[WIP2 01/12] lib: Only sync modified message documents

[David Bremner]

From: Austin Clements <amdragon at mit.edu>

Previously, we updated the database copy of a message on every call to
_notmuch_message_sync, even if nothing had changed.  In particular,
this always happens on a thaw, so a freeze/thaw pair with no
modifications between still caused a database update.

We only modify message documents in a handful of places, so keep track
of whether the document has been modified and only sync it when
necessary.  This will be particularly important when we add message
revision tracking.
---
 lib/message.cc | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/lib/message.cc b/lib/message.cc
index 5bc7aff..1ddce3c 100644
--- a/lib/message.cc
+++ b/lib/message.cc
@@ -43,6 +43,9 @@ struct visible _notmuch_message {
      * if each flag has been initialized. */
     unsigned long lazy_flags;
 
+    /* Message document modified since last sync */
+    notmuch_bool_t modified;
+
     Xapian::Document doc;
     Xapian::termcount termpos;
 };
@@ -539,6 +542,7 @@ _notmuch_message_remove_terms (notmuch_message_t *message, const char *prefix)
 
 	try {
 	    message->doc.remove_term ((*i));
+	    message->modified = TRUE;
 	} catch (const Xapian::InvalidArgumentError) {
 	    /* Ignore failure to remove non-existent term. */
 	}
@@ -793,6 +797,7 @@ void
 _notmuch_message_clear_data (notmuch_message_t *message)
 {
     message->doc.set_data ("");
+    message->modified = TRUE;
 }
 
 static void
@@ -990,6 +995,7 @@ _notmuch_message_set_header_values (notmuch_message_t *message,
 			    Xapian::sortable_serialise (time_value));
     message->doc.add_value (NOTMUCH_VALUE_FROM, from);
     message->doc.add_value (NOTMUCH_VALUE_SUBJECT, subject);
+    message->modified = TRUE;
 }
 
 /* Synchronize changes made to message->doc out into the database. */
@@ -1001,8 +1007,12 @@ _notmuch_message_sync (notmuch_message_t *message)
     if (message->notmuch->mode == NOTMUCH_DATABASE_MODE_READ_ONLY)
 	return;
 
+    if (! message->modified)
+	return;
+
     db = static_cast <Xapian::WritableDatabase *> (message->notmuch->xapian_db);
     db->replace_document (message->doc_id, message->doc);
+    message->modified = FALSE;
 }
 
 /* Delete a message document from the database. */
@@ -1077,6 +1087,7 @@ _notmuch_message_add_term (notmuch_message_t *message,
 	return NOTMUCH_PRIVATE_STATUS_TERM_TOO_LONG;
 
     message->doc.add_term (term, 0);
+    message->modified = TRUE;
 
     talloc_free (term);
 
@@ -1145,6 +1156,7 @@ _notmuch_message_remove_term (notmuch_message_t *message,
 
     try {
 	message->doc.remove_term (term);
+	message->modified = TRUE;
     } catch (const Xapian::InvalidArgumentError) {
 	/* We'll let the philosopher's try to wrestle with the
 	 * question of whether failing to remove that which was not
-- 
2.1.4