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Update the docs and comments for decoding of prepared xacts.

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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
This commit is contained in:
Amit Kapila 2021-03-01 08:14:33 +05:30
parent 6148656a0b
commit b4e3dc7fd4
2 changed files with 38 additions and 0 deletions
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24
doc/src/sgml/logicaldecoding.sgml
View File

@ -1228,5 +1228,29 @@ stream_commit_cb(...); <-- commit of the streamed transaction
that name pattern will not be decoded as a two-phase commit transaction.
</para>
<para>
The users that want to decode prepared transactions need to be careful about
below mentioned points:
<itemizedlist>
<listitem>
<para>
If the prepared transaction has locked [user] catalog tables exclusively
then decoding prepare can block till the main transaction is committed.
</para>
</listitem>
<listitem>
<para>
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 <command>LOCK</command> command) in
such transactions.
</para>
</listitem>
</itemizedlist>
</para>
</sect1>
</chapter>

14
src/backend/replication/logical/decode.c
View File

@ -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;
}