config: added ReadAndWatchFile function

This function will update configuration whenever a file changes.

This change introduces many data races (actually, it inherits these data
races from howeyc/fsnotify). This issue has been reported as
howeyc/fsnotify#29.

Closes #2.

config.go

@@ -8,6 +8,7 @@ package config
 
 import (
 	"fmt"
+	"github.com/howeyc/fsnotify"
 	"io"
 	"io/ioutil"
 	"launchpad.net/goyaml"
@@ -22,6 +23,10 @@ var (
 	mut     sync.RWMutex
 )
 
+func readConfigBytes(data []byte, out interface{}) error {
+	return goyaml.Unmarshal(data, out)
+}
+
 // ReadConfigBytes receives a slice of bytes and builds the internal
 // configuration object.
 //
@@ -30,7 +35,15 @@ var (
 func ReadConfigBytes(data []byte) error {
 	mut.Lock()
 	defer mut.Unlock()
-	return goyaml.Unmarshal(data, &configs)
+	return readConfigBytes(data, &configs)
+}
+
+func readConfigFile(filePath string, out interface{}) error {
+	data, err := ioutil.ReadFile(filePath)
+	if err != nil {
+		return err
+	}
+	return readConfigBytes(data, out)
 }
 
 // ReadConfigFile reads the content of a file and calls ReadConfigBytes to
@@ -39,11 +52,43 @@ func ReadConfigBytes(data []byte) error {
 // It returns error if it can not read the given file or if the file contents
 // is not valid yaml.
 func ReadConfigFile(filePath string) error {
-	data, err := ioutil.ReadFile(filePath)
+	return readConfigFile(filePath, &configs)
+}
+
+// ReadAndWatchConfigFile reads and watchs for changes in the configuration
+// file. Whenever the file change, and its contents are valid YAML, the
+// configuration gets updated. With this function, daemons that use this
+// package may reload configuration without restarting.
+func ReadAndWatchConfigFile(filePath string) error {
+	err := ReadConfigFile(filePath)
+	if err != nil {
+		return err
+	}
+	w, err := fsnotify.NewWatcher()
+	if err != nil {
+		return err
+	}
+	err = w.Watch(filePath)
 	if err != nil {
 		return err
 	}
-	return ReadConfigBytes(data)
+	go func() {
+		for {
+			select {
+			case e := <-w.Event:
+				if e.IsModify() {
+					var tmp map[interface{}]interface{}
+					if readConfigFile(filePath, &tmp) == nil {
+						mut.Lock()
+						configs = tmp
+						mut.Unlock()
+					}
+				}
+			case <-w.Error: // just ignore errors
+			}
+		}
+	}()
+	return nil
 }
 
 // WriteConfigFile writes the configuration to the disc, using the given path.

config_test.go

@@ -7,8 +7,10 @@ package config
 import (
 	. "launchpad.net/gocheck"
 	"os"
+	"os/exec"
 	"runtime"
 	"testing"
+	"time"
 )
 
 func Test(t *testing.T) { TestingT(t) }
@@ -71,6 +73,22 @@ func (s *S) TestConfigFile(c *C) {
 	c.Assert(configs, DeepEquals, expected)
 }
 
+func (s *S) TestWatchConfigFile(c *C) {
+	err := exec.Command("cp", "testdata/config.yml", "/tmp/config-test.yml").Run()
+	c.Assert(err, IsNil)
+	err = ReadAndWatchConfigFile("/tmp/config-test.yml")
+	c.Assert(err, IsNil)
+	c.Assert(configs, DeepEquals, expected)
+	err = exec.Command("cp", "testdata/config2.yml", "/tmp/config-test.yml").Run()
+	c.Assert(err, IsNil)
+	time.Sleep(1e9)
+	expectedAuth := map[interface{}]interface{}{
+		"salt": "xpta",
+		"key":  "sometoken1234",
+	}
+	c.Assert(configs["auth"], DeepEquals, expectedAuth)
+}
+
 func (s *S) TestWriteConfigFile(c *C) {
 	Set("database:host", "127.0.0.1")
 	Set("database:port", 3306)

testdata/config2.yml

@@ -0,0 +1,23 @@
+# Copyright 2013 config authors. All rights reserved.
+# Use of this source code is governed by a BSD-style
+# license that can be found in the LICENSE file.
+
+database:
+  host: 127.0.0.1
+  port: 8080
+auth:
+  salt: xpta
+  key: sometoken1234
+xpto: ble
+istrue: false
+fakebool: foo
+names:
+  - Mary
+  - John
+  - Anthony
+  - Gopher
+multiple-types:
+  - Mary
+  - 50
+  - 5.3
+  - true