Re: HOT chain validation in verify_heapam()

Hi,

On 2022-11-30 16:09:19 +0530, Himanshu Upadhyaya wrote:
> has been updated to handle cases of sub-transaction.

Thanks!

> + /* Loop over offsets and validate the data in the predecessor array. */
> + for (OffsetNumber currentoffnum = FirstOffsetNumber; currentoffnum <= maxoff;
> + currentoffnum = OffsetNumberNext(currentoffnum))
> + {
> + HeapTupleHeader pred_htup;
> + HeapTupleHeader curr_htup;
> + TransactionId pred_xmin;
> + TransactionId curr_xmin;
> + ItemId pred_lp;
> + ItemId curr_lp;
> + bool pred_in_progress;
> + XidCommitStatus xid_commit_status;
> + XidBoundsViolation xid_status;
> +
> + ctx.offnum = predecessor[currentoffnum];
> + ctx.attnum = -1;
> + curr_lp = PageGetItemId(ctx.page, currentoffnum);
> + if (!lp_valid[currentoffnum] || ItemIdIsRedirected(curr_lp))
> + continue;

I don't think we should do PageGetItemId(ctx.page, currentoffnum); if !lp_valid[currentoffnum].

> + curr_htup = (HeapTupleHeader) PageGetItem(ctx.page, curr_lp);
> + curr_xmin = HeapTupleHeaderGetXmin(curr_htup);
> + xid_status = get_xid_status(curr_xmin, &ctx, &xid_commit_status);
> + if (!(xid_status == XID_BOUNDS_OK || xid_status == XID_INVALID))
> + continue;

Why can we even get here if the xid status isn't XID_BOUNDS_OK?

> + if (ctx.offnum == 0)

For one, I think it'd be better to use InvalidOffsetNumber here. But more
generally, storing the predecessor in ctx.offnum seems quite confusing.

> + {
> + /*
> + * No harm in overriding value of ctx.offnum as we will always
> + * continue if we are here.
> + */
> + ctx.offnum = currentoffnum;
> + if (TransactionIdIsInProgress(curr_xmin) || TransactionIdDidCommit(curr_xmin))

Is it actually ok to call TransactionIdDidCommit() here? There's a reason
get_xid_status() exists after all. And we do have the xid status for xmin
already, so this could just check xid_commit_status, no?

Greetings,

Andres Freund