RE: [Proposal] Add foreign-server health checks infrastructure

Dear Osumi-san,

> I mainly followed the steps there and
> replaced the command "SELECT" for the remote table at 6-9 with "INSERT"
> command.
> Then, after waiting for few seconds, the "COMMIT" succeeded like below output,
> even after the server stop of the worker side.

> Additionally, the last reference "SELECT" which failed above can succeed,
> if I restart the worker server before the "SELECT" command to the remote table.
> This means the transaction looks successful but the data isn't there ?
> Could you please have a look at this issue ?

Good catch. This was occurred because we disconnected the crashed server.
 
Previously the failed server had been disconnected in the pgfdw_connection_check().
It was OK if the transaction(or statement) was surely cacneled.
But currently the statement might be not canceled because QueryCancelPending might be cleaned up[1].

If we failed to cancel statements and reached pgfdw_xact_callback(),
the connection would not be used because entry->conn is NULL.
That's why you sucseeded to do "COMMIT".
However, the backend did not send "COMMIT" command to the srever,
so inserted data was vanished on remote server.

I understood that we should not call disconnect_pg_server(entry->conn) even if we detect the disconnection.
If should be controlled in Xact callback. This will be modified in next version.

Moreover, I noticed that we should enable timer even if the QueryCancelMessage was not NULL.
If we do not turn on here, the checking will be never enabled again.


[1]: https://www.postgresql.org/message-id/TYAPR01MB5866CE34430424A588F60FC2F55D9%40TYAPR01MB5866.jpnprd01.prod.outlook.com


Best Regards,
Hayato Kuroda
FUJITSU LIMITED