Re: An improvement of ProcessTwoPhaseBuffer logic

On Thu, Dec 26, 2024 at 06:11:25PM +0300, Давыдов Виталий wrote:
> I concerned about twophase file name generation. The function
> TwoPhaseFilePath() is pretty straitforward and unambiguous in 16 and
> earlier versions. The logic of this function in 17+ seems to be more
> complex. I do not understand it clearly. But, I guess, it will work
> incorrectly after turning to a newer epoch, because the epoch is
> calculated from TransamVariables->nextXid, but not from problem
> xid. The same problem may happen if we are in epoch 1 or greater. It
> will produce a wrong file name, because the epoch will be obtained
> from the last xid, not from file name xid. In another words, the
> function AdjustToFullTransactionId assumes that if xid >
> TransamVariables->nextXid, then the xid from the previous epoch. I
> may be not the case in our scenario.

Yeah.  At this stage, we can pretty much say that the whole idea of
relying AdjustToFullTransactionId() is broken, because we would build
2PC file names based on wrong assumptions, while orphaned files could
be in the far past or far future depending on the epoch.

TBH, we'll live better if we remove AdjustToFullTransactionId() and
sprinkle a bit more the knowledge of FullTransactionIds to build
correctly the 2PC file path in v17~.  I've been playing with this code
for a couple of hours and finished with the attached patch.  I have
wondered if ReadTwoPhaseFile() should gain the same knowledge as
TwoPhaseFilePath(), but decided to limit the invasiness because we
always call ReadTwoPhaseFile() once we don't have any orphaned files,
for all the phases of recovery.  So while this requires a bit more
logic that depends on FullTransactionIdFromEpochAndXid() and
ReadNextFullTransactionId() to build a FullTransactionId and get the
current epoch, that's still acceptable as we only store an XID in the
2PC file.

So please see the attached.  You will note that RemoveTwoPhaseFile(),
ProcessTwoPhaseBuffer() and TwoPhaseFilePath() now require a
FullTransactionId, hence callers need to think about the epoch to use.
That should limit future errors compared to passing the file name as
optional argument.

What do you think?
--
Michael