[PATCH] Fix possible underflow in expression (maxoff - 1) - Mailing list pgsql-hackers

Hi,
The var OffsetNumber maxoff it's like uint16, see at include/storage/off.h
typedef uint16 OffsetNumber;

Within the function _bt_afternewitemoff, at line 641, maxoff is used in an dangerous expression,
without protection.:  (maxoff - 1)

The function: PageGetMaxOffsetNumber that initializes maxoff, can return zero.
See at storage/bufpage.h
 * PageGetMaxOffsetNumber
 *        Returns the maximum offset number used by the given page.
 *        Since offset numbers are 1-based, this is also the number
 *        of items on the page.
 *
 *        NOTE: if the page is not initialized (pd_lower == 0), we must
 *        return zero to ensure sane behavior.  Accept double evaluation
 *        of the argument so that we can ensure this.

Surely not the best solution, but it was the best I could think of.

best regards.
Ranier Vilela