update async query session pool

Author: vgvoleg

ydb/aio/query/pool.py

@@ -48,31 +48,33 @@ async def acquire(self, timeout: float) -> QuerySessionAsync:
             logger.error("An attempt to take session from closed session pool.")
             raise RuntimeError("An attempt to take session from closed session pool.")
 
+        session = None
         try:
             _, session = self._queue.get_nowait()
+        except asyncio.QueueEmpty:
+            pass
+
+        if session is None and self._current_size == self._size:
+            try:
+                self._waiters += 1
+                session = await self._get_session_with_timeout(timeout)
+            except asyncio.TimeoutError:
+                raise issues.SessionPoolEmpty("Timeout on acquire session")
+            finally:
+                self._waiters -= 1
+
+        if session is not None:
             if session._state.attached:
                 logger.debug(f"Acquired active session from queue: {session._state.session_id}")
                 return session
             else:
                 self._current_size -= 1
                 logger.debug(f"Acquired dead session from queue: {session._state.session_id}")
-        except asyncio.QueueEmpty:
-            pass
-
-        if self._current_size < self._size:
-            logger.debug(f"Session pool is not large enough: {self._current_size} < {self._size}, will create new one.")
-            session = await self._create_new_session()
-            self._current_size += 1
-            return session
 
-        try:
-            self._waiters += 1
-            session = await self._get_session_with_timeout(timeout)
-            return session if session._state.attached else await self._create_new_session()
-        except asyncio.TimeoutError:
-            raise issues.SessionPoolEmpty("Timeout on acquire session")
-        finally:
-            self._waiters -= 1
+        logger.debug(f"Session pool is not large enough: {self._current_size} < {self._size}, will create new one.")
+        session = await self._create_new_session()
+        self._current_size += 1
+        return session
 
     async def _get_session_with_timeout(self, timeout: float):
         task_wait = asyncio.ensure_future(asyncio.wait_for(self._queue.get(), timeout=timeout))