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
+}