[SOLVED] apt-get update fails on mounted iso of stretch DVD

[luvr]

Access with apt is now possible after doing:
<snip>

Slightly off-topic, but Ubuntu has been displaying “Permission denied” errors implicating the “_apt” user for quite some time now whenever it updates its “adobe-flashplugin” package:

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W: Can't drop privileges for downloading as file '/var/lib/update-notifier/packa
ge-data-downloads/partial/adobe-flashplugin_20170616.1.orig.tar.gz' couldn't be
accessed by user '_apt'. - pkgAcquire::Run (13: Permission denied)

The solution to your problem here made me realise how to get rid of these errors as well. I just did the following:

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# setfacl -m _apt:rwx /var/lib/update-notifier/package-data-downloads/partial

Then, to verify the result:

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$ getfacl -t /var/lib/update-notifier/package-data-downloads/partial
getfacl: Removing leading '/' from absolute path names
# file: var/lib/update-notifier/package-data-downloads/partial/
USER   root      rwx
user   _apt      rwx
GROUP  root      r-x
mask             rwx
other            r-x

The errors are gone now.