Re: Amcheck verification of GiST and GIN

> On Feb 21, 2025, at 12:16 PM, Mark Dilger <mark.dilger@enterprisedb.com> wrote:
>
> The pgbench script is not corrupting anything overtly, so this looks to either be a bug in gin or a bug in the check.


I suspected the AccessShareLock taken by verify_gin() might be too weak, and upgraded that to ShareRowExclusiveLock so
asto prevent the concurrent table modifications (and incidentally other concurrent verify_gin() calls), but to my
surprisethat didn't fix anything.  Even AccessExclusiveLock doesn't fix it.  So this seems to either be a bug in the
checkingcode complaining about perfectly valid tuple order, or a bug in Gin corrupting its own entry tree page. 

On successive runs, (instrumented to print out a bit more info), there doesn't seem to be any obvious pattern in where
thecorruption occurs.  The offset in the page changes, neither always being at the beginning, nor always at the maxoff;
likewisethe block where corruption is detected changes from run to run.  I've noticed that the rightlink for the page
isalways the page's block number plus one, but that might just be that I haven't run enough iterations yet to see
counter-examples.

Could one of the patch authors take a look?  I don't have the time to chase this to conclusion just now.  Thanks.

—
Mark Dilger
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company