[YSQL] Unable to set yb_read_from_followers and yb_follower_read_staleness_ms via startup parameters

[jameshartig]

Jira Link: DB-14886

Description

We set yb_read_from_followers=true and yb_follower_read_staleness_ms=3000 via PG Startup Message parameters. In 2.20.x this works fine but while testing 2024.2 internally we ran into the following error:

FATAL: SET yb_read_from_followers must be called before any query (SQLSTATE 25001)

I believe this is related to 79888cf. The commit mentions setting ysql_follower_reads_behavior_before_fixing_20482 gflag to restore the old behavior but that gflag doesn't seem to exist as the tserver crashes. See comments.

Here's the startup packet that's sent:

and here's the response to that packet:

Issue Type

kind/bug

Warning: Please confirm that this issue does not contain any sensitive information

• I confirm this issue does not contain any sensitive information.

[acube123]

The flag is ysql_yb_follower_reads_behavior_before_fixing_20482 (starting with ysql_yb_)

[jameshartig]

Adding ysql_yb_follower_reads_behavior_before_fixing_20482=true to the tserver gflags does seem to fix the issue. It doesn't seem intentional to disallow the follower read flags to be set at the connection-level though.