Re: Add Option To Check All Addresses For Matching target_session_attr

The previous patch had a mistake in a reference in the documentation. This
should fix it.

On Mon, Feb 24, 2025 at 10:07 AM Andrew Jackson <andrewjackson947(at)gmail(dot)com>
wrote:

> Looks like this needed another rebase to account for the oauth commit.
> Rebase attached.
>
> On Mon, Feb 24, 2025 at 9:38 AM Andrew Jackson <andrewjackson947(at)gmail(dot)com>
> wrote:
>
>> Hi,
>>
>> Thank you for the review!
>>
>> Review Response
>>
>> - Made a first pass at a real commit message
>> - Fixed the condition on the if statement to use strcmp
>> - Added a test suite in the files `src/interfaces/libpq/t/
>> 006_target_session_attr_dns.pl` and `src/interfaces/libpq/t/
>> 007_load_balance_dns_check_all_addrs.pl` which checks the
>> target_session_attrs as when used with and without load balancing.
>>
>> Regarding the name of the variable itself I am definitely open to
>> opinions on this. I didn't put too much thought initially and just chose
>> `check_all_addrs`. I feel like given that it modifies the behaviour of
>> `target_session_attrs` ideally it should reference that in the name but
>> that would make that variable name very long: something akin to
>> `target_session_attrs_check_all_addrs`.
>>
>> Context
>>
>> I tested some drivers as well and found that pgx, psycopg, and
>> rust-postgres all traverse every IP address when looking for a matching
>> target_session_attrs. Asyncpg and psycopg2 on the other hand follow libpq
>> and terminate additional attempts after the first failure. Given this it
>> seems like there is a decent amount of fragmentation in the ecosystem as to
>> how exactly to implement this feature. I believe some drivers choose to
>> traverse all addresses because they have users target the same use case
>> outlined above.
>>
>> Thanks again,
>> Andrew Jackson
>>
>> On Sun, Feb 16, 2025 at 6:03 AM Andrey Borodin <x4mmm(at)yandex-team(dot)ru>
>> wrote:
>>
>>> Hi Andrew!
>>>
>>> cc Jelte, I suspect he might be interested.
>>>
>>> > On 20 Nov 2024, at 20:51, Andrew Jackson <andrewjackson947(at)gmail(dot)com>
>>> wrote:
>>> >
>>> > Would appreciate any feedback on the applicability/relevancy of the
>>> goal here or the implementation.
>>>
>>> Thank you for raising the issue. Following our discussion in Discord I'm
>>> putting my thoughts to list.
>>>
>>>
>>> Context
>>>
>>> A DNS record might return several IPs. Consider we have a connection
>>> string with "host=A,B", A is resolved to 1.1.1.1,2.2.2.2, B to
>>> 3.3.3.3,4.4.4.4.
>>> If we connect with "target_session_attrs=read-write" IPs 1.1.1.1 and
>>> 3.3.3.3 will be probed, but 2.2.2.2 and 4.4.4.4 won't (if 1.1.1.1 and
>>> 3.3.3.3 responded).
>>>
>>> If we enable libpq load balancing some random 2 IPs will be probed.
>>>
>>> IMO it's a bug, at least when load balancing is enabled. Let's consider
>>> if we can change default behavior here. I suspect we can't do it for
>>> "load_balance_hosts=disable". And even for load balancing this might be too
>>> unexpected change for someone.
>>>
>>> Further I only consider proposal not as a bug fix, but as a feature.
>>>
>>> In Discord we have surveyed some other drivers.
>>> pgx treats all IPs as different servers [1]. npgsql goes through all IPs
>>> one-by-one always [2]. PGJDBC are somewhat in a decision process [3] (cc
>>> Dave and Vladimir, if they would like to provide some input).
>>>
>>>
>>> Review
>>>
>>> The patch needs a rebase. It's trivial, so please fine attached. The
>>> patch needs real commit message, it's not trivial :)
>>>
>>> We definitely need to adjust tests [0]. We need to change
>>> 004_load_balance_dns.pl so that it tests target_session_attrs too.
>>>
>>> Some documentation would be nice.
>>>
>>> I do not like how this check is performed
>>> + if
>>> (conn->check_all_addrs && conn->check_all_addrs[0] == '1')
>>> Let's make it like load balancing is done [4].
>>>
>>> Finally, let's think about naming alternatives for "check_all_addrs".
>>>
>>> I think that's enough for a first round of the review. If it's not a
>>> bug, but a feature - it's a very narrow window to get to 18. But we might
>>> be lucky...
>>>
>>> Thank you!
>>>
>>>
>>> Best regards, Andrey Borodin.
>>>
>>> [0]
>>> https://github.com/postgres/postgres/commit/7f5b19817eaf38e70ad1153db4e644ee9456853e#diff-b05b74d2a97d7f74d4311ba1702d732f0df1b101c6ac99c146b51215174cf3ffR94
>>> [1] https://github.com/jackc/pgx/blob/master/pgconn/pgconn.go#L177
>>> [2]
>>> https://github.com/npgsql/npgsql/blob/7f1a59fa8dc1ccc34a70154f49a768e1abf826ba/src/Npgsql/Internal/NpgsqlConnector.cs#L986
>>> [3] https://github.com/pgjdbc/pgjdbc/pull/3012#discussion_r1408069450
>>> [4]
>>> https://github.com/postgres/postgres/commit/7f5b19817eaf38e70ad1153db4e644ee9456853e#diff-8d819454e061b9d4cdae9c8922ded05753a629d70f2ac1de1d4f6d5a4aeb7f68R1660
>>>
>>