Comment 0 for bug 1803749

While triaging LP: #1803587 I found that originally autoremovable packages were handled as newly autoremovable ones by unattended-upgrades due to not finding all of them at the beginning of u-u's run.

The root cause seems to be cache.clear() resetting pkg.is_auto_removable to False under some circumstances I haven't fully narrowed down.

The reproduction is as follows:
Set up a Bionic (or later) system with autoremovable packages, packages upgradable from -security and blacklist at least one upgradable package and apply the following patch to u-u:

root@bb-1803587:~# diff -Naur /usr/bin/unattended-upgrade.orig /usr/bin/unattended-upgrade
--- /usr/bin/unattended-upgrade.orig 2018-11-16 16:17:58.522583254 +0000
+++ /usr/bin/unattended-upgrade 2018-11-16 16:36:12.226675870 +0000
@@ -948,7 +948,9 @@
 def rewind_cache(cache, pkgs_to_upgrade):
     # type: (apt.Cache, List[apt.Package]) -> None
     """ set the cache back to the state with packages_to_upgrade """
+ print([pkg.name for pkg in cache if pkg.is_auto_removable])
     cache.clear()
+ print([pkg.name for pkg in cache if pkg.is_auto_removable])
     for pkg2 in pkgs_to_upgrade:
         pkg2.mark_install(from_user=not pkg2.is_auto_installed)
     if cache.broken_count > 0:

Run u-u to observe cache.clear() resetting the list of autoremovable packages:

 ~# /usr/bin/unattended-upgrade --dry-run --verbose --debug
Initial blacklisted packages: systemd
Initial whitelisted packages:
Starting unattended upgrades script
Allowed origins are: o=Ubuntu,a=bionic, o=Ubuntu,a=bionic-security, o=UbuntuESM,a=bionic
Using (^linux-image|^linux-headers|^linux-image-extra|^linux-modules|^linux-modules-extra|^linux-signed-image|^kfreebsd-image|^kfreebsd-headers|^gnumach-image|^.*-modules|^.*-kernel|^linux-backports-modules-.*|^linux-modules-.*|^linux-tools|^linux-cloud-tools) regexp to find kernel packages
Using (^linux-image.*4.15.0-38-generic|^linux-headers.*4.15.0-38-generic|^linux-image-extra.*4.15.0-38-generic|^linux-modules.*4.15.0-38-generic|^linux-modules-extra.*4.15.0-38-generic|^linux-signed-image.*4.15.0-38-generic|^kfreebsd-image.*4.15.0-38-generic|^kfreebsd-headers.*4.15.0-38-generic|^gnumach-image.*4.15.0-38-generic|4.15.0-38-generic.*-modules|4.15.0-38-generic.*-kernel|^linux-backports-modules-.*.*4.15.0-38-generic|^linux-modules-.*.*4.15.0-38-generic|^linux-tools.*4.15.0-38-generic|^linux-cloud-tools.*4.15.0-38-generic) regexp to find running kernel packages
Checking: apport ([<Origin component:'main' archive:'bionic-updates' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>])
adjusting candidate version: apport=2.20.9-0ubuntu7.1
...
Checking: libnss-systemd ([<Origin component:'main' archive:'bionic-updates' origin:'Ubuntu' label:'Ubuntu' site:'archive.ubuntu.com' isTrusted:True>, <Origin component:'main' archive:'bionic-security' origin:'Ubuntu' label:'Ubuntu' site:'security.ubuntu.com' isTrusted:True>])
skipping blacklisted package systemd
pkg systemd package has been blacklisted
sanity check failed
['libfreetype6']
[]
...