Currently when an evalbroker is disabled, it still recieves delayed
enqueues via log application in the fsm. This causes an ever growing
heap of evaluations that will never be drained, and can cause memory
issues in larger clusters, or when left running for an extended period
of time without a leader election.
This commit prevents the enqueuing of evaluations while we are
disabled, and relies on the leader restoreEvals routine to handle
reconciling state during a leadership transition.
Existing dequeues during an Enabled->Disabled broker state transition are
handled by the enqueueLocked function dropping evals.
Primarily a cleanup commit, however, currently there is a potential race
condition (that I'm not sure we've ever actually hit) during a flapping
SetEnabled/Disabled state where we may never correctly restart the eval
broker, if it was being called from multiple routines.
Similar to previous commits the delayed eval update chan was set and
access from different goroutines causing a race. Passing the chan on the
stack resolves the race.
Race output from `go test -race -run 'Server_RPC$'` in nomad/
```
==================
WARNING: DATA RACE
Write at 0x00c000339150 by goroutine 63:
github.com/hashicorp/nomad/nomad.(*EvalBroker).flush()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/eval_broker.go:708
+0x3dc
github.com/hashicorp/nomad/nomad.(*EvalBroker).SetEnabled()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/eval_broker.go:174
+0xc4
github.com/hashicorp/nomad/nomad.(*Server).revokeLeadership()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:718
+0x1fd
github.com/hashicorp/nomad/nomad.(*Server).leaderLoop()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:122
+0x95d
github.com/hashicorp/nomad/nomad.(*Server).monitorLeadership.func1()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:72
+0x6c
Previous read at 0x00c000339150 by goroutine 73:
github.com/hashicorp/nomad/nomad.(*EvalBroker).runDelayedEvalsWatcher()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/eval_broker.go:771
+0x176
Goroutine 63 (running) created at:
github.com/hashicorp/nomad/nomad.(*Server).monitorLeadership()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:70
+0x269
Goroutine 73 (running) created at:
github.com/hashicorp/nomad/nomad.(*EvalBroker).SetEnabled()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/eval_broker.go:170
+0x173
github.com/hashicorp/nomad/nomad.(*Server).establishLeadership()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:207
+0x355
github.com/hashicorp/nomad/nomad.(*Server).leaderLoop()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:117
+0x82e
github.com/hashicorp/nomad/nomad.(*Server).monitorLeadership.func1()
/home/schmichael/go/src/github.com/hashicorp/nomad/nomad/leader.go:72
+0x6c
==================
```
