Add "cluster_unlocked" field (#764)

Add a field to an api to figure out if a master is there from patroni point
of view. It can be useful, when you have an alert, based on Auto Scaling
Groups, and then ASG decided to shutdown the current master, spin up a
new instance but the current master shutdown is stuck. In this situation
the current master is no longer a part of ASG, but patroni and Postgres
are still alive on the instance, which means a new replica will not be
promoted yet - this will lead to a false alert, saying that your cluster
doesn't have any master node.

patroni/api.py

@@ -415,6 +415,8 @@ class RestApiHandler(BaseHTTPRequestHandler):
 
     def get_postgresql_status(self, retry=False):
         try:
+            cluster = self.server.patroni.dcs.cluster
+
             if self.server.patroni.postgresql.state not in ('running', 'restarting', 'starting'):
                 raise RetryFailedError('')
             stmt = ("WITH replication_info AS ("
@@ -439,6 +441,7 @@ class RestApiHandler(BaseHTTPRequestHandler):
                 'postmaster_start_time': row[0],
                 'role': 'replica' if row[1] == 0 else 'master',
                 'server_version': self.server.patroni.postgresql.server_version,
+                'cluster_unlocked': bool(not cluster or cluster.is_unlocked()),
                 'xlog': ({
                     'received_location': row[3],
                     'replayed_location': row[4],
@@ -451,7 +454,6 @@ class RestApiHandler(BaseHTTPRequestHandler):
             if row[1] > 0:
                 result['timeline'] = row[1]
             else:
-                cluster = self.server.patroni.dcs.cluster
                 leader_timeline = None if not cluster or cluster.is_unlocked() else cluster.leader.timeline
                 result['timeline'] = self.server.patroni.postgresql.replica_cached_timeline(leader_timeline)