From 76acb09688d0fcc833fe5639272083e3f20b5c00 Mon Sep 17 00:00:00 2001 From: Amit Kapila Date: Tue, 23 Feb 2021 11:43:45 +0530 Subject: [PATCH v1] Update the docs and comments for decoding of prepared xacts. Commit a271a1b50e introduced decoding at prepare time in ReorderBuffer. This can lead to deadlock for out-of-core logical replication solutions that uses this feature to build distributed 2PC in case such transactions lock [user] catalog tables exclusively. They need to inform users to not have locks on catalog tables (via explicit LOCK command) in such transactions. Reported-by: Andres Freund Discussion: https://postgr.es/m/20210222222847.tpnb6eg3yiykzpky@alap3.anarazel.de --- doc/src/sgml/logicaldecoding.sgml | 24 ++++++++++++++++++++++++ src/backend/replication/logical/decode.c | 14 ++++++++++++++ 2 files changed, 38 insertions(+) diff --git a/doc/src/sgml/logicaldecoding.sgml b/doc/src/sgml/logicaldecoding.sgml index cf705ed9cd..430d234d65 100644 --- a/doc/src/sgml/logicaldecoding.sgml +++ b/doc/src/sgml/logicaldecoding.sgml @@ -1227,5 +1227,29 @@ stream_commit_cb(...); <-- commit of the streamed transaction that name pattern will not be decoded as a two-phase commit transaction. + + The users that want to decode prepared transactions need to be careful about + below mentioned points: + + + + + If the prepared transaction has locked [user] catalog tables exclusively + then decoding prepare can block till the main transaction is committed. + + + + + + The logical replication solution that builds distributed two phase commit + using this feature can deadlock if the prepared transaction has locked + [user] catalog tables exclusively. They need to inform users to not have + locks on catalog tables (via explicit LOCK command) in + such transactions. + + + + + diff --git a/src/backend/replication/logical/decode.c b/src/backend/replication/logical/decode.c index afa1df00d0..657cb4af1e 100644 --- a/src/backend/replication/logical/decode.c +++ b/src/backend/replication/logical/decode.c @@ -362,6 +362,20 @@ DecodeXactOp(LogicalDecodingContext *ctx, XLogRecordBuffer *buf) break; } + /* + * Note that if the prepared transaction has locked [user] + * catalog tables exclusively then decoding prepare can block + * till the main transaction is committed because it needs to + * lock the catalog tables. + * + * XXX Now, this can even lead to a deadlock if the prepare + * transaction is waiting to get it logically replicated for + * distributed 2PC. Currently, we don't have an in-core + * implementation of prepares for distributed 2PC but some + * out-of-core logical replication solution can have such an + * implementation. They need to inform users to not have locks + * on catalog tables in such transactions. + */ DecodePrepare(ctx, buf, &parsed); break; } -- 2.28.0.windows.1