Add checkpoint_create_hook

Allows an extension to hook into CheckPointCreate().

Author: tristan957

src/backend/access/transam/xlog.c

@@ -208,6 +208,8 @@ const struct config_enum_entry archive_mode_options[] = {
  */
 CheckpointStatsData CheckpointStats;
 
+checkpoint_create_hook_type checkpoint_create_hook = NULL;
+
 /*
  * During recovery, lastFullPageWrites keeps track of full_page_writes that
  * the replayed WAL records indicate. It's initialized with full_page_writes
@@ -7173,6 +7175,9 @@ CreateCheckPoint(int flags)
 	 */
 	END_CRIT_SECTION();
 
+	if (checkpoint_create_hook != NULL)
+		checkpoint_create_hook(&checkPoint);
+
 	/*
 	 * In some cases there are groups of actions that must all occur on one
 	 * side or the other of a checkpoint record. Before flushing the

src/include/access/xlog.h

@@ -13,6 +13,7 @@
 
 #include "access/xlogbackup.h"
 #include "access/xlogdefs.h"
+#include "catalog/pg_control.h"
 #include "datatype/timestamp.h"
 #include "lib/stringinfo.h"
 #include "nodes/pg_list.h"
@@ -59,6 +60,9 @@ extern PGDLLIMPORT int wal_decode_buffer_size;
 
 extern PGDLLIMPORT int CheckPointSegments;
 
+typedef void (*checkpoint_create_hook_type) (const CheckPoint *);
+extern PGDLLIMPORT checkpoint_create_hook_type checkpoint_create_hook;
+
 /* Archive modes */
 typedef enum ArchiveMode
 {