imapserver: split command handling

Split into two parts: parsing and processing. Some commands are
incompatible with this model (e.g. APPEND), so will have a no-op
second step.

The goal is to parallelize the second step with other processing:
reading the next command, and eventually handling the next command
as well.

Author: emersion

imapserver/conn.go

@@ -202,7 +202,10 @@ func (c *Conn) readCommand(dec *imapwire.Decoder) error {
 
 	// TODO: handle multiple commands concurrently
 	sendOK := true
-	var err error
+	var (
+		err error
+		cmd *command
+	)
 	switch name {
 	case "NOOP", "CHECK":
 		err = c.handleNoop(dec)
@@ -252,7 +255,7 @@ func (c *Conn) readCommand(dec *imapwire.Decoder) error {
 		err = c.handleAppend(tag, dec)
 		sendOK = false
 	case "FETCH", "UID FETCH":
-		err = c.handleFetch(dec, numKind)
+		cmd, err = c.handleFetch(dec, numKind)
 	case "EXPUNGE":
 		err = c.handleExpunge(dec)
 	case "UID EXPUNGE":
@@ -282,6 +285,10 @@ func (c *Conn) readCommand(dec *imapwire.Decoder) error {
 
 	dec.DiscardLine()
 
+	if err == nil && cmd != nil {
+		err = cmd.Wait()
+	}
+
 	var (
 		resp    *imap.StatusResponse
 		imapErr *imap.Error
@@ -474,6 +481,32 @@ func (c *Conn) poll(cmd string) error {
 	return c.session.Poll(w, allowExpunge)
 }
 
+type command struct {
+	done chan struct{}
+	err  error
+}
+
+func newCommand(f func() error) *command {
+	cmd := &command{done: make(chan struct{})}
+	go func() {
+		var err error
+		defer func() {
+			if v := recover(); v != nil {
+				err = fmt.Errorf("panic handling command: %v\n%s", v, debug.Stack())
+			}
+			cmd.err = err
+			close(cmd.done)
+		}()
+		err = f()
+	}()
+	return cmd
+}
+
+func (cmd *command) Wait() error {
+	<-cmd.done
+	return cmd.err
+}
+
 type responseEncoder struct {
 	*imapwire.Encoder
 	conn *Conn

imapserver/fetch.go

@@ -23,10 +23,10 @@ type fetchWriterOptions struct {
 	obsolete map[*imap.FetchItemBodySection]string
 }
 
-func (c *Conn) handleFetch(dec *imapwire.Decoder, numKind NumKind) error {
+func (c *Conn) handleFetch(dec *imapwire.Decoder, numKind NumKind) (*command, error) {
 	var seqSet imap.SeqSet
 	if !dec.ExpectSP() || !dec.ExpectSeqSet(&seqSet) || !dec.ExpectSP() {
-		return dec.Err()
+		return nil, dec.Err()
 	}
 
 	var options imap.FetchOptions
@@ -43,12 +43,12 @@ func (c *Conn) handleFetch(dec *imapwire.Decoder, numKind NumKind) error {
 		return handleFetchAtt(dec, name, &options, &writerOptions)
 	})
 	if err != nil {
-		return err
+		return nil, err
 	}
 	if !isList {
 		name, err := readFetchAttName(dec)
 		if err != nil {
-			return err
+			return nil, err
 		}
 
 		// Handle macros
@@ -70,28 +70,27 @@ func (c *Conn) handleFetch(dec *imapwire.Decoder, numKind NumKind) error {
 			handleFetchBodyStructure(&options, &writerOptions, false)
 		default:
 			if err := handleFetchAtt(dec, name, &options, &writerOptions); err != nil {
-				return err
+				return nil, err
 			}
 		}
 	}
 
 	if !dec.ExpectCRLF() {
-		return dec.Err()
+		return nil, dec.Err()
 	}
 
 	if err := c.checkState(imap.ConnStateSelected); err != nil {
-		return err
+		return nil, err
 	}
 
 	if numKind == NumKindUID {
 		options.UID = true
 	}
 
 	w := &FetchWriter{conn: c, options: writerOptions}
-	if err := c.session.Fetch(w, numKind, seqSet, &options); err != nil {
-		return err
-	}
-	return nil
+	return newCommand(func() error {
+		return c.session.Fetch(w, numKind, seqSet, &options)
+	}), nil
 }
 
 func handleFetchAtt(dec *imapwire.Decoder, attName string, options *imap.FetchOptions, writerOptions *fetchWriterOptions) error {