Cockroachdb unimplemented feature multiple active portals not supported

• asyncpg version: 0.20.1
• Cockroachdb version: v20.1.0 (connected to a 5-node on premise cluster)
• Python version: 3.8.0
• Platform: Linux
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: Yes

Example code:

    async with pool.acquire() as conn:
        async with conn.transaction():
            if not await conn.table_exists("sys_app_config"):
                await conn.execute("""
                    CREATE TABLE sys_app_config(
                        key  text NOT NULL PRIMARY KEY,
                        value jsonb
                    );
                """)

The table_exists method looks like this:

class MyConnection(Connection):
    async def table_exists(self, table_name: str) -> bool:
        """Tell if a table exists."""
        return await self.fetchval(
            "select table_name from information_schema.tables where table_name=$1", table_name) is not None

This code is actually called from tornado.ioloop.IOLoop.current().run_sync so it is guaranteed that nothing else is running queries concurrently.

Here is the traceback that I’m getting:

 File "/home/user/my_project/db/initialize.py", line 19, in init_database
    if not await conn.table_exists("sys_app_config"):
  File "/home/user/.local/share/virtualenvs/backend-YgzYW8Ky/lib/python3.8/site-packages/asyncpg/transaction.py", line 91, in __aexit__
    await self.__commit()
  File "/home/user/.local/share/virtualenvs/backend-YgzYW8Ky/lib/python3.8/site-packages/asyncpg/transaction.py", line 179, in __commit
    await self._connection.execute(query)
  File "/home/user/.local/share/virtualenvs/backend-YgzYW8Ky/lib/python3.8/site-packages/asyncpg/connection.py", line 272, in execute
    return await self._protocol.query(query, timeout)
  File "asyncpg/protocol/protocol.pyx", line 316, in query
asyncpg.exceptions.FeatureNotSupportedError: unimplemented: multiple active portals not supported
HINT:  You have attempted to use a feature that is not yet implemented.
See: https://github.com/cockroachdb/cockroach/issues/40195

The referenced issue tells that it is not possible to keep multiple queries open and fetch from them interleaved.

However, I’m not doing anything that should result in opening multiple queries and fetching from them that way. As far as I see, my code should not open a new query before closing the previous one.

I suspect that asyncpg itself forgot to close something in the background.

[rimadeodhar]

For the above code snippet by @hifi, one workaround that I noticed is that the error goes away by replace fetchval with fetch. By examining the difference over Wireshark between the two calls, I noticed since fetch results in fetching all rows, the execution step completes with a CommandComplete message as opposed to the PortalSuspended message by using fetchval. This doesn’t obviate the need for a general fix on the cockroachdb side though as we should still follow the unnamed portal semantics to be properly wire-compatible.

[jordanlewis]

Hi, I’m also a maintainer for CockroachDB.

The root cause of this is that Postgres permits leaving cursors open and beginning new cursors. CockroachDB does not.

The problem here is that asyncpg doesn’t explicitly close portals when it’s done using them. The example code from @hifi is correct in Postgres, but it actually leads to cursor leakage: if you run this against Postgres, you won’t get errors, but you will have leaked server-side cursors until you close your connection.

This is bad for performance in Postgres, and just plain doesn’t work in CockroachDB.

I think the best solution would be for asyncpg to provide a cursor.close() method that permits explicit closure of cursors. A call like conn.fetchRow should close the cursor it opens under the hood before exiting.

I believe this will improve performance in Postgres, and make the library compatible with CockroachDB.