WatchConfig and Kubernetes (#284)

Support override of symlink to config file
Include tests for WatchConfig of regular files, as well
as config file which links to a folder which is itself a
link to another folder in the same "watch dir" (the way
Kubernetes exposes config files from ConfigMaps mounted
on a volume in a Pod)

Also:
- Add synchronization with WaitGroup to ensure that the WatchConfig
is properly started before returning
- Remove the watcher when the Config file is removed.

Fixes #284

Signed-off-by: Xavier Coulon <xcoulon@redhat.com>

.gitignore

@@ -22,3 +22,8 @@ _testmain.go
 *.exe
 *.test
 *.bench
+
+.vscode
+
+# exclude dependencies in the `/vendor` folder
+vendor

viper.go

@@ -30,6 +30,7 @@ import (
 	"path/filepath"
 	"reflect"
 	"strings"
+	"sync"
 	"time"
 
 	yaml "gopkg.in/yaml.v2"
@@ -260,13 +261,14 @@ func (v *Viper) OnConfigChange(run func(in fsnotify.Event)) {
 
 func WatchConfig() { v.WatchConfig() }
 func (v *Viper) WatchConfig() {
+	wg := sync.WaitGroup{}
+	wg.Add(1)
 	go func() {
 		watcher, err := fsnotify.NewWatcher()
 		if err != nil {
 			log.Fatal(err)
 		}
 		defer watcher.Close()
-
 		// we have to watch the entire directory to pick up renames/atomic saves in a cross-platform way
 		filename, err := v.getConfigFile()
 		if err != nil {
@@ -276,31 +278,46 @@ func (v *Viper) WatchConfig() {
 
 		configFile := filepath.Clean(filename)
 		configDir, _ := filepath.Split(configFile)
+		realConfigFile, _ := filepath.EvalSymlinks(filename)
 
 		done := make(chan bool)
 		go func() {
+		loop:
 			for {
 				select {
 				case event := <-watcher.Events:
-					// we only care about the config file
+					currentConfigFile, _ := filepath.EvalSymlinks(filename)
-					if filepath.Clean(event.Name) == configFile {
+					// we only care about the config file with the following cases:
-						if event.Op&fsnotify.Write == fsnotify.Write || event.Op&fsnotify.Create == fsnotify.Create {
+					// 1 - if the config file was modified or created
+					// 2 - if the real path to the config file changed (eg: k8s ConfigMap replacement)
+					if (filepath.Clean(event.Name) == configFile &&
+						(event.Op&fsnotify.Write == fsnotify.Write || event.Op&fsnotify.Create == fsnotify.Create)) ||
+						(currentConfigFile != "" && currentConfigFile != realConfigFile) {
+						realConfigFile = currentConfigFile
 						err := v.ReadInConfig()
 						if err != nil {
-								log.Println("error:", err)
+							log.Println("error reading file:", err.Error())
 						}
+						if v.onConfigChange != nil {
 							v.onConfigChange(event)
 						}
+					} else if filepath.Clean(event.Name) == configFile &&
+						event.Op&fsnotify.Remove == fsnotify.Remove {
+						done <- true
+						break loop
 					}
-				case err := <-watcher.Errors:
-					log.Println("error:", err)
-				}
-			}
-		}()
 
-		watcher.Add(configDir)
+				case err := <-watcher.Errors:
-		<-done
+					log.Printf("watcher error: %v\n", err)
+				}
+			}
 		}()
+		watcher.Add(configDir)
+		wg.Done() // done initalizing the watch in this go routine, so the parent routine can move on...
+		<-done    // block until the watched file is removed...
+	}()
+	// make sure that the go routine above fully started before returning
+	wg.Wait()
 }
 
 // SetConfigFile explicitly defines the path, name and extension of the config file.

viper_test.go

@@ -11,18 +11,23 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"path"
 	"reflect"
+	"runtime"
 	"sort"
 	"strings"
+	"sync"
 	"testing"
 	"time"
 
+	"github.com/fsnotify/fsnotify"
 	"github.com/spf13/afero"
 	"github.com/spf13/cast"
 
 	"github.com/spf13/pflag"
 	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
 )
 
 var yamlExample = []byte(`Hacker: true
@@ -1368,6 +1373,104 @@ func doTestCaseInsensitive(t *testing.T, typ, config string) {
 
