Consul SD shutdown race condition resulting in crash

[juliusv]

As reported in https://groups.google.com/forum/#!topic/prometheus-developers/NbouN8u1S8Y, Prometheus 1.2.1 crashes with a panic due to nil-channel send upon shutdown in the Consul SD code. Looking at the code, the same problem seems to still exist in master.

The select in watch():

https://github.com/prometheus/prometheus/blob/master/retrieval/discovery/consul/consul.go#L302-L306

...may try to write into ch at the moment when ch is closed by Run() (which spawns the watch()es as goroutines). Run() should wait for the watch() goroutines to terminate before returning and closing ch.

[lock]

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