add minimal config interface and file implementation

config/file/config.go

@@ -0,0 +1,99 @@
+// inspired by / taken from github.com/spf13/viper
+
+package file
+
+import (
+	"context"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"sync"
+
+	"github.com/fsnotify/fsnotify"
+
+	"go.linka.cloud/grpc/config"
+	"go.linka.cloud/grpc/logger"
+)
+
+func NewConfig(path string) (config.Config, error) {
+	if _, err := os.Stat(path); err != nil {
+		return nil, err
+	}
+	return &file{path: path}, nil
+}
+
+type file struct {
+	path string
+}
+
+func (c *file) Read() ([]byte, error) {
+	return ioutil.ReadFile(c.path)
+}
+
+// Watch listen for config changes and send updated content to the updates channel
+func (c *file) Watch(ctx context.Context, updates chan<- []byte) error {
+	log := logger.From(ctx)
+	errs := make(chan error, 1)
+	go func() {
+		watcher, err := fsnotify.NewWatcher()
+		if err != nil {
+			errs <- err
+			return
+		}
+		defer watcher.Close()
+		// we have to watch the entire directory to pick up renames/atomic saves in a cross-platform way
+		configFile := filepath.Clean(c.path)
+		configDir, _ := filepath.Split(configFile)
+		realConfigFile, _ := filepath.EvalSymlinks(c.path)
+
+		eventsWG := sync.WaitGroup{}
+		eventsWG.Add(1)
+		go func() {
+			for {
+				select {
+				case event, ok := <-watcher.Events:
+					if !ok { // 'Events' channel is closed
+						eventsWG.Done()
+						return
+					}
+					currentConfigFile, _ := filepath.EvalSymlinks(c.path)
+					// we only care about the config file with the following cases:
+					// 1 - if the config file was modified or created
+					// 2 - if the real path to the config file changed (eg: k8s ConfigMap replacement)
+					const writeOrCreateMask = fsnotify.Write | fsnotify.Create
+					if (filepath.Clean(event.Name) == configFile &&
+						event.Op&writeOrCreateMask != 0) ||
+						(currentConfigFile != "" && currentConfigFile != realConfigFile) {
+						realConfigFile = currentConfigFile
+						b, err := c.Read()
+						if err != nil {
+							log.WithError(err).Error("failed to read config")
+							break
+						}
+						out := make([]byte, len(b))
+						copy(out, b)
+						updates <- out
+					} else if filepath.Clean(event.Name) == configFile &&
+						event.Op&fsnotify.Remove&fsnotify.Remove != 0 {
+						eventsWG.Done()
+						return
+					}
+
+				case err, ok := <-watcher.Errors:
+					if ok { // 'Errors' channel is not closed
+						log.WithError(err).Error("watcher failed")
+					}
+					eventsWG.Done()
+					return
+				case <-ctx.Done():
+					return
+				}
+			}
+		}()
+
+		errs <- watcher.Add(configDir) // done initializing the watch in this go routine, so the parent routine can move on...
+		eventsWG.Wait()                // now, wait for event loop to end in this go-routine...
+	}()
+	// initWG.Wait() // make sure that the go routine above fully ended before returning
+	return <-errs
+}