Re: [HACKERS] logical decoding of two-phase transactions

On Tue, Jun 8, 2021 at 4:12 PM Peter Smith <smithpb2250(at)gmail(dot)com> wrote:
>
> Please find attached the latest patch set v83*
>

Some feedback for the v83 patch set:

v83-0001:

(1) doc/src/sgml/protocol.sgml

(i) Remove extra space:

BEFORE:
+ The transaction will be decoded and transmitted at
AFTER:
+ The transaction will be decoded and transmitted at

(ii)
BEFORE:
+ contains Stream Commit or Stream Abort message.
AFTER:
+ contains a Stream Commit or Stream Abort message.

(iii)
BEFORE:
+ The LSN of the commit prepared.
AFTER:
+ The LSN of the commit prepared transaction.

(iv) Should documentation say "prepared transaction" as opposed to
"prepare transaction" ???

BEFORE:
+ The end LSN of the prepare transaction.
AFTER:
+ The end LSN of the prepared transaction.

(2) doc/src/sgml/ref/create_subscription.sgml

(i)
BEFORE:
+ The <literal>streaming</literal> option cannot be used along with
+ <literal>two_phase</literal> option.
AFTER:
+ The <literal>streaming</literal> option cannot be used with the
+ <literal>two_phase</literal> option.

(3) doc/src/sgml/ref/create_subscription.sgml

(i)
BEFORE:
+ prepared on publisher is decoded as normal transaction at commit.
AFTER:
+ prepared on the publisher is decoded as a normal
transaction at commit.

(ii)
BEFORE:
+ The <literal>two_phase</literal> option cannot be used along with
+ <literal>streaming</literal> option.
AFTER:
+ The <literal>two_phase</literal> option cannot be used with the
+ <literal>streaming</literal> option.

(4) src/backend/access/transam/twophase.c

(i)
BEFORE:
+ * Check if the prepared transaction with the given GID, lsn and timestamp
+ * is around.
AFTER:
+ * Check if the prepared transaction with the given GID, lsn and timestamp
+ * exists.

(5) src/backend/access/transam/twophase.c

Question:

Is:

+ * do this optimization if we encounter many collisions in GID

meant to be:

+ * do this optimization if we encounter any collisions in GID

???

(6) src/backend/replication/logical/decode.c

Grammar:

BEFORE:
+ * distributed 2PC. This can be avoided by disallowing to
+ * prepare transactions that have locked [user] catalog tables
+ * exclusively but as of now we ask users not to do such
+ * operation.
AFTER:
+ * distributed 2PC. This can be avoided by disallowing
+ * prepared transactions that have locked [user] catalog tables
+ * exclusively but as of now we ask users not to do such an
+ * operation.

(7) src/backend/replication/logical/logical.c

From the comment above it, it's not clear if the "&=" in the following
line is intentional:

+ ctx->twophase &= (ctx->twophase_opt_given || slot->data.two_phase);

Also, the boolean conditions tested are in the reverse order of what
is mentioned in that comment.
Based on the comment, I would expect the following code:

+ ctx->twophase = (slot->data.two_phase || ctx->twophase_opt_given);

Please check it, and maybe update the comment if "&=" is really intended.

There are TWO places where this same code is used.

(8) src/backend/replication/logical/tablesync.c

In the following code, "has_subrels" should be a bool, not an int.

+static bool
+FetchTableStates(bool *started_tx)
+{
+ static int has_subrels = false;

(9) src/backend/replication/logical/worker.c

Mixed current/past tense:

BEFORE:
+ * was still busy (see the condition of should_apply_changes_for_rel). The
AFTER:
+ * is still busy (see the condition of should_apply_changes_for_rel). The

(10)

2 places:

BEFORE:
+ /* there is no transaction when COMMIT PREPARED is called */
AFTER:
+ /* There is no transaction when COMMIT PREPARED is called */

v83-0002:

1) doc/src/sgml/protocol.sgml

BEFORE:
+ contains Stream Prepare or Stream Commit or Stream Abort message.
AFTER:
+ contains a Stream Prepare or Stream Commit or Stream Abort message.

v83-0003:

1) src/backend/replication/pgoutput/pgoutput.c

i) In pgoutput_commit_txn(), the following code that pfree()s a
pointer in a struct, without then NULLing it out, seems dangerous to
me (because what is to stop other code, either now or in the future,
from subsequently referencing that freed data or perhaps trying to
pfree() again?):

+ PGOutputTxnData *data = (PGOutputTxnData *) txn->output_plugin_private;
+ bool skip;
+
+ Assert(data);
+ skip = !data->sent_begin_txn;
+ pfree(data);

I suggest adding the following line of code after the pfree():
+ txn->output_plugin_private = NULL;

ii) In pgoutput_commit_prepared_txn(), there's the same type of code:

+ if (data)
+ {
+ bool skip = !data->sent_begin_txn;
+ pfree(data);
+ if (skip)
+ return;
+ }

I suggest adding the following line after the pfree() above:

+ txn->output_plugin_private = NULL;

iii) Again, same thing in pgoutput_rollback_prepared_txn():

I suggest adding the following line after the pfree() above:

+ txn->output_plugin_private = NULL;

Regards,
Greg Nancarrow
Fujitsu Australia