Use a single unified pass to fetch scalar message metadata.

This performs a single pass over a message's term list to fetch the
thread ID, message ID, and reply-to, rather than requiring a pass for
each.  Xapian decompresses the term list anew for each iteration, so
this reduces the amount of time spent decompressing message metadata.

This reduces my inbox search from 3.102 seconds to 2.555 seconds (1.2X
faster).

lib/message.cc

@@ -256,6 +256,92 @@ _notmuch_message_create_for_message_id (notmuch_database_t *notmuch,
     return message;
 }
 
+static char *
+_notmuch_message_get_term (notmuch_message_t *message,
+			   Xapian::TermIterator &i, Xapian::TermIterator &end,
+			   const char *prefix)
+{
+    int prefix_len = strlen (prefix);
+    const char *term = NULL;
+    char *value;
+
+    i.skip_to (prefix);
+
+    if (i != end)
+	term = (*i).c_str ();
+
+    if (!term || strncmp (term, prefix, prefix_len))
+	return NULL;
+
+    value = talloc_strdup (message, term + prefix_len);
+
+#if DEBUG_DATABASE_SANITY
+    i++;
+
+    if (i != end && strncmp ((*i).c_str (), prefix, prefix_len) == 0) {
+	INTERNAL_ERROR ("Mail (doc_id: %d) has duplicate %s terms: %s and %s\n",
+			message->doc_id, prefix, value,
+			(*i).c_str () + prefix_len);
+    }
+#endif
+
+    return value;
+}
+
+void
+_notmuch_message_ensure_metadata (notmuch_message_t *message)
+{
+    Xapian::TermIterator i, end;
+    const char *thread_prefix = _find_prefix ("thread"),
+	*id_prefix = _find_prefix ("id"),
+	*replyto_prefix = _find_prefix ("replyto");
+
+    /* We do this all in a single pass because Xapian decompresses the
+     * term list every time you iterate over it.  Thus, while this is
+     * slightly more costly than looking up individual fields if only
+     * one field of the message object is actually used, it's a huge
+     * win as more fields are used. */
+
+    i = message->doc.termlist_begin ();
+    end = message->doc.termlist_end ();
+
+    /* Get thread */
+    if (!message->thread_id)
+	message->thread_id =
+	    _notmuch_message_get_term (message, i, end, thread_prefix);
+
+    /* Get id */
+    assert (strcmp (thread_prefix, id_prefix) < 0);
+    if (!message->message_id)
+	message->message_id =
+	    _notmuch_message_get_term (message, i, end, id_prefix);
+
+    /* Get reply to */
+    assert (strcmp (id_prefix, replyto_prefix) < 0);
+    if (!message->in_reply_to)
+	message->in_reply_to =
+	    _notmuch_message_get_term (message, i, end, replyto_prefix);
+    /* It's perfectly valid for a message to have no In-Reply-To
+     * header. For these cases, we return an empty string. */
+    if (!message->in_reply_to)
+	message->in_reply_to = talloc_strdup (message, "");
+}
+
+static void
+_notmuch_message_invalidate_metadata (notmuch_message_t *message,
+				      const char *prefix_name)
+{
+    if (strcmp ("thread", prefix_name) == 0) {
+	talloc_free (message->thread_id);
+	message->thread_id = NULL;
+    }
+
+    if (strcmp ("replyto", prefix_name) == 0) {
+	talloc_free (message->in_reply_to);
+	message->in_reply_to = NULL;
+    }
+}
+
 unsigned int
 _notmuch_message_get_doc_id (notmuch_message_t *message)
 {
@@ -265,32 +351,11 @@ _notmuch_message_get_doc_id (notmuch_message_t *message)
 const char *
 notmuch_message_get_message_id (notmuch_message_t *message)
 {
-    Xapian::TermIterator i;
+    if (!message->message_id)
-
+	_notmuch_message_ensure_metadata (message);
-    if (message->message_id)
+    if (!message->message_id)
-	return message->message_id;
+	INTERNAL_ERROR ("Message with document ID of %u has no message ID.\n",
-
-    i = message->doc.termlist_begin ();
-    i.skip_to (_find_prefix ("id"));
-
-    if (i == message->doc.termlist_end ())
-	INTERNAL_ERROR ("Message with document ID of %d has no message ID.\n",
 			message->doc_id);
-
-    message->message_id = talloc_strdup (message, (*i).c_str () + 1);
-
-#if DEBUG_DATABASE_SANITY
-    i++;
-
-    if (i != message->doc.termlist_end () &&
-	strncmp ((*i).c_str (), _find_prefix ("id"),
-		 strlen (_find_prefix ("id"))) == 0)
-    {
-	INTERNAL_ERROR ("Mail (doc_id: %d) has duplicate message IDs",
-			message->doc_id);
-    }
-#endif
-
     return message->message_id;
 }
 
