worker/state: intermittent failure due to "Session already closed"

[Andrew Wilkins]

Two test runs failed in a row:
http://ci.jujucharms.com/job/github-merge-juju/733/console
http://ci.jujucharms.com/job/github-merge-juju/734/console

due to the same issue:

[xenial] panic: Session already closed
[xenial]
[xenial] goroutine 625 [running]:
[xenial] gopkg.in/mgo%2ev2.(*Session).cluster(0xc42051a4e0, 0x109c008)
[xenial] /home/ubuntu/juju-core_2.3.2/src/gopkg.in/mgo.v2/session.go:1624 +0x56
[xenial] gopkg.in/mgo%2ev2.(*Session).acquireSocket(0xc42051a4e0, 0xc42028d401, 0x0, 0x0, 0x0)
[xenial] /home/ubuntu/juju-core_2.3.2/src/gopkg.in/mgo.v2/session.go:4440 +0x201
[xenial] gopkg.in/mgo%2ev2.(*Query).One(0xc42015ba00, 0xe94cc0, 0xc420334860, 0x0, 0x0)
[xenial] /home/ubuntu/juju-core_2.3.2/src/gopkg.in/mgo.v2/session.go:3087 +0xce
[xenial] github.com/juju/juju/state/watcher.(*TxnWatcher).initLastId(0xc4206467e0, 0x2, 0xc42001c070)
[xenial] /home/ubuntu/juju-core_2.3.2/src/github.com/juju/juju/state/watcher/txnwatcher.go:226 +0xc5
[xenial] github.com/juju/juju/state/watcher.(*TxnWatcher).loop(0xc4206467e0, 0xc420084fc0, 0x96caab)
[xenial] /home/ubuntu/juju-core_2.3.2/src/github.com/juju/juju/state/watcher/txnwatcher.go:174 +0x40
[xenial] github.com/juju/juju/state/watcher.NewTxnWatcher.func1(0xc4206467e0)
[xenial] /home/ubuntu/juju-core_2.3.2/src/github.com/juju/juju/state/watcher/txnwatcher.go:129 +0x2f
[xenial] created by github.com/juju/juju/state/watcher.NewTxnWatcher
[xenial] /home/ubuntu/juju-core_2.3.2/src/github.com/juju/juju/state/watcher/txnwatcher.go:128 +0x166
[xenial] FAIL github.com/juju/juju/worker/state 2.299s

[Andrew Wilkins (axwalk)]

https://github.com/juju/juju/pull/8266

[Canonical Juju QA Bot (juju-qa-bot)]

This bug has not been updated in 2 years, so we're marking it Low importance. If you believe this is incorrect, please update the importance.