Re: pgstattuple locking fix

Heikki Linnakangas wrote:
> ITAGAKI Takahiro wrote:
>> Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
>>
>>> ITAGAKI Takahiro <itagaki(dot)takahiro(at)oss(dot)ntt(dot)co(dot)jp> writes:
>>>> Here is a trivial fix of locking issue in pgstattuple().
>>> Hmm, is this really a bug, and if so how far back does it go?
>>> I'm thinking that having a pin on the buffer should be enough to
>>> call PageGetHeapFreeSpace.
>> Hmm... we might use pd_upper and pd_lower at different times,
>> but the error is ok for pgstattuple purpose.
>> (It might be dangerous for tuple insertion, though.)
>> Inconsistent usage of locks is confusable -- remove them, please.
>
> No I think the original patch was right. You can indeed read
> inconsistent values for pd_upper and pd_lower, though the window is very
> small. But a bigger issue is that in 8.3 PageGetHeapFreeSpace counts the
> used line pointers to see if MaxHeapTuplesPerPage has been reached, and
> I'm not convinced you can do that safely without holding a lock.

On second thought, we do call PageGetHeapFreeSpace without holding a
lock in heap_page_prune_opt as well, so it better be safe. Looking
closer at PageGetHeapFreeSpace, I think it is. The
return value can be bogus, of course.

That's worth noting in the comments:

*** src/backend/storage/page/bufpage.c 21 Sep 2007 21:25:42 -0000 1.75
--- src/backend/storage/page/bufpage.c 22 Oct 2007 10:06:02 -0000
***************
*** 506,511 ****
--- 506,514 ----
* or dead line pointers it'd be possible to have too many line pointers.
* To avoid breaking code that assumes MaxHeapTuplesPerPage is a hard
limit
* on the number of line pointers, we make this extra check.)
+ *
+ * You don't need to hold a lock on the page to call this function,
but if
+ * you don't, the return value should be considered a hint only.
*/
Size
PageGetHeapFreeSpace(Page page)

--
Heikki Linnakangas
EnterpriseDB http://www.enterprisedb.com