr/lib/python3.7/subprocess.py", line 800, in __init__
[  112s]     restore_signals, start_new_session)
[  112s]   File "/usr/lib/python3.7/subprocess.py", line 1551, in
_execute_child
[  112s]     raise child_exception_type(errno_num, err_msg, err_filename)
[  112s] FileNotFoundError: [Errno 2] No such file or directory:
'apt-cache': 'apt-cache'

The line that attempts to run apt-cache is this one:
https://salsa.debian.org/debian/lsb/blob/debian/master/lsb_release.py#L174

I would suggest a check is added to see if apt-cache exists on the system,
and if not that check is bypassed. Possibly a check such as: if shutil.which
("apt-cache") != None:

This seems like a straightforward check, currently lsb_release does not
work at all on systems without apt-cache, such as those used by the Open
Build Service.

This appears to be present on all versions of lsb_release since at least
10.2019031300, I'm not sure if it is in older versions as well.
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