Detect DB connection timeout and retry connection

[PVince81]

Whenever "MySQL is gone" or other failure happen, catch them in the DB class and retry the connection a few times before giving up.

@DeepDiver1975

[DeepDiver1975]

This will work as long as we are not within a transaction. In case we loose the connection within a transaction we need to replay the full transaction because everything which is not commited yet will be lost.

[PVince81]

Argh, and most ownCloud operations aren't designed to be redoable... good luck.

[DeepDiver1975]

Anything which is http request driven can be replayed by submitting the request again (on a very high level 🙈 ).
With long running tasks like the file scanner this is a bit different.
We can either ensure to replay the transaction or give the command a way to recover from a termination.

[PVince81]

Right, then for the file scanner we could add some specific code there.
Might also be required in cron jobs.

[DeepDiver1975]

Another idea to get this sorted out would be to implement the scanner as multiple background jobs (split up by user and/of folder/mount etc).
The files:scan command will 'only' create them and monitor their execution.

The execution of these individual jobs is done by the background job execution system (9.2 topic)
which consists of multiple processes listening to the job queue and executes the jobs by forking itself.

[PVince81]

Or periodically reconnect in a controlled manner. After each job, check how long we've been running and decide whether to reset the connection. But would also need to be possible in the middle of a job, so the job would have to be able to trigger it by calling some kind of method.

[felixboehm]

Solution needed also for 8.2.8, please.

[DeepDiver1975]

backport are open

• stable9.1 - [9.1] Before a user is getting scanned the database connection is re-… #25888
• stable9.0 - [9.0] Before a user is getting scanned the database connection is re-… #25889
• stable8.2 - [8.2] Before a user is getting scanned the database connection is re-… #25890