Re: Patch: Implement failover on libpq connect level.

> I've tried to deal with some of these problems.
>
> My patch have support for following things:
>
> 1. Check whether database instance is in the recovery/standby mode and
> try to find another one if so.
> 2. Let cluster management software to have some time to promote one of
> the standbys to master. I.e. there can be failover timeout specified to
> allow retry after some time if no working master found.
>
> Really there is room for some improvements in handling of connect
> timeout (which became much more important thing when ability to try
> next host appears). Now it is handled only by blocking-mode connect
> functions, not by async state machine. But I decided to publish patch
> without these improvements to get feedback from community.
A bit of testing on this turns up a problem.

Consider a connection string that specifies two hosts and a read/write 
connection:
  postgresql://korry@127.0.0.1:5301,127.0.0.1:5300/edb?readonly=0

If the first host is a healthy standby (meaning that I can connect to it 
but pg_is_in_recovery() returns 't'), the state machine will never move 
on to the second host.

The problem seems to be in PQconnectPoll() in the case for 
CONNECTION_AUTH_OK, specifically this code:
  /* We can release the address list now. */  pg_freeaddrinfo_all(conn->addrlist_family, conn->addrlist);
conn->addrlist= NULL;  conn->addr_cur = NULL;
 

That frees up the list of alternative host addresses.  The state machine 
then progresses to CONNECTION_CHECK_RO (which invokes 
pg_is_in_recovery()), then CONNECTION_CHECK_RW (waiting for the response 
from the server).  Since we just connected to a standby replica, 
pg_is_in_recovery() returns 't' and the state changes to 
CONNECTION_NEEDED.  The next call to try_next_address() will fail to 
find a next address because we freed the list in the case for 
CONNECTION_AUTH_OK.

A related issue:  when the above sequence occurs, no error message is 
returned (because the case for CONNECTION_NEEDED thinks "an appropriate 
error message is already set up").

In short, if you successfully connect to standby replica (and specify 
readonly=0), the remaining hosts are ignored, even though one of those 
hosts is a master.

And one comment about the code itself - in connectDBStart(), you've 
added quite a bit of code to parse multiple hosts/ports. I would 
recommend adding a comment that shows the expected format, and then 
choosing better variable names (better than 'p', 'q', and 'r'); perhaps 
the variable names could refer to components of the connection string 
that you are parsing (like 'host', 'port', 'delimiter', ...).  That 
would make the code much easier to read/maintain.

Thanks.

                -- Korry