Adding Varnish HTTP Cache input plugin (#1173)

* Adding Varnish HTTP Cache input plugin

* Applying PR feedback

- Linked to varnish in input README
- Updated README/CHANGELOG
- Cleaned up sampleConfig to remove formatting
- Shorted lines to 80 chars (except where test input requires long strings)
- Using internal.RunTimeout to wrap call to varnishtat
- Added dummy file for windows

plugins/inputs/varnish/varnish.go

@@ -0,0 +1,164 @@
+// +build !windows
+
+package varnish
+
+import (
+	"bufio"
+	"bytes"
+	"fmt"
+	"os"
+	"os/exec"
+	"strconv"
+	"strings"
+
+	"github.com/influxdata/telegraf"
+	"github.com/influxdata/telegraf/internal"
+	"github.com/influxdata/telegraf/plugins/inputs"
+	"time"
+)
+
+const (
+	kwAll = "all"
+)
+
+// Varnish is used to store configuration values
+type Varnish struct {
+	Stats  []string
+	Binary string
+}
+
+var defaultStats = []string{"MAIN.cache_hit", "MAIN.cache_miss", "MAIN.uptime"}
+var defaultBinary = "/usr/bin/varnishstat"
+
+var varnishSampleConfig = `
+  ## The default location of the varnishstat binary can be overridden with:
+  binary = "/usr/bin/varnishstat"
+
+  ## By default, telegraf gather stats for 3 metric points.
+  ## Setting stats will override the defaults shown below.
+  ## stats may also be set to ["all"], which will collect all stats
+  stats = ["MAIN.cache_hit", "MAIN.cache_miss", "MAIN.uptime"]
+`
+
+func (s *Varnish) Description() string {
+	return "A plugin to collect stats from Varnish HTTP Cache"
+}
+
+// SampleConfig displays configuration instructions
+func (s *Varnish) SampleConfig() string {
+	return fmt.Sprintf(varnishSampleConfig, strings.Join(defaultStats, "\",\""))
+}
+
+func (s *Varnish) setDefaults() {
+	if len(s.Stats) == 0 {
+		s.Stats = defaultStats
+	}
+
+	if s.Binary == "" {
+		s.Binary = defaultBinary
+	}
+}
+
+// Builds a filter function that will indicate whether a given stat should
+// be reported
+func (s *Varnish) statsFilter() func(string) bool {
+	s.setDefaults()
+
+	// Build a set for constant-time lookup of whether stats should be included
+	filter := make(map[string]struct{})
+	for _, s := range s.Stats {
+		filter[s] = struct{}{}
+	}
+
+	// Create a function that respects the kwAll by always returning true
+	// if it is set
+	return func(stat string) bool {
+		if s.Stats[0] == kwAll {
+			return true
+		}
+
+		_, found := filter[stat]
+		return found
+	}
+}
+
+// Shell out to varnish_stat and return the output
+var varnishStat = func(cmdName string) (*bytes.Buffer, error) {
+	cmdArgs := []string{"-1"}
+
+	cmd := exec.Command(cmdName, cmdArgs...)
+	var out bytes.Buffer
+	cmd.Stdout = &out
+	err := internal.RunTimeout(cmd, time.Millisecond*200)
+	if err != nil {
+		return &out, fmt.Errorf("error running varnishstat: %s", err)
+	}
+
+	return &out, nil
+}
+
+// Gather collects the configured stats from varnish_stat and adds them to the
+// Accumulator
+//
+// The prefix of each stat (eg MAIN, MEMPOOL, LCK, etc) will be used as a
+// 'section' tag and all stats that share that prefix will be reported as fields
+// with that tag
+func (s *Varnish) Gather(acc telegraf.Accumulator) error {
+	s.setDefaults()
+	out, err := varnishStat(s.Binary)
+	if err != nil {
+		return fmt.Errorf("error gathering metrics: %s", err)
+	}
+
+	statsFilter := s.statsFilter()
+	sectionMap := make(map[string]map[string]interface{})
+	scanner := bufio.NewScanner(out)
+	for scanner.Scan() {
+		cols := strings.Fields(scanner.Text())
+		if len(cols) < 2 {
+			continue
+		}
+		if !strings.Contains(cols[0], ".") {
+			continue
+		}
+
+		stat := cols[0]
+		value := cols[1]
+
+		if !statsFilter(stat) {
+			continue
+		}
+
+		parts := strings.SplitN(stat, ".", 2)
+		section := parts[0]
+		field := parts[1]
+
+		// Init the section if necessary
+		if _, ok := sectionMap[section]; !ok {
+			sectionMap[section] = make(map[string]interface{})
+		}
+
+		sectionMap[section][field], err = strconv.Atoi(value)
+		if err != nil {
+			fmt.Fprintf(os.Stderr, "Expected a numeric value for %s = %v\n",
+				stat, value)
+		}
+	}
+
+	for section, fields := range sectionMap {
+		tags := map[string]string{
+			"section": section,
+		}
+		if len(fields) == 0 {
+			continue
+		}
+
+		acc.AddFields("varnish", fields, tags)
+	}
+
+	return nil
+}
+
+func init() {
+	inputs.Add("varnish", func() telegraf.Input { return &Varnish{} })
+}