core/pppd-plugin: wait to recover port settings before notifying death
pppd restores the previous settings for the serial port it uses right
before exiting. It is especially important to do so because otherwise
ModemManager is not able to recover the port as it can receive a hangup
event from the port due to CLOCAL not being restored. However, there is
currently a race condition that produces this issue. This is because
when PHASE_DEAD is notified, pppd still has not restored the port
settings - it does that a bit later, in the die() function.
This patch delays notifying PHASE_DEAD until when the exitnotify() hook
is called by pppd: when this happens the port settings have already been
restored.
There were previously efforts to fix this in commit fe090c3, so
PHASE_DEAD was used instead of PHASE_DISCONNECT to notify MM that the
port was disconnected, but that still early to ensure that the port
settings are restored.
The MM traces seen when the bug is triggered are:
ModemManager[2158]: <warn> (ttyACM1): could not re-acquire serial port lock: (5) Input/output error
ModemManager[2158]: <warn> Couldn't load Operator Code: 'Cannot run sequence: 'Could not open serial device
ttyACM1: it has been forced close'
Note the even though the master branch does not contain the recipe for
the core18 or core20 snaps, we prefer it to update the documentation.
The reason is that the master branch is the one pulled from the repo
configuration in https://github.com/canonical-web-and-design/ubuntu-core-docs
