Watch method to track config file changes (#15)

* Watch to track config file changes

* Check missed error in test

Author: DenisPalnitsky

config.go

@@ -2,15 +2,18 @@ package config
 
 import (
 	"encoding/base64"
-	"fmt"
+	"log"
 	"os"
 	"reflect"
 	"strings"
+	"sync"
 
 	"github.com/creasty/defaults"
+	"github.com/fsnotify/fsnotify"
 	"github.com/go-playground/validator/v10"
 	"github.com/iamolegga/enviper"
 	"github.com/pkg/errors"
+	jww "github.com/spf13/jwalterweatherman"
 	"github.com/spf13/pflag"
 	"github.com/spf13/viper"
 )
@@ -41,6 +44,7 @@ type ConfReader struct {
 	configDirs   []string
 	envVarPrefix string
 	Verbose      bool
+	configStruct any
 }
 
 // NewConfReader creates new instance of ConfReader
@@ -70,8 +74,8 @@ func (c *ConfReader) Read(configStruct interface{}) error {
 		return errors.Wrap(err, "failed to set default values")
 	}
 
-	//jww.SetLogThreshold(jww.LevelTrace)
-	//jww.SetStdoutThreshold(jww.LevelTrace)
+	jww.SetLogThreshold(jww.LevelTrace)
+	jww.SetStdoutThreshold(jww.LevelTrace)
 
 	c.viper.SetConfigFile(c.configName)
 
@@ -113,6 +117,8 @@ func (c *ConfReader) Read(configStruct interface{}) error {
 		}
 		return err
 	}
+
+	c.configStruct = configStruct
 	return nil
 }
 
@@ -225,7 +231,7 @@ type flagInfo struct {
 
 func (c *ConfReader) dumpStruct(t reflect.Type, path string, res map[string]*flagInfo) map[string]*flagInfo {
 	if c.Verbose {
-		fmt.Printf("%s: %s", path, t.Name())
+		log.Printf("%s: %s", path, t.Name())
 	}
 	switch t.Kind() {
 	case reflect.Ptr:
@@ -282,8 +288,32 @@ func (c *ConfReader) WithSearchDirs(s ...string) *ConfReader {
 	return c
 }
 
-// WithPrefix sets the prefix for environment variables
+// WithPrefix sets the prefix for environment variables. It adds '_' to the end of the prefix.
+// For example, if prefix is "MYAPP", then environment variable for field "Name" will be "MYAPP_NAME".
 func (c *ConfReader) WithPrefix(prefix string) *ConfReader {
 	c.envVarPrefix = prefix
 	return c
 }
+
+// Watch watches for config changes and reloads config. This method should be called after Read() to make sure that ConfReader konws which struct to reload.
+// Returns a mutex that can be used to synchronize access to the config.
+// If you care about thread safety, call RLock() on the mutex while accessing the config and the RUnlock().
+// This will ensure that the config is not reloaded while you are accessing it.
+func (c *ConfReader) Watch() *sync.RWMutex {
+	if c.configStruct == nil {
+		log.Fatalln("ConfReader: config struct is not set. Call Read before Watch")
+	}
+	rwmutex := &sync.RWMutex{}
+
+	c.viper.WatchConfig()
+	c.viper.OnConfigChange(func(e fsnotify.Event) {
+		rwmutex.Lock()
+		defer rwmutex.Unlock()
+		err := c.Read(c.configStruct)
+		if err != nil {
+			log.Printf("failed to reload config: %s\n", err)
+		}
+
+	})
+	return rwmutex
+}

config_test.go

@@ -121,6 +121,51 @@ func Test_ReadFromJsonFile(t *testing.T) {
 	}
 }
 
+func Test_WatchWithFile(t *testing.T) {
+	resetFlags()
+	nc := &FullConfig{}
+
+	err := os.Mkdir("testdata/tmp", 0755)
+	if err != nil {
+		t.Fatal(err)
+	}
+	err = os.WriteFile("testdata/tmp/changing_file.json", []byte(`{"verbose":"true"}`), 0644)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer os.RemoveAll("testdata/tmp")
+	confReader := NewConfReader("changing_file")
+	confReader.configDirs = []string{"testdata/tmp"}
+	err = confReader.Read(nc)
+	if assert.NoError(t, err) {
+		assert.Equal(t, true, nc.Verbose)
+	}
+	mutex := confReader.Watch()
+
+	t.Run("configChanges", func(t *testing.T) {
+		err = os.WriteFile("testdata/tmp/changing_file.json", []byte(`{"verbose":"false"}`), 0644)
+		if assert.NoError(t, err) {
+			time.Sleep(10 * time.Millisecond)
+			assert.Equal(t, false, nc.Verbose)
+		}
+	})
+
+	t.Run("configStructLock", func(t *testing.T) {
+		mutex.RLock()
+		err = os.WriteFile("testdata/tmp/changing_file.json", []byte(`{"verbose":"true"}`), 0644)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		//time.Sleep(time.Second)
+		assert.Equal(t, false, nc.Verbose)
+
+		mutex.RUnlock()
+		time.Sleep(10 * time.Millisecond)
+		assert.Equal(t, true, nc.Verbose)
+	})
+}
+
 type dmParent struct {
 	GlobalConfig `mapstructure:",squash"`
 	Conf         dmSibling `flag:"notAllowed"`

go.mod

@@ -1,6 +1,6 @@
 module github.com/num30/config
 
-go 1.18
+go 1.19
 
 require (
 	github.com/go-playground/validator/v10 v10.10.1