With plain Debian
debdelta-upgrade works correctly: it gets a list of upgradeable files, downloads all available deltas, and creates the desired .deb upgrade files. So if packages foo, bar, and baz have available deltas, then
debdelta-upgrade will fetch foo, bar, and baz, then quit.
But under Ubuntu distros, (tested with Mint and Lubuntu), when there are several packages with available deltas,
debdelta-upgrade fetches only one delta at a time. If run once it fetches foo, then quits; if run again it fetches bar; if run again, it fetches baz.
Under Ubuntu, this kludge works; it fetches all available deltas, by running
debdelta-upgrade once for each upgradable package:
apt list --upgradable 2> /dev/null | sed -n '1!s#/.*##p' | \ xargs -n 1 debdelta-upgrade
But it's much slower than Debian, since each time
debdelta-upgrade runs it needs 5 seconds or so to initialize ("updating apt cache") -- if 10 packages need upgrading, that's an added 50 second delay (5 seconds x 10 packages) under Ubuntu variants.
Is there a fix?