SUMMARY
Sometimes, an agent is in a weird state. If you go to KMail, right-click and select Restart Account, or in Akonadiconsole select Restart Agent, the agent doesn't exit.

STEPS TO REPRODUCE
1. Get into this condition (unknown how; probably related to network interfaces going up and down)
2. Open Akonadiconsole, select affected agent, get its Identifier and find it in the ps process list
3. Right-click the agent, Restart Account

OBSERVED RESULT
The same process is still running.


EXPECTED RESULT
A new process shows up in the process list

SOFTWARE/OS VERSIONS
Qt: 5.15.0
KDE Frameworks: 5.71.0
kf5-config: 1.0

ADDITIONAL INFORMATION
strace shows the process received a D-Bus message asking it to exit:

[pid  2894] recvmsg(9<socket:[43406]>, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="l\1\0\1\0\0\0\0q\210\2\0\236\0\0\0\1\1o\0\1\0\0\0/\0\0\0\0\0\0\0\6\1s\0005\0\0\0org.freedesktop.Akonadi.Agent.akonadi_imap_resource_5\0\0\0\2\1s\0%\0\0\0org.freedesktop.Akonadi.Agent.Control\0\0\0\3\1s\0\4\0\0\0quit\0\0\0\0\7\1s\0\5\0\0\0:1.31\0\0\0", iov_len=2048}], msg_iovlen=1, msg_controllen=0, msg_flags=MSG_CMSG_CLOEXEC}, MSG_CMSG_CLOEXEC) = 176

It replied confirming it received this message:

[pid  2894] sendmsg(9<socket:[43406]>, {msg_name=NULL, msg_namelen=0, msg_iov=[{iov_base="l\2\1\1\0\0\0\0\5#\1\0\30\0\0\0\6\1s\0\5\0\0\0:1.31\0\0\0\5\1u\0q\210\2\0", iov_len=40}, {iov_base="", iov_len=0}], msg_iovlen=2, msg_controllen=0, msg_flags=0}, MSG_NOSIGNAL <unfinished ...>

But didn't exit.

Hypothesis: there's a job inside the process that is holding the event loop counter above 0. So the event loop doesn't exit.