Provide a pg_truncate_freespacemap function

Makefile

@@ -11,7 +11,7 @@ RPM_MINOR_VERSION_SUFFIX ?=
 # Extension packaging
 EXTENSION = aiven_extras
 MODULE_big = aiven_extras
-OBJS = src/standby_slots.o
+OBJS = src/aiven_extras.o
 PG_CONFIG ?= pg_config
 DATA = $(filter-out $(generated),$(wildcard sql/*--*.sql))
 DATA_built = $(generated)

sql/aiven_extras.sql

@@ -580,3 +580,9 @@ CREATE FUNCTION aiven_extras.pg_create_logical_replication_slot_on_standby(
 	OUT slot_name name, OUT lsn pg_lsn)
 AS 'MODULE_PATHNAME', 'standby_slot_create'
 LANGUAGE C;
+
+DROP FUNCTION IF EXISTS aiven_extras.aiven_truncate_freespace_map(regclass);
+CREATE FUNCTION aiven_extras.truncate_freespace_map(regclass)
+RETURNS void 
+AS 'MODULE_PATHNAME', 'aiven_truncate_freespace_map'
+LANGUAGE C;

src/standby_slots.c → src/aiven_extras.c

@@ -2,19 +2,28 @@
 #include "fmgr.h"
 #include "funcapi.h"
 #include "miscadmin.h"
+#include "storage/freespace.h"
+#include "storage/smgr.h"
+#if PG_VERSION_NUM >= 160000
+#include "access/relation.h"
+#endif
+#include "access/xloginsert.h"
 #include "access/xlogreader.h"
 #include "access/xlogutils.h"
 #include "replication/logical.h"
+#include "catalog/storage_xlog.h"
 #include "replication/slot.h"
 #include "replication/walreceiver.h"
 #if PG_VERSION_NUM >= 120000 && PG_VERSION_NUM < 130000
 #include "replication/logicalfuncs.h"
 #endif
 #include "utils/pg_lsn.h"
+#include "utils/rel.h"
 
 PG_MODULE_MAGIC;
 
 PG_FUNCTION_INFO_V1(standby_slot_create);
+PG_FUNCTION_INFO_V1(aiven_truncate_freespace_map);
 	Datum
 standby_slot_create(PG_FUNCTION_ARGS)
 {
@@ -120,3 +129,63 @@ standby_slot_create(PG_FUNCTION_ARGS)
 	ReplicationSlotRelease();
 	PG_RETURN_DATUM(result);
 }
+
+
+
+#if PG_VERSION_NUM >= 160000
+Datum
+aiven_truncate_freespace_map(PG_FUNCTION_ARGS)
+{
+	Oid			relid = PG_GETARG_OID(0);
+	Relation	rel;
+	ForkNumber	fork;
+	BlockNumber block;
+
+	rel = relation_open(relid, AccessExclusiveLock);
+
+	/* Only some relkinds have a visibility map */
+	if (!RELKIND_HAS_TABLE_AM(rel->rd_rel->relkind))
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("relation \"%s\" is of wrong relation kind",
+						RelationGetRelationName(rel)),
+				 errdetail_relkind_not_supported(rel->rd_rel->relkind)));
+
+
+	/* Forcibly reset cached file size */
+	RelationGetSmgr(rel)->smgr_cached_nblocks[FSM_FORKNUM] = InvalidBlockNumber;
+
+	/* Just pretend we're going to wipeout the whole rel */
+	block = FreeSpaceMapPrepareTruncateRel(rel, 0);
+
+	if (BlockNumberIsValid(block))
+	{
+		fork = FSM_FORKNUM;
+		smgrtruncate(RelationGetSmgr(rel), &fork, 1, &block);
+	}
+
+	if (RelationNeedsWAL(rel))
+	{
+		xl_smgr_truncate xlrec;
+
+		xlrec.blkno = 0;
+		xlrec.rlocator = rel->rd_locator;
+		xlrec.flags = SMGR_TRUNCATE_FSM;
+
+		XLogBeginInsert();
+		XLogRegisterData((char *) &xlrec, sizeof(xlrec));
+
+		XLogInsert(RM_SMGR_ID, XLOG_SMGR_TRUNCATE | XLR_SPECIAL_REL_UPDATE);
+	}
+
+	relation_close(rel, AccessExclusiveLock);
+
+	PG_RETURN_VOID();
+}
+#else 
+Datum
+aiven_truncate_freespace_map(PG_FUNCTION_ARGS)
+{
+	elog(ERROR, "aiven_truncate_freespace_map is not supported on this version.");
+}
+#endif