On 2017-09-27 14:46, Stas Kelvich wrote:
On 7 Sep 2017, at 18:58, Nikhil Sontakke <nikh...@2ndquadrant.com> wrote:Hi, FYI all, wanted to mention that I am working on an updated version of the latest patch that I plan to submit to a later CF.Cool! So what kind of architecture do you have in mind? Same way as is it was implemented before? As far as I remember there were two main issues: * Decodong of aborted prepared transaction. If such transaction modified catalog then we can’t read reliable info with our historic snapshot, since clog already have aborted bit for our tx it will brake visibility logic. There are some way to deal with that — by doing catalog seq scan two times and counting number of tuples (details upthread) or by hijacking clog values in historic visibility function. But ISTM it is better not solve this issue at all =) In most cases intended usage of decoding of 2PC transaction is to do some form of distributed commit, so naturally decoding will happens only with in-progress transactions and we commit/abort will happen only after it is decoded, sent and response is received. So we can just have atomic flag that prevents commit/abort of tx currently being decoded. And we can filter interesting prepared transactions based on GID, to prevent holding this lock for ordinary 2pc. * Possible deadlocks that Andres was talking about. I spend some time trying to find that, but didn’t find any. If locking pg_class in prepared tx is the only example then (imho) it is better to just forbid to prepare such transactions. Otherwise if some realistic examples that can block decoding are actually exist, then we probably need to reconsider the way tx being decoded. Anyway this part probably need Andres blessing.
Just rebased patch logical_twophase_v6 to master. Fixed small issues: - XactLogAbortRecord wrote DBINFO twice, but it was decoded in ParseAbortRecord only once. Second DBINFO were parsed as ORIGIN. Fixed by removing second write of DBINFO. - SnapBuildPrepareTxnFinish tried to remove xid from `running` instead of `committed`. And it removed only xid, without subxids.- test_decoding skipped returning "COMMIT PREPARED" and "ABORT PREPARED",
Big issue were with decoding ddl-including two-phase transactions: - prepared.out were misleading. We could not reproduce decoding body of "test_prepared#3" with logical_twophase_v6.diff. It was skipped if `pg_logical_slot_get_changes` were called without `twophase-decode-with-catalog-changes` set, and only "COMMIT PREPARED test_prepared#3" were decoded. The reason is "PREPARE TRANSACTION" is passed to `pg_filter_prepare` twice: - first on "PREPARE" itself, - second - on "COMMIT PREPARED". In v6, `pg_filter_prepare` without `with-catalog-changes` first time answered "true" (ie it should not be decoded), and second time (when transaction became committed) it answered "false" (ie it should bedecoded). But second time in DecodePrepare `ctx->snapshot_builder->start_decoding_at`
is already in a future compared to `buf->origptr` (because it is on "COMMIT PREPARED" lsn). Therefore DecodePrepare just called ReorderBufferForget. If `pg_filter_prepare` is called with `with-catalog-changes`, then it returns "false" both times, thus DeocdePrepare decodes transaction in first time, and calls `ReorderBufferForget` in second time.I didn't found a way to fix it gracefully. I just change `pg_filter_prepare` to return same answer both times: "false" if called `with-catalog-changes`
(ie need to call DecodePrepare), and "true" otherwise. With this change, catalog changing two-phase transaction is decoded as simple one-phase transaction, if `pg_logical_slot_get_changes` is called without `with-catalog-changes`. -- With regards, Sokolov Yura Postgres Professional: https://postgrespro.ru The Russian Postgres Company
logical_twophase_v7.diff.gz
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