maildir: Provide nicer error message on invalid url

If accounts.conf contains an invalid maildir url, return a nice
error instead of panicking.

Log a couple of different error cases to provide extra
information about the error to the user.

worker/maildir/container.go

@@ -21,15 +21,30 @@ type Container struct {
 }
 
 // NewContainer creates a new container at the specified directory
-// TODO: return an error if the provided directory is not accessible
-func NewContainer(dir string, l *log.Logger) *Container {
-	return &Container{dir: dir, uids: uidstore.NewStore(), log: l}
+func NewContainer(dir string, l *log.Logger) (*Container, error) {
+	f, err := os.Open(dir)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	s, err := f.Stat()
+	if err != nil {
+		return nil, err
+	}
+	if !s.IsDir() {
+		return nil, fmt.Errorf("Given maildir '%s' not a directory", dir)
+	}
+	return &Container{dir: dir, uids: uidstore.NewStore(), log: l}, nil
 }
 
 // ListFolders returns a list of maildir folders in the container
 func (c *Container) ListFolders() ([]string, error) {
 	folders := []string{}
 	err := filepath.Walk(c.dir, func(path string, info os.FileInfo, err error) error {
+		if err != nil {
+			return fmt.Errorf("Invalid path '%s': error: %v", path, err)
+
+		}
 		if !info.IsDir() {
 			return nil
 		}

worker/maildir/worker.go

@@ -1,6 +1,7 @@
 package maildir
 
 import (
+	"errors"
 	"fmt"
 	"io"
 	"net/url"
@@ -221,7 +222,15 @@ func (w *Worker) handleConfigure(msg *types.Configure) error {
 		}
 		dir = filepath.Join(home, u.Path)
 	}
-	w.c = NewContainer(dir, w.worker.Logger)
+	if len(dir) == 0 {
+		return fmt.Errorf("could not resolve maildir from URL '%s'", msg.Config.Source)
+	}
+	c, err := NewContainer(dir, w.worker.Logger)
+	if err != nil {
+		w.worker.Logger.Printf("could not configure maildir: %s", dir)
+		return err
+	}
+	w.c = c
 	w.worker.Logger.Printf("configured base maildir: %s", dir)
 	return nil
 }
@@ -231,6 +240,12 @@ func (w *Worker) handleConnect(msg *types.Connect) error {
 }
 
 func (w *Worker) handleListDirectories(msg *types.ListDirectories) error {
+	// TODO If handleConfigure has returned error, w.c is nil.
+	// It could be better if we skip directory listing completely
+	// when configure fails.
+	if w.c == nil {
+		return errors.New("Incorrect maildir directory")
+	}
 	dirs, err := w.c.ListFolders()
 	if err != nil {
 		w.worker.Logger.Printf("error listing directories: %v", err)