Set a timeout for calls to input.Gather

currently the input interface does not have any methods for killing a
running Gather call, so there is nothing we can do but log a "FATAL
ERROR" and move on. This will at least give some visibility into the
plugin that is acting up.

Open questions:

- should the telegraf process die and exit when this happens? This might
  be a better idea than leaving around the dead process.
- should the input interface have a Kill() method? I suspect not, since
  most inputs wouldn't have a way of killing themselves anyways.

closes #1230

agent/accumulator.go

@@ -4,7 +4,6 @@ import (
 	"fmt"
 	"log"
 	"math"
-	"sync"
 	"time"
 
 	"github.com/influxdata/telegraf"
@@ -22,8 +21,6 @@ func NewAccumulator(
 }
 
 type accumulator struct {
-	sync.Mutex
-
 	metrics chan telegraf.Metric
 
 	defaultTags map[string]string

agent/agent.go

@@ -136,10 +136,7 @@ func (a *Agent) gatherParallel(metricC chan telegraf.Metric) error {
 				}
 			}
 
-			if err := input.Input.Gather(acc); err != nil {
-				log.Printf("Error in input [%s]: %s", input.Name, err)
-			}
-
+			gatherWithTimeout(input, acc, a.Config.Agent.Interval.Duration)
 		}(input)
 	}
 
@@ -176,9 +173,7 @@ func (a *Agent) gatherSeparate(
 		acc.SetDebug(a.Config.Agent.Debug)
 		acc.setDefaultTags(a.Config.Tags)
 
-		if err := input.Input.Gather(acc); err != nil {
-			log.Printf("Error in input [%s]: %s", input.Name, err)
-		}
+		gatherWithTimeout(input, acc, input.Config.Interval)
 
 		elapsed := time.Since(start)
 		if !a.Config.Agent.Quiet {
@@ -199,6 +194,34 @@ func (a *Agent) gatherSeparate(
 	}
 }
 
+// gatherWithTimeout gathers from the given input, with the given timeout.
+//   if Gather() times out, unfortunately there isn't much it can do, and so it
+//   simply returns and logs a "FATAL ERROR"
+func gatherWithTimeout(
+	input *internal_models.RunningInput,
+	acc *accumulator,
+	timeout time.Duration,
+) {
+	timer := time.NewTimer(timeout)
+	done := make(chan error)
+	go func() {
+		done <- input.Input.Gather(acc)
+	}()
+
+	select {
+	case err := <-done:
+		if err != nil {
+			log.Printf("ERROR in input [%s]: %s", input.Name, err)
+		}
+		return
+	case <-timer.C:
+		log.Printf("FATAL ERROR: input [%s] took longer than interval (%s)"+
+			" to run. Leaving behind a hung process and moving on.",
+			input.Name, timeout)
+		return
+	}
+}
+
 // Test verifies that we can 'Gather' from all inputs with their configured
 // Config struct
 func (a *Agent) Test() error {