Fix possible deadlock when output cannot write. (#2610)

CHANGELOG.md

@@ -89,6 +89,7 @@ be deprecated eventually.
 - [#1827](https://github.com/influxdata/telegraf/issues/1827): Fix Windows Performance Counters multi instance identifier
 - [#2576](https://github.com/influxdata/telegraf/pull/2576): Add write timeout to Riemann output
 - [#2596](https://github.com/influxdata/telegraf/pull/2596): fix timestamp parsing on prometheus plugin
+- [#2610](https://github.com/influxdata/telegraf/pull/2610): Fix deadlock when output cannot write
 
 
 ## v1.2.1 [2017-02-01]

internal/buffer/buffer.go

@@ -45,9 +45,11 @@ func (b *Buffer) Add(metrics ...telegraf.Metric) {
 		select {
 		case b.buf <- metrics[i]:
 		default:
+			b.mu.Lock()
 			MetricsDropped.Incr(1)
 			<-b.buf
 			b.buf <- metrics[i]
+			b.mu.Unlock()
 		}
 	}
 }