notmuch_message_tags_to_maildir_flags() unconditionally moves messages from
maildir directory "new/" to maildir directory "cur/", which makes messages lose
their "new" status in the MUA. However some users want to keep this "new"
status after, for instance, an auto-tagging of new messages.

However, as Austin mentioned and according to the maildir specification,
messages living in "new/" are not allowed to have flags, even if mutt allows it
to happen. For this reason, this patch prevents moving messages from "new/" to
"cur/", only if no flags have to be changed. It's hopefully enough to satisfy
mutt (and maybe other MUAs showing the "new" status) users checking the "new"
status.

Changelog:
* v2: Fix bool type as well as NULL returned despite having no errors (Austin
      Clements)

Signed-off-by: Louis Rilling <l.rilling@av7.net>
---
 lib/message.cc |   12 +++++++++++-
 1 files changed, 11 insertions(+), 1 deletions(-)

diff --git a/lib/message.cc b/lib/message.cc
index b1b2942..c003729 100644
--- a/lib/message.cc
+++ b/lib/message.cc
@@ -1139,7 +1139,7 @@ _get_maildir_flag_actions (notmuch_message_t *message,
  * compute the new maildir filename.
  *
  * If the existing filename is in the directory "new", the new
- * filename will be in the directory "cur".
+ * filename will be in the directory "cur", unless no flags are changed.
  *
  * After a sequence of ":2," in the filename, any subsequent
  * single-character flags will be added or removed according to the
@@ -1162,6 +1162,7 @@ _new_maildir_filename (void *ctx,
     char *filename_new, *dir;
     char flag_map[128];
     int flags_in_map = 0;
+    notmuch_bool_t flags_changed = FALSE;
     unsigned int i;
     char *s;
 
@@ -1202,6 +1203,7 @@ _new_maildir_filename (void *ctx,
 	if (flag_map[flag] == 0) {
 	    flag_map[flag] = 1;
 	    flags_in_map++;
+	    flags_changed = TRUE;
 	}
     }
 
@@ -1210,9 +1212,17 @@ _new_maildir_filename (void *ctx,
 	if (flag_map[flag]) {
 	    flag_map[flag] = 0;
 	    flags_in_map--;
+	    flags_changed = TRUE;
 	}
     }
 
+    /* No need to rename. Messages in new/ can be kept in new/.
+     * Note: We don't even try to fix buggy messages having flags and living in
+     * new/. It's not our business.
+     */
+    if (!flags_changed)
+	return talloc_strdup (ctx, filename);
+
     filename_new = (char *) talloc_size (ctx,
 					 info - filename +
 					 strlen (":2,") + flags_in_map + 1);
-- 
1.7.2.5