imapserver: prevent unbounded memory allocations when handling a command

some commands, like search, can specify any number of literals, of arbitrary
size.  we already limited individual literals to 100kb. but you could specify
many of them, causing unbounded memory consumption. this change adds a limit of
1000 literals in a command, and a limit of 1mb of total combined memory for
literals. once the limits are exceeded, a TOOBIG error code is returned.

unbounded memory use could only be triggered on authenticated connections.

this addresses the same issue as CVE-2024-34055 for cyrus-imap, by damian
poddebniak.

imapclient/client.go

@@ -238,19 +238,27 @@ func (c *Conn) Write(buf []byte) (n int, rerr error) {
 
 // WriteSyncLiteral first writes the synchronous literal size, then read the
 // continuation "+" and finally writes the data.
-func (c *Conn) WriteSyncLiteral(s string) (rerr error) {
+func (c *Conn) WriteSyncLiteral(s string) (untagged []Untagged, rerr error) {
 	defer c.recover(&rerr)
 
 	_, err := fmt.Fprintf(c.conn, "{%d}\r\n", len(s))
 	c.xcheckf(err, "write sync literal size")
-	line, err := c.Readline()
-	c.xcheckf(err, "read line")
-	if !strings.HasPrefix(line, "+") {
-		c.xerrorf("no continuation received for sync literal")
+
+	plus, err := c.r.Peek(1)
+	c.xcheckf(err, "read continuation")
+	if plus[0] == '+' {
+		_, err = c.Readline()
+		c.xcheckf(err, "read continuation line")
+
+		_, err = c.conn.Write([]byte(s))
+		c.xcheckf(err, "write literal data")
+		return nil, nil
 	}
-	_, err = c.conn.Write([]byte(s))
-	c.xcheckf(err, "write literal data")
-	return nil
+	untagged, result, err := c.Response()
+	if err == nil && result.Status == OK {
+		c.xerrorf("no continuation, but invalid ok response (%q)", result.More)
+	}
+	return untagged, fmt.Errorf("no continuation (%s)", result.Status)
 }
 
 // Transactf writes format and args as an IMAP command, using Commandf with an