Add initial support for YAML configuration files

Signed-off-by: Ben Reedy <breed808@breed808.com>

config/config.go

@@ -0,0 +1,82 @@
+// Copyright 2018 Prometheus Team
+// 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.
+
+package config
+
+import (
+	"io/ioutil"
+	"os"
+
+	"github.com/prometheus/common/log"
+	"gopkg.in/alecthomas/kingpin.v2"
+	"gopkg.in/yaml.v2"
+)
+
+type getFlagger interface {
+	GetFlag(name string) *kingpin.FlagClause
+}
+
+// Resolver represents a configuration file resolver for kingpin.
+type Resolver struct {
+	flags map[string]string
+}
+
+// NewResolver returns a Resolver structure.
+func NewResolver(file string) (*Resolver, error) {
+	flags := map[string]string{}
+	log.Infof("Loading configuration file: %v", file)
+	if _, err := os.Stat(file); err != nil {
+		return nil, err
+	}
+	b, err := ioutil.ReadFile(file)
+	if err != nil {
+		return nil, err
+	}
+
+	var m map[string]string
+	err = yaml.Unmarshal(b, &m)
+	if err != nil {
+		return nil, err
+	}
+	for k, v := range m {
+		if _, ok := flags[k]; !ok {
+			flags[k] = v
+		}
+	}
+	return &Resolver{flags: flags}, nil
+}
+
+func (c *Resolver) setDefault(v getFlagger) {
+	for name, value := range c.flags {
+		f := v.GetFlag(name)
+		if f != nil {
+			f.Default(value)
+		}
+	}
+}
+
+// Bind sets active flags with their default values from the configuration file(s).
+func (c *Resolver) Bind(app *kingpin.Application, args []string) error {
+	// Parse the command line arguments to get the selected command.
+	pc, err := app.ParseContext(args)
+	if err != nil {
+		return err
+	}
+
+	c.setDefault(app)
+	if pc.SelectedCommand != nil {
+		c.setDefault(pc.SelectedCommand)
+	}
+
+	return nil
+}

exporter.go

@@ -18,6 +18,7 @@ import (
 
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus-community/windows_exporter/collector"
+	"github.com/prometheus-community/windows_exporter/config"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/prometheus/common/log"
@@ -269,7 +270,11 @@ func initWbem() {
 
 func main() {
 	var (
-		app           = kingpin.New("windows_exporter", "")
+		app        = kingpin.New("windows_exporter", "")
+		configFile = app.Flag(
+			"config.file",
+			"YAML configuration file to use. Values set in this file will override flags.",
+		).Default(os.ExpandEnv("$ProgramFiles\\windows_exporter\\config.yml")).String()
 		listenAddress = app.Flag(
 			"telemetry.addr",
 			"host:port for exporter.",
@@ -299,7 +304,26 @@ func main() {
 	log.AddFlags(app)
 	app.Version(version.Print("windows_exporter"))
 	app.HelpFlag.Short('h')
-	app.Parse(os.Args[1:])
+
+	// Load values from configuration file(s). Executable flags must first be parsed, in order
+	// to load the specified file(s).
+	_, err := app.Parse(os.Args[1:])
+	if err != nil {
+		log.Fatalf("%v\n", err)
+	}
+	resolver, err := config.NewResolver(*configFile)
+	if err != nil {
+		log.Fatalf("could not load config file: %v\n", err)
+	}
+	err = resolver.Bind(app, os.Args[1:])
+	if err != nil {
+		log.Fatalf("%v\n", err)
+	}
+	// Parse flags once more to include those discovered in configuration file(s).
+	_, err = app.Parse(os.Args[1:])
+	if err != nil {
+		log.Fatalf("%v\n", err)
+	}
 
 	if *printCollectors {
 		collectors := collector.Available()

go.mod

@@ -14,4 +14,5 @@ require (
 	github.com/prometheus/common v0.2.0
 	golang.org/x/sys v0.0.0-20190507160741-ecd444e8653b
 	gopkg.in/alecthomas/kingpin.v2 v2.2.6
+	gopkg.in/yaml.v2 v2.2.1
 )

go.sum

@@ -69,4 +69,5 @@ golang.org/x/sys v0.0.0-20190507160741-ecd444e8653b/go.mod h1:h1NjWce9XRLGQEsW7w
 gopkg.in/alecthomas/kingpin.v2 v2.2.6 h1:jMFz6MfLP0/4fUyZle81rXUoxOBFi19VUFKVDOQfozc=
 gopkg.in/alecthomas/kingpin.v2 v2.2.6/go.mod h1:FMv+mEhP44yOT+4EoQTLFTRgOQ1FBLkstjWtayDeSgw=
 gopkg.in/check.v1 v0.0.0-20161208181325-20d25e280405/go.mod h1:Co6ibVJAznAaIkqp8huTwlJQCZ016jof/cbN4VW5Yz0=
+gopkg.in/yaml.v2 v2.2.1 h1:mUhvW9EsL+naU5Q3cakzfE91YhliOondGd6ZrsDBHQE=
 gopkg.in/yaml.v2 v2.2.1/go.mod h1:hI93XBmqTisBFMUTm0b8Fm+jr3Dg1NNxqwp+5A1VGuI=