[PATCH] libpq: try all addresses for a host before moving to next on target_session_attrs mismatch

Hi hackers,
This is my first time submitting a patch to PostgreSQL, so please bear with me if I've missed anything in the process.
We've been running into an issue with "target_session_attrs" when using dns-based service discovery. Currently, when libpq connects to a host with multiple A-records and the connection succeeds but is rejected due to target_session_attrs mismatch (e.g., connecting to a read-only server with target_session_attrs=read-write), it skips all remaining addresses for that hostname and moves directly to the next host in the connection string.

Looking at git history, I found this was a deliberate choice by Robert Haas in commit 721f7bd3cbc (2016), where he noted "I changed Mithun's patch to skip all remaining IPs for a host if we reject a connection based on this new parameter." The original mailing list discussion is at [1], though I wasn't able to find a clear explanation of why this approach was preferred over trying all addresses.

This makes it impractical to use a single multi-A-record DNS name pointing to all cluster members with target_session_attrs=read-write to find the primary - only the first responding IP is tried before giving up on that hostname.
The attached patch changes the behavior to try all addresses for a hostname before moving to the next host, matching the existing behavior for connection failures. This would enable simpler DNS-based service discovery without requiring external tools like Consul or explicit multi-host connection strings.
If there was a specific reason for the original design that I'm missing, I'd be happy to learn more.
Happy to address any feedback or rework the patch as needed.


[1] http://postgr.es/m/CAD__OuhqPRGpcsfwPHz_PDqAGkoqS1UvnUnOnAB-LBWBW=wu4A@mail.gmail.com

Thanks,
Evgeny