Re: [PoC] Improve dead tuple storage for lazy vacuum

While creating a benchmark for inserting into node128-inner, I found a bug.
If a caller deletes from a node128, the slot index is set to invalid, but
the child pointer is still valid. Do that a few times, and every child
pointer is valid, even if no slot index points to it. When the next
inserter comes along, something surprising happens. This function:

/* Return an unused slot in node-128 */
static int
node_inner_128_find_unused_slot(rt_node_inner_128 *node, uint8 chunk)
{
int slotpos = 0;

Assert(!NODE_IS_LEAF(node));
while (node_inner_128_is_slot_used(node, slotpos))
slotpos++;

return slotpos;
}

...passes an integer to this function, whose parameter is a uint8:

/* Is the slot in the node used? */
static inline bool
node_inner_128_is_slot_used(rt_node_inner_128 *node, uint8 slot)
{
Assert(!NODE_IS_LEAF(node));
return (node->children[slot] != NULL);
}

...so instead of growing the node unnecessarily or segfaulting, it enters
an infinite loop doing this:

add eax, 1
movzx ecx, al
cmp QWORD PTR [rbx+264+rcx*8], 0
jne .L147

The fix is easy enough -- set the child pointer to null upon deletion, but
I'm somewhat astonished that the regression tests didn't hit this. I do
still intend to replace this code with something faster, but before I do so
the tests should probably exercise the deletion paths more. Since VACUUM

--
John Naylor
EDB: http://www.enterprisedb.com