rules/manager.go: Fix race between reload and stop

On one relatively large Prometheus instance (1.7M series), I noticed
that upgrades were frequently resulting in Prometheus undergoing crash
recovery on start-up.

On closer examination, I found that Prometheus was panicking on
shutdown.

It seems that our configuration management (or misconfiguration thereof)
is reloading Prometheus then immediately restarting it, which I suspect
is causing this race:

    Sep 21 15:12:42 host systemd[1]: Reloading prometheus monitoring system.
    Sep 21 15:12:42 host prometheus[18734]: time="2016-09-21T15:12:42Z" level=info msg="Loading configuration file /etc/prometheus/config.yaml" source="main.go:221"
    Sep 21 15:12:42 host systemd[1]: Reloaded prometheus monitoring system.
    Sep 21 15:12:44 host systemd[1]: Stopping prometheus monitoring system...
    Sep 21 15:12:44 host prometheus[18734]: time="2016-09-21T15:12:44Z" level=warning msg="Received SIGTERM, exiting gracefully..." source="main.go:203"
    Sep 21 15:12:44 host prometheus[18734]: time="2016-09-21T15:12:44Z" level=info msg="See you next time!" source="main.go:210"
    Sep 21 15:12:44 host prometheus[18734]: time="2016-09-21T15:12:44Z" level=info msg="Stopping target manager..." source="targetmanager.go:90"
    Sep 21 15:12:52 host prometheus[18734]: time="2016-09-21T15:12:52Z" level=info msg="Checkpointing in-memory metrics and chunks..." source="persistence.go:548"
    Sep 21 15:12:56 host prometheus[18734]: time="2016-09-21T15:12:56Z" level=warning msg="Error on ingesting out-of-order samples" numDropped=1 source="scrape.go:467"
    Sep 21 15:12:56 host prometheus[18734]: time="2016-09-21T15:12:56Z" level=error msg="Error adding file watch for \"/etc/prometheus/targets\": no such file or directory" source="file.go:84"
    Sep 21 15:12:56 host prometheus[18734]: time="2016-09-21T15:12:56Z" level=error msg="Error adding file watch for \"/etc/prometheus/targets\": no such file or directory" source="file.go:84"
    Sep 21 15:13:01 host prometheus[18734]: time="2016-09-21T15:13:01Z" level=info msg="Stopping rule manager..." source="manager.go:366"
    Sep 21 15:13:01 host prometheus[18734]: time="2016-09-21T15:13:01Z" level=info msg="Rule manager stopped." source="manager.go:372"
    Sep 21 15:13:01 host prometheus[18734]: time="2016-09-21T15:13:01Z" level=info msg="Stopping notification handler..." source="notifier.go:325"
    Sep 21 15:13:01 host prometheus[18734]: time="2016-09-21T15:13:01Z" level=info msg="Stopping local storage..." source="storage.go:381"
    Sep 21 15:13:01 host prometheus[18734]: time="2016-09-21T15:13:01Z" level=info msg="Stopping maintenance loop..." source="storage.go:383"
    Sep 21 15:13:01 host prometheus[18734]: panic: close of closed channel
    Sep 21 15:13:01 host prometheus[18734]: goroutine 7686074 [running]:
    Sep 21 15:13:01 host prometheus[18734]: panic(0xba57a0, 0xc60c42b500)
    Sep 21 15:13:01 host prometheus[18734]: /usr/local/go/src/runtime/panic.go:500 +0x1a1
    Sep 21 15:13:01 host prometheus[18734]: github.com/prometheus/prometheus/rules.(*Manager).ApplyConfig.func1(0xc6645a9901, 0xc420271ef0, 0xc420338ed0, 0xc60c42b4f0, 0xc6645a9900)
    Sep 21 15:13:01 host prometheus[18734]: /home/build/packages/prometheus/tmp/build/gopath/src/github.com/prometheus/prometheus/rules/manager.go:412 +0x3c
    Sep 21 15:13:01 host prometheus[18734]: created by github.com/prometheus/prometheus/rules.(*Manager).ApplyConfig
    Sep 21 15:13:01 host prometheus[18734]: /home/build/packages/prometheus/tmp/build/gopath/src/github.com/prometheus/prometheus/rules/manager.go:423 +0x56b
    Sep 21 15:13:03 host systemd[1]: prometheus.service: main process exited, code=exited, status=2/INVALIDARGUMENT

rules/manager.go

@@ -365,6 +365,9 @@ func (m *Manager) Run() {
 
 // Stop the rule manager's rule evaluation cycles.
 func (m *Manager) Stop() {
+	m.mtx.Lock()
+	defer m.mtx.Unlock()
+
 	log.Info("Stopping rule manager...")
 
 	for _, eg := range m.groups {