 }
 
+func newViperWithConfigFile(t *testing.T) (*Viper, string, func()) {
+	watchDir, err := ioutil.TempDir("", "")
+	require.Nil(t, err)
+	configFile := path.Join(watchDir, "config.yaml")
+	err = ioutil.WriteFile(configFile, []byte("foo: bar\n"), 0640)
+	require.Nil(t, err)
+	cleanup := func() {
+		os.RemoveAll(watchDir)
+	}
+	v := New()
+	v.SetConfigFile(configFile)
+	err = v.ReadInConfig()
+	require.Nil(t, err)
+	require.Equal(t, "bar", v.Get("foo"))
+	return v, configFile, cleanup
+}
+
+func newViperWithSymlinkedConfigFile(t *testing.T) (*Viper, string, string, func()) {
+	watchDir, err := ioutil.TempDir("", "")
+	require.Nil(t, err)
+	dataDir1 := path.Join(watchDir, "data1")
+	err = os.Mkdir(dataDir1, 0777)
+	require.Nil(t, err)
+	realConfigFile := path.Join(dataDir1, "config.yaml")
+	t.Logf("Real config file location: %s\n", realConfigFile)
+	err = ioutil.WriteFile(realConfigFile, []byte("foo: bar\n"), 0640)
+	require.Nil(t, err)
+	cleanup := func() {
+		os.RemoveAll(watchDir)
+	}
+	// now, symlink the tm `data1` dir to `data` in the baseDir
+	os.Symlink(dataDir1, path.Join(watchDir, "data"))
+	// and link the `<watchdir>/datadir1/config.yaml` to `<watchdir>/config.yaml`
+	configFile := path.Join(watchDir, "config.yaml")
+	os.Symlink(path.Join(watchDir, "data", "config.yaml"), configFile)
+	fmt.Printf("Config file location: %s\n", path.Join(watchDir, "config.yaml"))
+	// init Viper
+	v := New()
+	v.SetConfigFile(configFile)
+	err = v.ReadInConfig()
+	require.Nil(t, err)
+	require.Equal(t, "bar", v.Get("foo"))
+	return v, watchDir, configFile, cleanup
+}
+
+func TestWatchFile(t *testing.T) {
+	t.Run("file content changed", func(t *testing.T) {
+		// given a `config.yaml` file being watched
+		v, configFile, cleanup := newViperWithConfigFile(t)
+		defer cleanup()
+		wg := sync.WaitGroup{}
+		v.WatchConfig()
+		v.OnConfigChange(func(in fsnotify.Event) {
+			t.Logf("config file changed")
+			wg.Done()
+		})
+		wg.Add(1)
+		// when overwriting the file and waiting for the custom change notification handler to be triggered
+		err := ioutil.WriteFile(configFile, []byte("foo: baz\n"), 0640)
+		wg.Wait()
+		// then the config value should have changed
+		require.Nil(t, err)
+		assert.Equal(t, "baz", v.Get("foo"))
+	})
+
+	t.Run("link to real file changed (à la Kubernetes)", func(t *testing.T) {
+		// skip if not executed on Linux
+		if runtime.GOOS != "linux" {
+			t.Skipf("Skipping test as symlink replacements don't work on non-linux environment...")
+		}
+		v, watchDir, _, _ := newViperWithSymlinkedConfigFile(t)
+		// defer cleanup()
+		wg := sync.WaitGroup{}
+		v.WatchConfig()
+		v.OnConfigChange(func(in fsnotify.Event) {
+			t.Logf("config file changed")
+			wg.Done()
+		})
+		wg.Add(1)
+		// when link to another `config.yaml` file
+		dataDir2 := path.Join(watchDir, "data2")
+		err := os.Mkdir(dataDir2, 0777)
+		require.Nil(t, err)
+		configFile2 := path.Join(dataDir2, "config.yaml")
+		err = ioutil.WriteFile(configFile2, []byte("foo: baz\n"), 0640)
+		require.Nil(t, err)
+		// change the symlink using the `ln -sfn` command
+		err = exec.Command("ln", "-sfn", dataDir2, path.Join(watchDir, "data")).Run()
+		require.Nil(t, err)
+		wg.Wait()
+		// then
+		require.Nil(t, err)
+		assert.Equal(t, "baz", v.Get("foo"))
+
+	})
+
+}
+
 func BenchmarkGetBool(b *testing.B) {
 	key := "BenchmarkGetBool"
 	v = New()