Rework config directives iteration

Create more high-level wrapper (config.Map) instead of walking syntax
tree directly.

config.go

@@ -0,0 +1,153 @@
+package maddy
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"log"
+	"os"
+
+	"github.com/emersion/maddy/config"
+	"github.com/emersion/maddy/module"
+)
+
+/*
+Config matchers for module interfaces.
+*/
+
+func authProvider(modName string) (module.AuthProvider, error) {
+	modObj := module.GetInstance(modName)
+	if modObj == nil {
+		return nil, fmt.Errorf("unknown auth. provider instance: %s", modName)
+	}
+
+	provider, ok := modObj.(module.AuthProvider)
+	if !ok {
+		return nil, fmt.Errorf("module %s doesn't implements auth. provider interface", modObj.Name())
+	}
+	return provider, nil
+}
+
+func authDirective(m *config.Map, node *config.Node) (interface{}, error) {
+	if len(node.Args) != 1 {
+		return nil, m.MatchErr("expected 1 argument")
+	}
+	if len(node.Children) != 0 {
+		return nil, m.MatchErr("can't declare block here")
+	}
+
+	modObj, err := authProvider(node.Args[0])
+	if err != nil {
+		return nil, m.MatchErr("%s", err.Error())
+	}
+
+	return modObj, nil
+}
+
+func storageBackend(modName string) (module.Storage, error) {
+	modObj := module.GetInstance(modName)
+	if modObj == nil {
+		return nil, fmt.Errorf("unknown storage backend instance: %s", modName)
+	}
+
+	backend, ok := modObj.(module.Storage)
+	if !ok {
+		return nil, fmt.Errorf("module %s doesn't implements storage interface", modObj.Name())
+	}
+	return backend, nil
+}
+
+func storageDirective(m *config.Map, node *config.Node) (interface{}, error) {
+	if len(node.Args) != 1 {
+		return nil, m.MatchErr("expected 1 argument")
+	}
+	if len(node.Children) != 0 {
+		return nil, m.MatchErr("can't declare block here")
+	}
+
+	modObj, err := storageBackend(node.Args[0])
+	if err != nil {
+		return nil, m.MatchErr("%s", err.Error())
+	}
+
+	return modObj, nil
+}
+
+func deliveryTarget(modName string) (module.DeliveryTarget, error) {
+	mod := module.GetInstance(modName)
+	if mod == nil {
+		return nil, fmt.Errorf("unknown delivery target instance: %s", modName)
+	}
+
+	target, ok := mod.(module.DeliveryTarget)
+	if !ok {
+		return nil, fmt.Errorf("module %s doesn't implements delivery target interface", mod.Name())
+	}
+	return target, nil
+}
+
+func deliveryDirective(m *config.Map, node *config.Node) (interface{}, error) {
+	if len(node.Args) != 1 {
+		return nil, m.MatchErr("expected 1 argument")
+	}
+	if len(node.Children) != 0 {
+		return nil, m.MatchErr("can't declare block here")
+	}
+
+	modObj, err := deliveryTarget(node.Args[0])
+	if err != nil {
+		return nil, m.MatchErr("%s", err.Error())
+	}
+	return modObj, nil
+}
+
+func defaultAuthProvider() (interface{}, error) {
+	res, err := authProvider("default_auth")
+	if err != nil {
+		res, err = authProvider("default")
+		if err != nil {
+			return nil, errors.New("missing default auth. provider, must set custom")
+		}
+	}
+	return res, nil
+}
+
+func defaultStorage() (interface{}, error) {
+	res, err := storageBackend("default_storage")
+	if err != nil {
+		res, err = storageBackend("default")
+		if err != nil {
+			return nil, errors.New("missing default storage backend, must set custom")
+		}
+	}
+	return res, nil
+}
+
+func errorsDirective(m *config.Map, node *config.Node) (interface{}, error) {
+	if len(node.Args) != 1 {
+		return nil, m.MatchErr("expected 1 argument")
+	}
+	if len(node.Children) != 0 {
+		return nil, m.MatchErr("can't declare block here")
+	}
+
+	output := node.Args[0]
+	var w io.Writer
+	switch output {
+	case "off":
+		w = ioutil.Discard
+	case "stdout":
+		w = os.Stdout
+	case "stderr":
+		w = os.Stderr
+	default:
+		f, err := os.OpenFile(output, os.O_WRONLY|os.O_APPEND|os.O_CREATE, 0666)
+		if err != nil {
+			return nil, err
+		}
+		w = f
+	}
+
+	return log.New(w, "imap ", log.LstdFlags), nil
+}