Extend the output plugin API to allow decoding of prepared xacts.

This adds six methods to the output plugin API, adding support for
streaming changes of two-phase transactions at prepare time.

* begin_prepare
* filter_prepare
* prepare
* commit_prepared
* rollback_prepared
* stream_prepare

Most of this is a simple extension of the existing methods, with the
semantic difference that the transaction is not yet committed and maybe
aborted later.

Until now two-phase transactions were translated into regular transactions
on the subscriber, and the GID was not forwarded to it. None of the
two-phase commands were communicated to the subscriber.

This patch provides the infrastructure for logical decoding plugins to be
informed of two-phase commands Like PREPARE TRANSACTION, COMMIT PREPARED
and ROLLBACK PREPARED commands with the corresponding GID.

This also extends the 'test_decoding' plugin, implementing these new
methods.

This commit simply adds these new APIs and the upcoming patch to "allow
the decoding at prepare time in ReorderBuffer" will use these APIs.

Author: Ajin Cherian and Amit Kapila based on previous work by Nikhil Sontakke and Stas Kelvich
Reviewed-by: Amit Kapila, Peter Smith, Sawada Masahiko, and Dilip Kumar
Discussion:
https://postgr.es/m/02DA5F5E-CECE-4D9C-8B4B-418077E2C010@postgrespro.ru
https://postgr.es/m/CAMGcDxeqEpWj3fTXwqhSwBdXd2RS9jzwWscO-XbeCfso6ts3+Q@mail.gmail.com

Author: Amit Kapila

contrib/test_decoding/test_decoding.c

@@ -76,6 +76,20 @@ static void pg_decode_message(LogicalDecodingContext *ctx,
 							  ReorderBufferTXN *txn, XLogRecPtr message_lsn,
 							  bool transactional, const char *prefix,
 							  Size sz, const char *message);
+static bool pg_decode_filter_prepare(LogicalDecodingContext *ctx,
+									 const char *gid);
+static void pg_decode_begin_prepare_txn(LogicalDecodingContext *ctx,
+										ReorderBufferTXN *txn);
+static void pg_decode_prepare_txn(LogicalDecodingContext *ctx,
+								  ReorderBufferTXN *txn,
+								  XLogRecPtr prepare_lsn);
+static void pg_decode_commit_prepared_txn(LogicalDecodingContext *ctx,
+										  ReorderBufferTXN *txn,
+										  XLogRecPtr commit_lsn);
+static void pg_decode_rollback_prepared_txn(LogicalDecodingContext *ctx,
+											ReorderBufferTXN *txn,
+											XLogRecPtr prepare_end_lsn,
+											TimestampTz prepare_time);
 static void pg_decode_stream_start(LogicalDecodingContext *ctx,
 								   ReorderBufferTXN *txn);
 static void pg_output_stream_start(LogicalDecodingContext *ctx,
@@ -87,6 +101,9 @@ static void pg_decode_stream_stop(LogicalDecodingContext *ctx,
 static void pg_decode_stream_abort(LogicalDecodingContext *ctx,
 								   ReorderBufferTXN *txn,
 								   XLogRecPtr abort_lsn);
+static void pg_decode_stream_prepare(LogicalDecodingContext *ctx,
+									 ReorderBufferTXN *txn,
+									 XLogRecPtr prepare_lsn);
 static void pg_decode_stream_commit(LogicalDecodingContext *ctx,
 									ReorderBufferTXN *txn,
 									XLogRecPtr commit_lsn);
@@ -123,9 +140,15 @@ _PG_output_plugin_init(OutputPluginCallbacks *cb)
 	cb->filter_by_origin_cb = pg_decode_filter;
 	cb->shutdown_cb = pg_decode_shutdown;
 	cb->message_cb = pg_decode_message;
+	cb->filter_prepare_cb = pg_decode_filter_prepare;
+	cb->begin_prepare_cb = pg_decode_begin_prepare_txn;
+	cb->prepare_cb = pg_decode_prepare_txn;
+	cb->commit_prepared_cb = pg_decode_commit_prepared_txn;
+	cb->rollback_prepared_cb = pg_decode_rollback_prepared_txn;
 	cb->stream_start_cb = pg_decode_stream_start;
 	cb->stream_stop_cb = pg_decode_stream_stop;
 	cb->stream_abort_cb = pg_decode_stream_abort;
+	cb->stream_prepare_cb = pg_decode_stream_prepare;
 	cb->stream_commit_cb = pg_decode_stream_commit;
 	cb->stream_change_cb = pg_decode_stream_change;
 	cb->stream_message_cb = pg_decode_stream_message;
@@ -141,6 +164,7 @@ pg_decode_startup(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
 	ListCell   *option;
 	TestDecodingData *data;
 	bool		enable_streaming = false;
+	bool		enable_twophase = false;
 
 	data = palloc0(sizeof(TestDecodingData));
 	data->context = AllocSetContextCreate(ctx->context,
@@ -241,6 +265,16 @@ pg_decode_startup(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
 						 errmsg("could not parse value \"%s\" for parameter \"%s\"",
 								strVal(elem->arg), elem->defname)));
 		}
+		else if (strcmp(elem->defname, "two-phase-commit") == 0)
+		{
+			if (elem->arg == NULL)
+				continue;
+			else if (!parse_bool(strVal(elem->arg), &enable_twophase))
+				ereport(ERROR,
+						(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
+						 errmsg("could not parse value \"%s\" for parameter \"%s\"",
+								strVal(elem->arg), elem->defname)));
+		}
 		else
 		{
 			ereport(ERROR,
@@ -252,6 +286,7 @@ pg_decode_startup(LogicalDecodingContext *ctx, OutputPluginOptions *opt,
 	}
 
 	ctx->streaming &= enable_streaming;
+	ctx->twophase &= enable_twophase;
 }
 
 /* cleanup this plugin's resources */
@@ -320,6 +355,111 @@ pg_decode_commit_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
 	OutputPluginWrite(ctx, true);
 }
 