@@ -329,89 +394,19 @@ notmuch_message_get_header (notmuch_message_t *message, const char *header)
 const char *
 _notmuch_message_get_in_reply_to (notmuch_message_t *message)
 {
-    const char *prefix = _find_prefix ("replyto");
+    if (!message->in_reply_to)
-    int prefix_len = strlen (prefix);
+	_notmuch_message_ensure_metadata (message);
-    Xapian::TermIterator i;
-    std::string in_reply_to;
-
-    if (message->in_reply_to)
-	return message->in_reply_to;
-
-    i = message->doc.termlist_begin ();
-    i.skip_to (prefix);
-
-    if (i != message->doc.termlist_end ())
-	in_reply_to = *i;
-
-    /* It's perfectly valid for a message to have no In-Reply-To
-     * header. For these cases, we return an empty string. */
-    if (i == message->doc.termlist_end () ||
-	strncmp (in_reply_to.c_str (), prefix, prefix_len))
-    {
-	message->in_reply_to = talloc_strdup (message, "");
-	return message->in_reply_to;
-    }
-
-    message->in_reply_to = talloc_strdup (message,
-					  in_reply_to.c_str () + prefix_len);
-
-#if DEBUG_DATABASE_SANITY
-    i++;
-
-    in_reply_to = *i;
-
-    if (i != message->doc.termlist_end () &&
-	strncmp ((*i).c_str (), prefix, prefix_len) == 0)
-    {
-       INTERNAL_ERROR ("Message %s has duplicate In-Reply-To IDs: %s and %s\n",
-			notmuch_message_get_message_id (message),
-			message->in_reply_to,
-			(*i).c_str () + prefix_len);
-    }
-#endif
-
     return message->in_reply_to;
 }
 
 const char *
 notmuch_message_get_thread_id (notmuch_message_t *message)
 {
-    const char *prefix = _find_prefix ("thread");
+    if (!message->thread_id)
-    Xapian::TermIterator i;
+	_notmuch_message_ensure_metadata (message);
-    std::string id;
+    if (!message->thread_id)
-
+	INTERNAL_ERROR ("Message with document ID of %u has no thread ID.\n",
-    /* This code is written with the assumption that "thread" has a
-     * single-character prefix. */
-    assert (strlen (prefix) == 1);
-
-    if (message->thread_id)
-	return message->thread_id;
-
-    i = message->doc.termlist_begin ();
-    i.skip_to (prefix);
-
-    if (i != message->doc.termlist_end ())
-	id = *i;
-
-    if (i == message->doc.termlist_end () || id[0] != *prefix)
-	INTERNAL_ERROR ("Message with document ID of %d has no thread ID.\n",
 			message->doc_id);
-
-    message->thread_id = talloc_strdup (message, id.c_str () + 1);
-
-#if DEBUG_DATABASE_SANITY
-    i++;
-    id = *i;
-
-    if (i != message->doc.termlist_end () && id[0] == *prefix)
-    {
-	INTERNAL_ERROR ("Message %s has duplicate thread IDs: %s and %s\n",
-			notmuch_message_get_message_id (message),
-			message->thread_id,
-			id.c_str () + 1);
-    }
-#endif
-
     return message->thread_id;
 }
 
@@ -809,6 +804,8 @@ _notmuch_message_add_term (notmuch_message_t *message,
 
     talloc_free (term);
 
+    _notmuch_message_invalidate_metadata (message, prefix_name);
+
     return NOTMUCH_PRIVATE_STATUS_SUCCESS;
 }
 
@@ -874,6 +871,8 @@ _notmuch_message_remove_term (notmuch_message_t *message,
 
     talloc_free (term);
 
+    _notmuch_message_invalidate_metadata (message, prefix_name);
+
     return NOTMUCH_PRIVATE_STATUS_SUCCESS;
 }