Allow extensions to override the global storage manager

Author: tristan957

src/backend/storage/smgr/smgr.c

@@ -69,6 +69,8 @@ static int	NSmgr = 0;
 
 static Size LargestSMgrRelationSize = 0;
 
+SMgrId		storage_manager_id;
+
 /*
  * Each backend has a hashtable that stores all extant SMgrRelation objects.
  * In addition, "unpinned" SMgrRelation objects are chained together in a list.
@@ -227,7 +229,7 @@ smgropen(RelFileLocator rlocator, ProcNumber backend)
 		for (int i = 0; i <= MAX_FORKNUM; ++i)
 			reln->smgr_cached_nblocks[i] = InvalidBlockNumber;
 
-		reln->smgr_which = MdSMgrId;	/* we only have md.c at present */
+		reln->smgr_which = storage_manager_id;
 
 		/* implementation-specific initialization */
 		smgrsw[reln->smgr_which].smgr_open(reln);

src/backend/utils/init/miscinit.c

@@ -1931,6 +1931,8 @@ void
 register_builtin_dynamic_managers(void)
 {
 	mdsmgr_register();
+
+	storage_manager_id = MdSMgrId;
 }
 
 /*

src/include/storage/smgr.h

@@ -20,6 +20,8 @@
 
 typedef uint8 SMgrId;
 
+extern PGDLLIMPORT SMgrId storage_manager_id;
+
 /*
  * smgr.c maintains a table of SMgrRelation objects, which are essentially
  * cached file handles.  An SMgrRelation is created (if not already present)