Replace Session.ConnectSMTP with Session.DoSMTP

This gives more flexibility in Session for optimizations, e.g. keep the
SMTP connection around for some time if possible.

plugins/base/handlers.go

@@ -13,6 +13,7 @@ import (
 	"github.com/emersion/go-imap"
 	imapclient "github.com/emersion/go-imap/client"
 	"github.com/emersion/go-message"
+	"github.com/emersion/go-smtp"
 	"github.com/labstack/echo/v4"
 )
 
@@ -257,7 +258,9 @@ func handleCompose(ectx echo.Context) error {
 		msg.Text = ctx.FormValue("text")
 		msg.InReplyTo = ctx.FormValue("in_reply_to")
 
-		c, err := ctx.Session.ConnectSMTP()
+		err := ctx.Session.DoSMTP(func(c *smtp.Client) error {
+			return sendMessage(c, &msg)
+		})
 		if err != nil {
 			if _, ok := err.(koushin.AuthError); ok {
 				return echo.NewHTTPError(http.StatusForbidden, err)
@@ -265,14 +268,6 @@ func handleCompose(ectx echo.Context) error {
 			return err
 		}
 
-		if err := sendMessage(c, &msg); err != nil {
-			return err
-		}
-
-		if err := c.Quit(); err != nil {
-			return fmt.Errorf("QUIT failed: %v", err)
-		}
-
 		// TODO: append to IMAP Sent mailbox
 
 		return ctx.Redirect(http.StatusFound, "/mailbox/INBOX")

session.go

@@ -76,21 +76,29 @@ func (s *Session) DoIMAP(f func(*imapclient.Client) error) error {
 	return f(s.imapConn)
 }
 
-// ConnectSMTP connects to the upstream SMTP server and authenticates this
-// session.
-func (s *Session) ConnectSMTP() (*smtp.Client, error) {
+// DoSMTP executes an SMTP operation on this session. The SMTP client can only
+// be used from inside f.
+func (s *Session) DoSMTP(f func(*smtp.Client) error) error {
 	c, err := s.manager.dialSMTP()
 	if err != nil {
-		return nil, err
+		return err
 	}
+	defer c.Close()
 
 	auth := sasl.NewPlainClient("", s.username, s.password)
 	if err := c.Auth(auth); err != nil {
-		c.Close()
-		return nil, AuthError{err}
+		return AuthError{err}
 	}
 
-	return c, nil
+	if err := f(c); err != nil {
+		return err
+	}
+
+	if err := c.Quit(); err != nil {
+		return fmt.Errorf("QUIT failed: %v", err)
+	}
+
+	return nil
 }
 
 // Close destroys the session. This can be used to log the user out.