[PATCH 09/10] lib: Add notmuch_database_{begin,end}_atomic.

[Austin Clements]

These operations translate into non-flushed Xapian transactions,
allowing arbitrary groups of database operations to be performed
atomically.
---
 lib/database.cc |   44 ++++++++++++++++++++++++++++++++++++++++++++
 lib/notmuch.h   |   30 ++++++++++++++++++++++++++++++
 2 files changed, 74 insertions(+), 0 deletions(-)

diff --git a/lib/database.cc b/lib/database.cc
index bee1e96..0103a56 100644
--- a/lib/database.cc
+++ b/lib/database.cc
@@ -974,6 +974,50 @@ notmuch_database_upgrade (notmuch_database_t *notmuch,
     return NOTMUCH_STATUS_SUCCESS;
 }
 
+notmuch_status_t
+notmuch_database_begin_atomic (notmuch_database_t *notmuch)
+{
+    if (notmuch->mode == NOTMUCH_DATABASE_MODE_READ_ONLY)
+	return NOTMUCH_STATUS_SUCCESS;
+
+    try {
+	(static_cast <Xapian::WritableDatabase *> (notmuch->xapian_db))->begin_transaction (false);
+    } catch (const Xapian::Error &error) {
+	fprintf (stderr, "A Xapian exception occurred beginning transaction: %s.\n",
+		 error.get_msg().c_str());
+	notmuch->exception_reported = TRUE;
+	return NOTMUCH_STATUS_XAPIAN_EXCEPTION;
+    }
+    return NOTMUCH_STATUS_SUCCESS;
+}
+
+notmuch_status_t
+notmuch_database_end_atomic (notmuch_database_t *notmuch)
+{
+    Xapian::WritableDatabase *db;
+
+    if (notmuch->mode == NOTMUCH_DATABASE_MODE_READ_ONLY)
+	return NOTMUCH_STATUS_SUCCESS;
+
+    db = static_cast <Xapian::WritableDatabase *> (notmuch->xapian_db);
+    try {
+	db->commit_transaction ();
+
+	/* This is a hack for testing.  Xapian never flushes on a
+	 * non-flushed commit, even if the flush threshold is 1.
+	 * However, we rely on flushing to test atomicity. */
+	const char *thresh = getenv ("XAPIAN_FLUSH_THRESHOLD");
+	if (thresh && atoi (thresh) == 1)
+	    db->commit ();
+    } catch (const Xapian::Error &error) {
+	fprintf (stderr, "A Xapian exception occurred committing transaction: %s.\n",
+		 error.get_msg().c_str());
+	notmuch->exception_reported = TRUE;
+	return NOTMUCH_STATUS_XAPIAN_EXCEPTION;
+    }
+    return NOTMUCH_STATUS_SUCCESS;
+}
+
 /* We allow the user to use arbitrarily long paths for directories. But
  * we have a term-length limit. So if we exceed that, we'll use the
  * SHA-1 of the path for the database term.
diff --git a/lib/notmuch.h b/lib/notmuch.h
index 61030cb..9cdcec0 100644
--- a/lib/notmuch.h
+++ b/lib/notmuch.h
@@ -214,6 +214,36 @@ notmuch_database_upgrade (notmuch_database_t *database,
 						   double progress),
 			  void *closure);
 
+/* Begin an atomic database operation.
+ *
+ * Any modifications performed between a successful begin and a
+ * notmuch_database_end_atomic will be applied to the database
+ * atomically.  Note that, unlike a typical database transaction, this
+ * only ensures atomicity, not durability; neither begin nor end
+ * necessarily flush modifications to disk.
+ *
+ * Return value:
+ *
+ * NOTMUCH_STATUS_SUCCESS: Successfully entered atomic section.
+ *
+ * NOTMUCH_STATUS_XAPIAN_EXCEPTION: A Xapian exception occurred;
+ *	atomic section not entered.
+ */
+notmuch_status_t
+notmuch_database_begin_atomic (notmuch_database_t *notmuch);
+
+/* Indicate the end of an atomic database operation.
+ *
+ * Return value:
+ *
+ * NOTMUCH_STATUS_SUCCESS: Successfully completed atomic section.
+ *
+ * NOTMUCH_STATUS_XAPIAN_EXCEPTION: A Xapian exception occurred;
+ *	atomic section not ended.
+ */
+notmuch_status_t
+notmuch_database_end_atomic (notmuch_database_t *notmuch);
+
 /* Retrieve a directory object from the database for 'path'.
  *
  * Here, 'path' should be a path relative to the path of 'database'
-- 
1.7.2.3