core: call dynamic_user_acquire() only when 'group' is non-null
When unit is reloaded, and the reloaded unit has bad-setting, then
unit_patch_contexts() is not called and exec_context::user and group
may not be configured.
A minimum reproducer for the case is:
- step 1.
$ sudo systemctl edit --full hoge.service
[Service]
oneshot
ExecStart=sleep 1h
- step 2.
$ sudo systemctl start hoge.service
- step 3.
$ sudo systemctl edit --full hoge.service
[Service]
Type=oneshot
ExecStart=@bindir@/sleep 1h
DynamicUser=yes
Then pid1 crashed.
Fixes #14733.
(cherry picked from commit
50152bb1c5c311e97e9eeec3b09044925b6e3663)
Gbp-Pq: Name core-call-dynamic_user_acquire-only-when-group-is-non-nul.patch