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* MM-27648: Fix a hub deadlock while revoking session
This is a bug which has always been there in the codebase.
And it can only occur in the extreme of edge-cases.
Following is the call trace due to which this happens:
```
0 0x0000000001dfea68 in github.com/mattermost/mattermost-server/v5/app.(*Hub).InvalidateUser // deadlock
at ./app/web_hub.go:369
1 0x0000000001dfc0bd in github.com/mattermost/mattermost-server/v5/app.(*App).InvalidateWebConnSessionCacheForUser
at ./app/web_hub.go:109
2 0x0000000001db1be5 in github.com/mattermost/mattermost-server/v5/app.(*App).ClearSessionCacheForUserSkipClusterSend
at ./app/session.go:209
3 0x0000000001db1763 in github.com/mattermost/mattermost-server/v5/app.(*App).ClearSessionCacheForUser
at ./app/session.go:170
4 0x0000000001db2d2f in github.com/mattermost/mattermost-server/v5/app.(*App).RevokeSession
at ./app/session.go:275
5 0x0000000001db2c09 in github.com/mattermost/mattermost-server/v5/app.(*App).RevokeSessionById
at ./app/session.go:260
6 0x0000000001daf442 in github.com/mattermost/mattermost-server/v5/app.(*App).GetSession
at ./app/session.go:93
7 0x0000000001df93f4 in github.com/mattermost/mattermost-server/v5/app.(*WebConn).IsAuthenticated
at ./app/web_conn.go:271
8 0x0000000001dfa29b in github.com/mattermost/mattermost-server/v5/app.(*WebConn).shouldSendEvent
at ./app/web_conn.go:323
9 0x0000000001e2667e in github.com/mattermost/mattermost-server/v5/app.(*Hub).Start.func1.3 // starting from hub
at ./app/web_hub.go:491
10 0x0000000001e27c01 in github.com/mattermost/mattermost-server/v5/app.(*Hub).Start.func1
at ./app/web_hub.go:504
11 0x0000000001e27ee2 in github.com/mattermost/mattermost-server/v5/app.(*Hub).Start.func2
at ./app/web_hub.go:528
12 0x0000000000473811 in runtime.goexit
at /usr/local/go/src/runtime/asm_amd64.s:1373
```
The stars have to align in such a way that the session idle timeout _has_ to happen
_exactly_ when a broadcast is happening for that user. Only then, this code path gets
triggered.
Since this is an extreme rabbit hole of calls, I have not attempted any big
refactors and went with the most sensible approach which is to make the RevokeSessionById
call asynchronous.
There are 2 main reasons:
- It was already treated as an asynchronous call because it happened during an error condition
and we were not checking for the return value anyways.
- Session idle timeout is a relatively infrequent event, so creating unbounded goroutines is not a concern.
As a bonus, we also get to check the error return and log it.
https://mattermost.atlassian.net/browse/MM-27648
* Add a test case
* Fix an incorrect comment
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