fix(spanner): return sessions from pool in LIFO order (#9454)

Closes #9392.

Author: Gurov Ilya

spanner/google/cloud/spanner_v1/pool.py

@@ -156,7 +156,7 @@ def __init__(self, size=DEFAULT_SIZE, default_timeout=DEFAULT_TIMEOUT, labels=No
         super(FixedSizePool, self).__init__(labels=labels)
         self.size = size
         self.default_timeout = default_timeout
-        self._sessions = queue.Queue(size)
+        self._sessions = queue.LifoQueue(size)
 
     def bind(self, database):
         """Associate the pool with a database.
@@ -251,7 +251,7 @@ def __init__(self, target_size=10, labels=None):
         super(BurstyPool, self).__init__(labels=labels)
         self.target_size = target_size
         self._database = None
-        self._sessions = queue.Queue(target_size)
+        self._sessions = queue.LifoQueue(target_size)
 
     def bind(self, database):
         """Associate the pool with a database.

spanner/tests/unit/test_pool.py

@@ -164,15 +164,16 @@ def test_bind(self):
     def test_get_non_expired(self):
         pool = self._make_one(size=4)
         database = _Database("name")
-        SESSIONS = [_Session(database)] * 4
+        SESSIONS = sorted([_Session(database) for i in range(0, 4)])
         database._sessions.extend(SESSIONS)
         pool.bind(database)
 
-        session = pool.get()
-
-        self.assertIs(session, SESSIONS[0])
-        self.assertTrue(session._exists_checked)
-        self.assertFalse(pool._sessions.full())
+        # check if sessions returned in LIFO order
+        for i in (3, 2, 1, 0):
+            session = pool.get()
+            self.assertIs(session, SESSIONS[i])
+            self.assertTrue(session._exists_checked)
+            self.assertFalse(pool._sessions.full())
 
     def test_get_expired(self):
         pool = self._make_one(size=4)
@@ -898,7 +899,10 @@ def mock_batch_create_sessions(db, session_count=10, timeout=10, metadata=[]):
         self.spanner_api.batch_create_sessions.side_effect = mock_batch_create_sessions
 
     def session(self):
-        return self._sessions.pop()
+        # always return first session in the list
+        # to avoid reversing the order of putting
+        # sessions into pool (important for order tests)
+        return self._sessions.pop(0)
 
 
 class _Queue(object):