Verify configuration before adding data node

This change will call a function on a remote database to validate
its configuration before following through with an add_data_node
call.  Right now the check will ensure that the data is able to use
prepared transactions, but further checks can be easily added in
the future.

Since this uses the timescaledb extension to validate the remote
database, it runs at the end of bootstrapping.  We may want to
consider adding code to undo our bootstrapping changes if this check
fails.

sql/dist_internal.sql

@@ -25,3 +25,6 @@ RETURNS TABLE (id   int,
     toast_bytes     bigint,
     total_bytes     bigint)
 AS '@MODULE_PATHNAME@', 'ts_dist_remote_hypertable_info' LANGUAGE C VOLATILE STRICT;
+
+CREATE OR REPLACE FUNCTION _timescaledb_internal.validate_as_data_node() RETURNS BOOL
+AS '@MODULE_PATHNAME@', 'ts_dist_validate_as_data_node' LANGUAGE C VOLATILE STRICT;

src/cross_module_fn.c

@@ -62,6 +62,7 @@ TS_FUNCTION_INFO_V1(ts_dist_set_id);
 TS_FUNCTION_INFO_V1(ts_dist_remove_id);
 TS_FUNCTION_INFO_V1(ts_dist_set_peer_id);
 TS_FUNCTION_INFO_V1(ts_dist_remote_hypertable_info);
+TS_FUNCTION_INFO_V1(ts_dist_validate_as_data_node);
 
 Datum
 ts_add_drop_chunks_policy(PG_FUNCTION_ARGS)
@@ -226,6 +227,12 @@ ts_dist_remote_hypertable_info(PG_FUNCTION_ARGS)
 	PG_RETURN_DATUM(ts_cm_functions->remote_hypertable_info(fcinfo));
 }
 
+Datum
+ts_dist_validate_as_data_node(PG_FUNCTION_ARGS)
+{
+	PG_RETURN_BOOL(ts_cm_functions->validate_as_data_node(fcinfo));
+}
+
 /*
  * stub function to trigger aggregate util functions.
  */
@@ -631,6 +638,7 @@ TSDLLEXPORT CrossModuleFunctions ts_cm_functions_default = {
 	.is_frontend_session = error_no_default_fn_bool_void_community,
 	.remove_from_distributed_db = error_no_default_fn_bool_void_community,
 	.remote_hypertable_info = error_no_default_fn_pg_community,
+	.validate_as_data_node = error_no_default_fn_bool_void_community,
 };
 
 TSDLLEXPORT CrossModuleFunctions *ts_cm_functions = &ts_cm_functions_default;

src/cross_module_fn.h

@@ -139,6 +139,7 @@ typedef struct CrossModuleFunctions
 	bool (*is_frontend_session)(void);
 	bool (*remove_from_distributed_db)(void);
 	PGFunction remote_hypertable_info;
+	bool (*validate_as_data_node)();
 } CrossModuleFunctions;
 
 extern TSDLLEXPORT CrossModuleFunctions *ts_cm_functions;

src/errors.h

@@ -46,6 +46,7 @@
 #define ERRCODE_TS_DATA_NODE_ASSIGNMENT_ALREADY_EXISTS MAKE_SQLSTATE('T', 'S', '1', '7', '1')
 #define ERRCODE_TS_DATA_NODE_ALREADY_ATTACHED MAKE_SQLSTATE('T', 'S', '1', '7', '2')
 #define ERRCODE_TS_DATA_NODE_NOT_ATTACHED MAKE_SQLSTATE('T', 'S', '1', '7', '3')
+#define ERRCODE_TS_DATA_NODE_INVALID_CONFIG MAKE_SQLSTATE('T', 'S', '1', '7', '4')
 
 /*
 --IO500 - GROUP: internal error

tsl/src/data_node.c

@@ -387,6 +387,7 @@ data_node_bootstrap_extension(const char *node_name, const char *host, int32 por
 		const char *schema_name_quoted = quote_identifier(schema_name);
 		Oid schema_oid = get_namespace_oid(schema_name, true);
 		bool extension_exists = false;
+		bool data_node_valid = false;
 
 		request = psprintf("SELECT 1 FROM pg_extension WHERE extname = %s",
 						   quote_literal_cstr(EXTENSION_NAME));
@@ -395,6 +396,8 @@ data_node_bootstrap_extension(const char *node_name, const char *host, int32 por
 		if (PQntuples(res) > 0)
 			extension_exists = true;
 
+		remote_connection_result_close(res);
+
 		if (!extension_exists)
 		{
 			if (schema_oid != PG_PUBLIC_NAMESPACE)
@@ -414,6 +417,20 @@ data_node_bootstrap_extension(const char *node_name, const char *host, int32 por
 
 			created = true;
 		}
+
+		request = "SELECT _timescaledb_internal.validate_as_data_node()";
+		res = remote_connection_query_any_result(conn, request);
+		data_node_valid =
+			PQntuples(res) == 1 && PQnfields(res) == 1 && PQgetvalue(res, 0, 0)[0] == 't';
+		remote_connection_result_close(res);
+
+		if (!data_node_valid)
+			ereport(ERROR,
+					(errcode(ERRCODE_TS_DATA_NODE_INVALID_CONFIG),
+					 (errmsg("postgres instance for data node is not properly configured for "
+							 "TimescaleDB distributed operations"),
+					  errhint("max_prepared_transactions should be >= max_connections, note that "
+							  "changing these setting requires a postgres restart"))));
 	}
 	PG_CATCH();
 	{

tsl/src/dist_util.c

@@ -15,6 +15,8 @@
 #include <utils/fmgrprotos.h>
 #include "remote/dist_commands.h"
 #include "funcapi.h"
+#include <access/twophase.h>
+#include <miscadmin.h>
 
 /*
  * When added to a distributed database, this key in the metadata table will be set to match the
@@ -205,3 +207,9 @@ dist_util_remote_hypertable_info(PG_FUNCTION_ARGS)
 	ts_dist_cmd_close_response(funcctx->user_fctx);
 	SRF_RETURN_DONE(funcctx);
 }
+
+bool
+validate_data_node_settings(void)
+{
+	return MaxConnections > 0 && max_prepared_xacts >= MaxConnections;
+}

tsl/src/dist_util.h

@@ -28,8 +28,8 @@ const char *dist_util_internal_key_name(void);
 void dist_util_set_peer_id(Datum dist_id);
 bool dist_util_is_frontend_session(void);
 
-#if !PG96
 Datum dist_util_remote_hypertable_info(PG_FUNCTION_ARGS);
-#endif
+
+bool validate_data_node_settings(void);
 
 #endif /* TIMESCALEDB_TSL_CHUNK_API_H */

tsl/src/init.c

@@ -253,6 +253,7 @@ CrossModuleFunctions tsl_cm_functions = {
 	.is_frontend_session = NULL,
 	.remove_from_distributed_db = NULL,
 	.remote_hypertable_info = error_not_supported_default_fn,
+	.validate_as_data_node = NULL,
 #else
 	.add_data_node = data_node_add,
 	.delete_data_node = data_node_delete,
@@ -282,6 +283,7 @@ CrossModuleFunctions tsl_cm_functions = {
 	.is_frontend_session = dist_util_is_frontend_session,
 	.remove_from_distributed_db = dist_util_remove_from_db,
 	.remote_hypertable_info = dist_util_remote_hypertable_info,
+	.validate_as_data_node = validate_data_node_settings,
 #endif
 	.cache_syscache_invalidate = cache_syscache_invalidate,
 };