Release the leader key when the leader restarts with an empty data dir (#420)

* If the leader has an empty data directory it must have been recreated, so release the leader key

patroni/ha.py

@@ -886,6 +886,11 @@ class Ha(object):
 
             # is data directory empty?
             if self.state_handler.data_directory_empty():
+                # is this instance the leader?
+                if self.has_lock():
+                    self.release_leader_key_voluntarily()
+                    return 'released leader key voluntarily as data dir empty and currently leader'
+
                 return self.bootstrap()  # new node
             # "bootstrap", but data directory is not empty
             elif not self.sysid_valid(self.cluster.initialize) and self.cluster.is_unlocked() and not self.is_paused():

tests/test_ha.py

@@ -771,3 +771,14 @@ class TestHa(unittest.TestCase):
 
     def test_wakup(self):
         self.ha.wakeup()
+
+    def test_leader_with_empty_directory(self):
+        self.ha.cluster = get_cluster_initialized_with_leader()
+        self.ha.has_lock = true
+        self.p.data_directory_empty = true
+        self.assertEquals(self.ha.run_cycle(), 'released leader key voluntarily as data dir empty and currently leader')
+
+        # as has_lock is mocked out, we need to fake the leader key release
+        self.ha.has_lock = false
+        # will not say bootstrap from leader as replica can't self elect
+        self.assertEquals(self.ha.run_cycle(), "trying to bootstrap from replica 'other'")