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

On Mon, Sep 7, 2020 at 10:54 AM Ajin Cherian <itsajin(at)gmail(dot)com> wrote:
>
> Hello,
>
> Trying to revive this patch which attempts to support logical decoding of two phase transactions. I've rebased and polished Nikhil's patch on the current HEAD. Some of the logic in the previous patchset has already been committed as part of large-in-progress transaction commits, like the handling of concurrent aborts, so all that logic has been left out.
>

I am not sure about your point related to concurrent aborts. I think
we need some changes related to this patch. Have you tried to test
this behavior? Basically, we have the below code in
ReorderBufferProcessTXN() which will be hit for concurrent aborts, and
currently, the Asserts shown below will fail.

if (errdata->sqlerrcode == ERRCODE_TRANSACTION_ROLLBACK)
{
/*
* This error can only occur when we are sending the data in
* streaming mode and the streaming is not finished yet.
*/
Assert(streaming);
Assert(stream_started);

Nikhil has a test for the same
(0004-Teach-test_decoding-plugin-to-work-with-2PC.Jan4) in his last
email [1]. You might want to use it to test this behavior. I think you
can also keep the tests as a separate patch as Nikhil had.

One other comment:
===================
@@ -27,6 +27,7 @@ typedef struct OutputPluginOptions
{
OutputPluginOutputType output_type;
bool receive_rewrites;
+ bool enable_twophase;
} OutputPluginOptions;
..
..
@@ -684,6 +699,33 @@ startup_cb_wrapper(LogicalDecodingContext *ctx,
OutputPluginOptions *opt, bool i
/* do the actual work: call callback */
ctx->callbacks.startup_cb(ctx, opt, is_init);

+ /*
+ * If the plugin claims to support two-phase transactions, then
+ * check that the plugin implements all callbacks necessary to decode
+ * two-phase transactions - we either have to have all of them or none.
+ * The filter_prepare callback is optional, but can only be defined when
+ * two-phase decoding is enabled (i.e. the three other callbacks are
+ * defined).
+ */
+ if (opt->enable_twophase)
+ {
+ int twophase_callbacks = (ctx->callbacks.prepare_cb != NULL) +
+ (ctx->callbacks.commit_prepared_cb != NULL) +
+ (ctx->callbacks.abort_prepared_cb != NULL);
+
+ /* Plugins with incorrect number of two-phase callbacks are broken. */
+ if ((twophase_callbacks != 3) && (twophase_callbacks != 0))
+ ereport(ERROR,
+ (errmsg("Output plugin registered only %d twophase callbacks. ",
+ twophase_callbacks)));
+ }

I don't know why the patch has used this way to implement an option to
enable two-phase. Can't we use how we implement 'stream-changes'
option in commit 7259736a6e? Just refer how we set ctx->streaming and
you can use a similar way to set this parameter.

--
With Regards,
Amit Kapila.