Add ARP collector for Linux (#540)

* Implement commonalities and linux support for ARP collection

* Add ARP collector to fixtures and run as part of e2e tests

* Bubble up scanner errors

* Use single return values where it makes sense

* Add missing annotation

* Move arp_common into arp_linux

* Add license header to arp_linux.go

* Address initial feedback

* Use strings.Fields instead of strings.Split

* Deal with scanner.Err() rather than throwing away errors

* Check for scan errors in-line before interacting with the entries map

* Don't interact with potentially empty text from scan

* Check for scan errors outside the scan loop

* Add comment about moving procfs parsing

* Add more direct comment

* Update initialism style to match go style guide

* Put function args on the same line

* Add TODO in front of comment about procfs extraction

* Guard against strings.Fields returning an empty slice

* Be more defensive about ARP table format and use upcase more broadly

* Enable the ARP collector by default

* Add ARP collector to the README

* Remove 'entry'

collector/arp_linux.go

@@ -0,0 +1,102 @@
+// 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
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+// +build !noarp
+
+package collector
+
+import (
+	"bufio"
+	"fmt"
+	"io"
+	"os"
+	"strings"
+
+	"github.com/prometheus/client_golang/prometheus"
+)
+
+type arpCollector struct {
+	entries *prometheus.Desc
+}
+
+func init() {
+	Factories["arp"] = NewARPCollector
+}
+
+// NewARPCollector returns a new Collector exposing ARP stats.
+func NewARPCollector() (Collector, error) {
+	return &arpCollector{
+		entries: prometheus.NewDesc(
+			prometheus.BuildFQName(Namespace, "arp", "entries"),
+			"ARP entries by device",
+			[]string{"device"}, nil,
+		),
+	}, nil
+}
+
+func getARPEntries() (map[string]uint32, error) {
+	file, err := os.Open(procFilePath("net/arp"))
+	if err != nil {
+		return nil, err
+	}
+	defer file.Close()
+
+	entries, err := parseARPEntries(file)
+	if err != nil {
+		return nil, err
+	}
+
+	return entries, nil
+}
+
+// TODO: This should get extracted to the github.com/prometheus/procfs package
+// to support more complete parsing of /proc/net/arp. Instead of adding
+// more fields to this function's return values it should get moved and
+// changed to support each field.
+func parseARPEntries(data io.Reader) (map[string]uint32, error) {
+	scanner := bufio.NewScanner(data)
+	entries := make(map[string]uint32)
+
+	for scanner.Scan() {
+		columns := strings.Fields(scanner.Text())
+
+		if len(columns) < 6 {
+			return nil, fmt.Errorf("unexpected ARP table format")
+		}
+
+		if columns[0] != "IP" {
+			deviceIndex := len(columns) - 1
+			entries[columns[deviceIndex]]++
+		}
+	}
+
+	if err := scanner.Err(); err != nil {
+		return nil, fmt.Errorf("failed to parse ARP info: %s", err)
+	}
+
+	return entries, nil
+}
+
+func (c *arpCollector) Update(ch chan<- prometheus.Metric) error {
+	entries, err := getARPEntries()
+	if err != nil {
+		return fmt.Errorf("could not get ARP entries: %s", err)
+	}
+
+	for device, entryCount := range entries {
+		ch <- prometheus.MustNewConstMetric(
+			c.entries, prometheus.GaugeValue, float64(entryCount), device)
+	}
+
+	return nil
+}