Re: BUG #17462: Invalid memory access in heapam_tuple_lock

I wrote:
> As written here, even though there's no obvious API break, there is
> a subtle change: heap_fetch will now return tuple->t_data = NULL after
> a snapshot failure. I'm of two minds whether to back-patch that part
> (not doing so would require more changes in heapam_tuple_lock, though).

After further thought I think we shouldn't back-patch that part: it
could easily break code that's functionally correct today, and there
isn't any case that it makes better without caller-code changes.

(I'm not sure what made me think it'd affect heapam_tuple_lock; that
function only cares about the behavior with keep_buf = true.)

Hence, I end up with two different patches for HEAD and the back branches.
In HEAD, we explicitly break heap_fetch compatibility by adding a new
argument, and we can make the commit message note the more subtle change
too. In the back branches, the behavior of heap_fetch is unchanged
and heapam_tuple_lock has to use heap_fetch_extended.

regards, tom lane