# HG changeset patch
# User Paul Boddie <paul@boddie.org.uk>
# Date 1509401685 -3600
# Node ID 0d6768d83dd79c23f40e270608abf924bbdaa2b4
# Parent  683059dc2e7bdd2d32cf98ff4048ba9d35c4617e
Changed the structure of wrapped messages to employ multipart/alternative and to
collect calendar system information into a text part rather than the MIME
preamble.

diff -r 683059dc2e7b -r 0d6768d83dd7 imiptools/mail.py
--- a/imiptools/mail.py	Fri Oct 27 22:26:10 2017 +0200
+++ b/imiptools/mail.py	Mon Oct 30 23:14:45 2017 +0100
@@ -144,7 +144,7 @@
         """
         Return a simple summary using details from 'msg' and the given 'parts'.
         Information messages provided amongst the parts by the handlers will be
-        merged into the preamble so that mail programs will show them
+        merged into a single text part so that mail programs will show them
         immediately.
         """
 
@@ -157,12 +157,16 @@
         """
         Wrap 'msg' and provide the given 'parts' as the primary content.
         Information messages provided amongst the parts by the handlers will be
-        merged into the preamble so that mail programs will show them
+        merged into a single text part so that mail programs will show them
         immediately.
         """
 
         message = self._make_container_for_parts(parts, True)
         payload = message.get_payload()
+
+        # The original message is placed in a mailbox part for reference,
+        # attached to the rest of the primary content parts.
+
         payload.append(MIMEMessage(msg))
         self._copy_headers(message, msg)
         return message
@@ -193,14 +197,30 @@
         else:
             info = []
 
-        # Insert a preamble message before any calendar information messages.
+        # Provide a preamble message before any other data.
+
+        preamble = self.preamble_text or \
+                   self.gettext and self.gettext(PREAMBLE_TEXT) or PREAMBLE_TEXT
+
+        # Provide a mixture of aggregated message parts.
+
+        message = parts and MIMEMultipart("mixed", _subparts=parts) or None
+
+        # Provide an informational message using the aggregated information.
 
-        info.insert(0, self.preamble_text or
-                       self.gettext and self.gettext(PREAMBLE_TEXT) or PREAMBLE_TEXT)
+        info_message = info and MIMEText("\n\n".join(info)) or None
+
+        # Wrap the information and message parts.
 
-        message = MIMEMultipart("mixed", _subparts=parts)
-        message.preamble = "\n\n".join(info)
-        return message
+        subparts = []
+        if info_message:
+            subparts.append(info_message)
+        if message:
+            subparts.append(message)
+
+        top = MIMEMultipart("alternative", _subparts=subparts)
+        top.preamble = preamble
+        return top
 
     def _merge_calendar_info_parts(self, parts):