Fix #8799: BUGCHECK "decompression overran buffer (179)" when WITH LOCK clause is used

[ilya071294]

VIO_writelock creates new_rpb by copying from org_rpb which has rpb_delta flag set. new_rpb is passed to prepare_update. It creates a delta but its size exceeds the limit (1024) so the current version is stored as a regular record. The problem is that rpb_delta flag remains set for new_rpb, and then replace_record sets it in the header of the primary version. From this moment any attempt to get data of older versions fails.
The fix is to let prepare_update clear rpb_delta flag from new_rpb just before an attempt to generate a delta.

[hvlad]

Looks ok.

To be completely sure: have you considered to change VIO_writelock() instead ?
What about other flags, copied from old record (rpb_incomplete, for example) ?

[ilya071294]

To be completely sure: have you considered to change VIO_writelock() instead ?

Yes, but that leaves open the possibility of facing a similar bug again, so I see it as a worse solution.

What about other flags, copied from old record (rpb_incomplete, for example) ?

I thought about it and came to this logic: if prepare_update determines how the backversion is stored then it should have full control over rpb_delta flag. It doesn't do anything else with new_rpb, so it should not care about other flags. The caller should. And both VIO_writelock and VIO_modify handle other flags correctly as I can see.