Re: Reviewing freeze map code

On Wed, Jun 15, 2016 at 9:56 PM, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> On Wed, Jun 15, 2016 at 2:41 AM, Thomas Munro
> <thomas(dot)munro(at)enterprisedb(dot)com> wrote:
>> I spent some time chasing down the exact circumstances. I suspect
>> that there may be an interlocking problem in heap_update. Using the
>> line numbers from cae1c788 [1], I see the following interaction
>> between the VACUUM, UPDATE and SELECT (pg_check_visible) backends, all
>> in reference to the same block number:
>>
>> [VACUUM] sets all visible bit
>>
>> [UPDATE] heapam.c:3931 HeapTupleHeaderSetXmax(oldtup.t_data, xmax_old_tuple);
>> [UPDATE] heapam.c:3938 LockBuffer(buffer, BUFFER_LOCK_UNLOCK);
>>
>> [SELECT] LockBuffer(buffer, BUFFER_LOCK_SHARE);
>> [SELECT] observes VM_ALL_VISIBLE as true
>> [SELECT] observes tuple in HEAPTUPLE_DELETE_IN_PROGRESS state
>> [SELECT] barfs
>>
>> [UPDATE] heapam.c:4116 visibilitymap_clear(...)
>
> Yikes: heap_update() sets the tuple's XMAX, CMAX, infomask, infomask2,
> and CTID without logging anything or clearing the all-visible flag and
> then releases the lock on the heap page to go do some more work that
> might even ERROR out. Only if that other work all goes OK do we
> relock the page and perform the WAL-logged actions.
>
> That doesn't seem like a good idea even in existing releases, because
> you've taken a tuple on an all-visible page and made it not
> all-visible, and you've made a page modification that is not
> necessarily atomic without logging it. This is is particularly bad in
> 9.6, because if that page is also all-frozen then XMAX will eventually
> be pointing into space and VACUUM will never visit the page to
> re-freeze it the way it would have done in earlier releases. However,
> even in older releases, I think there's a remote possibility of data
> corruption. Backend #1 makes these changes to the page, releases the
> lock, and errors out. Backend #2 writes the page to the OS. DBA
> takes a hot backup, tearing the page in the middle of XMAX. Oops.
>
> I'm not sure what to do about this: this part of the heap_update()
> logic has been like this forever, and I assume that if it were easy to
> refactor this away, somebody would have done it by now.
>

How about changing collect_corrupt_items to acquire
AccessExclusiveLock for safely checking?

Regards,

--
Masahiko Sawada