+/* BEGIN PREPARE callback */
+static void
+pg_decode_begin_prepare_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn)
+{
+	TestDecodingData *data = ctx->output_plugin_private;
+	TestDecodingTxnData *txndata =
+	MemoryContextAllocZero(ctx->context, sizeof(TestDecodingTxnData));
+
+	txndata->xact_wrote_changes = false;
+	txn->output_plugin_private = txndata;
+
+	if (data->skip_empty_xacts)
+		return;
+
+	pg_output_begin(ctx, data, txn, true);
+}
+
+/* PREPARE callback */
+static void
+pg_decode_prepare_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
+					  XLogRecPtr prepare_lsn)
+{
+	TestDecodingData *data = ctx->output_plugin_private;
+	TestDecodingTxnData *txndata = txn->output_plugin_private;
+
+	if (data->skip_empty_xacts && !txndata->xact_wrote_changes)
+		return;
+
+	OutputPluginPrepareWrite(ctx, true);
+
+	appendStringInfo(ctx->out, "PREPARE TRANSACTION %s",
+					 quote_literal_cstr(txn->gid));
+
+	if (data->include_xids)
+		appendStringInfo(ctx->out, ", txid %u", txn->xid);
+
+	if (data->include_timestamp)
+		appendStringInfo(ctx->out, " (at %s)",
+						 timestamptz_to_str(txn->commit_time));
+
+	OutputPluginWrite(ctx, true);
+}
+
+/* COMMIT PREPARED callback */
+static void
+pg_decode_commit_prepared_txn(LogicalDecodingContext *ctx, ReorderBufferTXN *txn,
+							  XLogRecPtr commit_lsn)
+{
+	TestDecodingData *data = ctx->output_plugin_private;
+
+	OutputPluginPrepareWrite(ctx, true);
+
+	appendStringInfo(ctx->out, "COMMIT PREPARED %s",
+					 quote_literal_cstr(txn->gid));
+
+	if (data->include_xids)
+		appendStringInfo(ctx->out, ", txid %u", txn->xid);
+
+	if (data->include_timestamp)
+		appendStringInfo(ctx->out, " (at %s)",
+						 timestamptz_to_str(txn->commit_time));
+
+	OutputPluginWrite(ctx, true);
+}
+
+/* ROLLBACK PREPARED callback */
+static void
+pg_decode_rollback_prepared_txn(LogicalDecodingContext *ctx,
+								ReorderBufferTXN *txn,
+								XLogRecPtr prepare_end_lsn,
+								TimestampTz prepare_time)
+{
+	TestDecodingData *data = ctx->output_plugin_private;
+
+	OutputPluginPrepareWrite(ctx, true);
+
+	appendStringInfo(ctx->out, "ROLLBACK PREPARED %s",
+					 quote_literal_cstr(txn->gid));
+
+	if (data->include_xids)
+		appendStringInfo(ctx->out, ", txid %u", txn->xid);
+
+	if (data->include_timestamp)
+		appendStringInfo(ctx->out, " (at %s)",
+						 timestamptz_to_str(txn->commit_time));
+
+	OutputPluginWrite(ctx, true);
+}
+
+/*
+ * Filter out two-phase transactions.
+ *
+ * Each plugin can implement its own filtering logic. Here we demonstrate a
+ * simple logic by checking the GID. If the GID contains the "_nodecode"
+ * substring, then we filter it out.
+ */
+static bool
+pg_decode_filter_prepare(LogicalDecodingContext *ctx, const char *gid)
+{
+	if (strstr(gid, "_nodecode") != NULL)
+		return true;
+
+	return false;
+}
+
 static bool
 pg_decode_filter(LogicalDecodingContext *ctx,
 				 RepOriginId origin_id)
@@ -701,6 +841,33 @@ pg_decode_stream_abort(LogicalDecodingContext *ctx,
 	OutputPluginWrite(ctx, true);
 }
 
+static void
+pg_decode_stream_prepare(LogicalDecodingContext *ctx,
+						 ReorderBufferTXN *txn,
+						 XLogRecPtr prepare_lsn)
+{
+	TestDecodingData *data = ctx->output_plugin_private;
+	TestDecodingTxnData *txndata = txn->output_plugin_private;
+
+	if (data->skip_empty_xacts && !txndata->xact_wrote_changes)
+		return;
+
+	OutputPluginPrepareWrite(ctx, true);
+
+	if (data->include_xids)
+		appendStringInfo(ctx->out, "preparing streamed transaction TXN %s, txid %u",
+						 quote_literal_cstr(txn->gid), txn->xid);
+	else
+		appendStringInfo(ctx->out, "preparing streamed transaction %s",
+						 quote_literal_cstr(txn->gid));
+
+	if (data->include_timestamp)
+		appendStringInfo(ctx->out, " (at %s)",
+						 timestamptz_to_str(txn->commit_time));
+
+	OutputPluginWrite(ctx, true);
+}
+
 static void
 pg_decode_stream_commit(LogicalDecodingContext *ctx,
 						ReorderBufferTXN *txn,