[Bug 1816045] Re: secureboot-db: release upgrade from 16.04 to 18.04 modifies changelog.gz but resets mtime

[Oliver O.]

Thanks for explaining. I am using an internally developed, Python-based
backup system. It makes backups from Btrfs snapshots and regularly
verifies the integrity of the entire restore process. The verification
shows discrepancies such as the one above.

As most backup systems rely on mtime to detect changes for performance,
I suppose this is really a widespread problem. It probably goes
undetected in the majority of cases as backup systems usually don't do
100% integrity verification lacking file system snapshots.

My backup system uses regexp-based filtering to switch from mtime to
content change detection. In this case the relevant line in its YAML
configuration file is:

      - { pattern: '^usr/share/doc/[^/]*/changelog[^/]*$', filter:
compare-contents }

The problem is not just limited to package installation files as this
excerpt shows:

      - { pattern: '^var/lib/sudo/', filter: compare-contents }
      - { pattern: '^var/lib/machines$', filter: compare-contents }
      - { pattern: '^var/cache/man/(.*/)?index\.db$', filter: compare-contents }

And there are even more entries in my list. I wonder whether tinkering
with mtime is really such a good idea...

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https://bugs.launchpad.net/bugs/1816045

Title:
  secureboot-db: release upgrade from 16.04 to 18.04 modifies
  changelog.gz but resets mtime

Status in secureboot-db package in Ubuntu:
  Won't Fix

Bug description:
  During a release-upgrade from Ubuntu 16.04 to 18.04, the file
  /usr/share/doc/secureboot-db/changelog.gz is modified, but retains its
  original mtime. This will interfere with incremental backups, which
  will fail to pick up the updated file.

  The file's first line has changed
  from: secureboot-db (1.4~ubuntu0.16.04.1) xenial; urgency=medium
  to:   secureboot-db (1.4~ubuntu0.18.04.1) bionic; urgency=medium

  ... but the file time for both file versions is identical:
  2018-10-19 20:20:34.000000000 +0200 changelog.gz

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