lib/database.cc: change how the parent of a message is calculated

Presently, the code which finds the parent of a message as it is being
added to the database assumes that the first Message-ID-like substring
of the In-Reply-To header is the parent Message ID.  Some mail clients,
however, put stuff other than the Message-ID of the parent in the
In-Reply-To header, such as the email address of the sender of the
parent.  This can fool notmuch.

The updated algorithm prefers the last Message ID in the References
header.  The References header lists messages oldest-first, so the last
Message ID is the parent (RFC2822, p. 24).  The References header is
also less likely to be in a non-standard
syntax (http://cr.yp.to/immhf/thread.html,
http://www.jwz.org/doc/threading.html).  In case the References header
is not to be found, fall back to the old behavior.

V2 of this patch, incorporating feedback from Jani and (indirectly)
Austin.

1 files changed, 33 insertions, 15 deletions

diff --git a/lib/database.cc b/lib/database.cc
index 91d43298..52ed618b 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -501,8 +501,10 @@ _parse_message_id (void *ctx, const char *message_id, const char **next)
  * 'message_id' in the result (to avoid mass confusion when a single
  * message references itself cyclically---and yes, mail messages are
  * not infrequent in the wild that do this---don't ask me why).
-*/
-static void
+ *
+ * Return the last reference parsed, if it is not equal to message_id.
+ */
+static char *
 parse_references (void *ctx,
 		  const char *message_id,
 		  GHashTable *hash,
@@ -511,7 +513,7 @@ parse_references (void *ctx,
     char *ref;
 
     if (refs == NULL || *refs == '\0')
-	return;
+	return NULL;
 
     while (*refs) {
 	ref = _parse_message_id (ctx, refs, &refs);
@@ -519,6 +521,17 @@ parse_references (void *ctx,
 	if (ref && strcmp (ref, message_id))
 	    g_hash_table_insert (hash, ref, NULL);
     }
+
+    /* The return value of this function is used to add a parent
+     * reference to the database.  We should avoid making a message
+     * its own parent, thus the following check.
+     */
+
+    if (ref && strcmp(ref, message_id)) {
+	return ref;
+    } else {
+	return NULL;
+    }
 }
 
 notmuch_status_t
@@ -1510,28 +1523,33 @@ _notmuch_database_link_message_to_parents (notmuch_database_t *notmuch,
 {
     GHashTable *parents = NULL;
     const char *refs, *in_reply_to, *in_reply_to_message_id;
+    const char *last_ref_message_id, *this_message_id;
     GList *l, *keys = NULL;
     notmuch_status_t ret = NOTMUCH_STATUS_SUCCESS;
 
     parents = g_hash_table_new_full (g_str_hash, g_str_equal,
 				     _my_talloc_free_for_g_hash, NULL);
+    this_message_id = notmuch_message_get_message_id (message);
 
     refs = notmuch_message_file_get_header (message_file, "references");
-    parse_references (message, notmuch_message_get_message_id (message),
-		      parents, refs);
+    last_ref_message_id = parse_references (message,
+					    this_message_id,
+					    parents, refs);
 
     in_reply_to = notmuch_message_file_get_header (message_file, "in-reply-to");
-    parse_references (message, notmuch_message_get_message_id (message),
-		      parents, in_reply_to);
-
-    /* Carefully avoid adding any self-referential in-reply-to term. */
-    in_reply_to_message_id = _parse_message_id (message, in_reply_to, NULL);
-    if (in_reply_to_message_id &&
-	strcmp (in_reply_to_message_id,
-		notmuch_message_get_message_id (message)))
-    {
+    in_reply_to_message_id = parse_references (message,
+					       this_message_id,
+					       parents, in_reply_to);
+
+    /* For the parent of this message, use the last message ID of the
+     * References header, if available.  If not, fall back to the
+     * first message ID in the In-Reply-To header. */
+    if (last_ref_message_id) {
+        _notmuch_message_add_term (message, "replyto",
+                                   last_ref_message_id);
+    } else if (in_reply_to_message_id) {
 	_notmuch_message_add_term (message, "replyto",
-			     _parse_message_id (message, in_reply_to, NULL));
+			     in_reply_to_message_id);
     }
 
     keys = g_hash_table_get_keys (parents);