Handle multiple NFS device mounts

It's possible to mount an NFS share in multiple locations.
* Duplicates contain the same metric values, so they can be ignored.
* Update fixture.

collector/fixtures/proc/10/mountstats

@@ -17,3 +17,18 @@ device 192.168.1.1:/srv/test mounted on /mnt/nfs/test with fstype nfs4 statvers=
 	        READ: 1298 1298 0 207680 1210292152 6 79386 79407
 	       WRITE: 0 0 0 0 0 0 0 0
 
+device 192.168.1.1:/srv/test mounted on /mnt/nfs/test-dupe with fstype nfs4 statvers=1.1
+	opts:	rw,vers=4.0,rsize=1048576,wsize=1048576,namlen=255,acregmin=3,acregmax=60,acdirmin=30,acdirmax=60,hard,proto=tcp,port=0,timeo=600,retrans=2,sec=sys,clientaddr=192.168.1.5,local_lock=none
+	age:	13968
+	caps:	caps=0xfff7,wtmult=512,dtsize=32768,bsize=0,namlen=255
+	nfsv4:	bm0=0xfdffafff,bm1=0xf9be3e,bm2=0x0,acl=0x0,pnfs=not configured
+	sec:	flavor=1,pseudoflavor=1
+	events:	52 226 0 0 1 13 398 0 0 331 0 47 0 0 77 0 0 77 0 0 0 0 0 0 0 0 0
+	bytes:	1207640230 0 0 0 1210214218 0 295483 0
+	RPC iostats version: 1.0  p/v: 100003/4 (nfs)
+	xprt:	tcp 832 0 1 0 11 6428 6428 0 12154 0 24 26 5726
+	per-op statistics
+	        NULL: 0 0 0 0 0 0 0 0
+	        READ: 1298 1298 0 207680 1210292152 6 79386 79407
+	       WRITE: 0 0 0 0 0 0 0 0
+

collector/mountstats_linux.go

@@ -1,4 +1,4 @@
-// Copyright 2016 The Prometheus Authors
+// Copyright 2017 The Prometheus Authors
 // Licensed under the Apache License, Version 2.0 (the "License");
 // you may not use this file except in compliance with the License.
 // You may obtain a copy of the License at
@@ -15,8 +15,10 @@ package collector
 
 import (
 	"fmt"
+	"sort"
 
 	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/common/log"
 	"github.com/prometheus/procfs"
 )
 
@@ -493,6 +495,7 @@ func (c *mountStatsCollector) Update(ch chan<- prometheus.Metric) error {
 	if err != nil {
 		return fmt.Errorf("failed to parse mountstats: %v", err)
 	}
+	var deviceList []string
 
 	for _, m := range mounts {
 		// For the time being, only NFS statistics are available via this mechanism
@@ -501,8 +504,15 @@ func (c *mountStatsCollector) Update(ch chan<- prometheus.Metric) error {
 			continue
 		}
 
+		sort.Strings(deviceList)
+		i := sort.SearchStrings(deviceList, m.Device)
+		if i < len(deviceList) && deviceList[i] == m.Device {
+			log.Debugf("Skipping duplicate device entry %q", m.Device)
+		} else {
+			deviceList = append(deviceList, m.Device)
 			c.updateNFSStats(ch, m.Device, stats)
 		}
+	}
 
 	return nil
 }