On Wed, Jun 15, 2016 at 11:43 AM, Thomas Munro
<thomas.mu...@enterprisedb.com> wrote:
> On Wed, Jun 15, 2016 at 12:44 AM, Robert Haas <robertmh...@gmail.com> wrote:
>> On Tue, Jun 14, 2016 at 8:11 AM, Robert Haas <robertmh...@gmail.com> wrote:
>>>>> I noticed that the tuples that it reported were always offset 1 in a
>>>>> page, and that the page always had a maxoff over a couple of hundred,
>>>>> and that we called record_corrupt_item because VM_ALL_VISIBLE returned
>>>>> true but HeapTupleSatisfiesVacuum on the first tuple returned
>>>>> It did that because HEAP_XMAX_COMMITTED was not set and
>>>>> TransactionIdIsInProgress returned true for xmax.
>>>> So this seems like it might be a visibility map bug rather than a bug
>>>> in the test code, but I'm not completely sure of that.  How was it
>>>> legitimate to mark the page as all-visible if a tuple on the page
>>>> still had a live xmax?  If xmax is live and not just a locker then the
>>>> tuple is not visible to the transaction that wrote xmax, at least.
>>> Ah, wait a minute.  I see how this could happen.  Hang on, let me
>>> update the pg_visibility patch.
>> The problem should be fixed in the attached revision of
>> pg_check_visible.  I think what happened is:
>> 1. pg_check_visible computed an OldestXmin.
>> 2. Some transaction committed.
>> 3. VACUUM computed a newer OldestXmin and marked a page all-visible with it.
>> 4. pg_check_visible then used its older OldestXmin to check the
>> visibility of tuples on that page, and saw delete-in-progress as a
>> result.
>> I added a guard against a similar scenario involving xmin in the last
>> version of this patch, but forgot that we need to protect xmax in the
>> same way.  With this version of the patch, I can no longer get any
>> TIDs to pop up out of pg_check_visible in my testing.  (I haven't run
>> your test script for lack of the proper Python environment...)
> I can still reproduce the problem with this new patch.  What I see is
> that the OldestXmin, the new RecomputedOldestXmin and the tuple's xmax
> are all the same.

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(...)


Thomas Munro

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