Handle process termination during read from /proc

Fixes influxdata#2815.

plugins/inputs/system/processes.go

@@ -12,6 +12,7 @@ import (
 	"path/filepath"
 	"runtime"
 	"strconv"
+	"syscall"
 
 	"github.com/influxdata/telegraf"
 	"github.com/influxdata/telegraf/plugins/inputs"
@@ -195,6 +196,13 @@ func readProcFile(filename string) ([]byte, error) {
 		if os.IsNotExist(err) {
 			return nil, nil
 		}
+
+		// Reading from /proc/<PID> fails with ESRCH if the process has
+		// been terminated between open() and read().
+		if perr, ok := err.(*os.PathError); ok && perr.Err == syscall.ESRCH {
+			return nil, nil
+		}
+
 		return nil, err
 	}