Introduce access/{table.h, relation.h}, for generic functions from heapam.h.

access/heapam contains functions that are very storage specific (say
heap_insert() and a lot of lower level functions), and fairly generic
infrastructure like relation_open(), heap_open() etc.  In the upcoming
pluggable storage work we're introducing a layer between table
accesses in general and heapam, to allow for different storage
methods. For a bit cleaner separation it thus seems advantageous to
move generic functions like the aforementioned to their own headers.

access/relation.h will contain relation_open() etc, and access/table.h
will contain table_open() (formerly known as heap_open()). I've decided
for table.h not to include relation.h, but we might change that at a
later stage.

relation.h already exists in another directory, but the other
plausible name (rel.h) also conflicts. It'd be nice if there were a
non-conflicting name, but nobody came up with a suggestion. It's
possible that the appropriate way to address the naming conflict would
be to rename nodes/relation.h, which isn't particularly well named.

To avoid breaking a lot of extensions that just use heap_open() etc,
table.h has macros mapping the old names to the new ones, and heapam.h
includes relation, table.h.  That also allows to keep the
bulk renaming of existing callers in a separate commit.

Author: Andres Freund
Discussion: https://postgr.es/m/20190111000539.xbv7s6w7ilcvm7dp@alap3.anarazel.de

src/include/access/heapam.h

@@ -14,8 +14,10 @@
 #ifndef HEAPAM_H
 #define HEAPAM_H
 
+#include "access/relation.h"	/* for backward compatibility */
 #include "access/sdir.h"
 #include "access/skey.h"
+#include "access/table.h"		/* for backward compatibility */
 #include "nodes/lockoptions.h"
 #include "nodes/primnodes.h"
 #include "storage/bufpage.h"
@@ -67,20 +69,6 @@ typedef struct HeapUpdateFailureData
  * ----------------
  */
 
-/* in heap/heapam.c */
-extern Relation relation_open(Oid relationId, LOCKMODE lockmode);
-extern Relation try_relation_open(Oid relationId, LOCKMODE lockmode);
-extern Relation relation_openrv(const RangeVar *relation, LOCKMODE lockmode);
-extern Relation relation_openrv_extended(const RangeVar *relation,
-						 LOCKMODE lockmode, bool missing_ok);
-extern void relation_close(Relation relation, LOCKMODE lockmode);
-
-extern Relation heap_open(Oid relationId, LOCKMODE lockmode);
-extern Relation heap_openrv(const RangeVar *relation, LOCKMODE lockmode);
-extern Relation heap_openrv_extended(const RangeVar *relation,
-					 LOCKMODE lockmode, bool missing_ok);
-
-#define heap_close(r,l)  relation_close(r,l)
 
 /* struct definitions appear in relscan.h */
 typedef struct HeapScanDescData *HeapScanDesc;

src/include/access/relation.h

@@ -0,0 +1,29 @@
+/*-------------------------------------------------------------------------
+ *
+ * relation.h
+ *	  Generic relation related routines.
+ *
+ *
+ * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/access/relation.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef ACCESS_RELATION_H
+#define ACCESS_RELATION_H
+
+#include "nodes/primnodes.h"
+#include "utils/relcache.h"
+#include "storage/lockdefs.h"
+
+
+extern Relation relation_open(Oid relationId, LOCKMODE lockmode);
+extern Relation try_relation_open(Oid relationId, LOCKMODE lockmode);
+extern Relation relation_openrv(const RangeVar *relation, LOCKMODE lockmode);
+extern Relation relation_openrv_extended(const RangeVar *relation,
+						 LOCKMODE lockmode, bool missing_ok);
+extern void relation_close(Relation relation, LOCKMODE lockmode);
+
+#endif							/* ACCESS_RELATION_H */

src/include/access/table.h

@@ -0,0 +1,38 @@
+/*-------------------------------------------------------------------------
+ *
+ * table.h
+ *	  Generic routines for table related code.
+ *
+ *
+ * Portions Copyright (c) 1996-2019, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ * src/include/access/table.h
+ *
+ *-------------------------------------------------------------------------
+ */
+#ifndef TABLE_H
+#define TABLE_H
+
+#include "nodes/primnodes.h"
+#include "utils/relcache.h"
+#include "storage/lockdefs.h"
+
+
+extern Relation table_open(Oid relationId, LOCKMODE lockmode);
+extern Relation table_openrv(const RangeVar *relation, LOCKMODE lockmode);
+extern Relation table_openrv_extended(const RangeVar *relation,
+					  LOCKMODE lockmode, bool missing_ok);
+extern void table_close(Relation relation, LOCKMODE lockmode);
+
+/*
+ * heap_ used to be the prefix for these routines, and a lot of code will just
+ * continue to work without adaptions after the introduction of pluggable
+ * storage, therefore just map these names.
+ */
+#define heap_open(r, l)					table_open(r, l)
+#define heap_openrv(r, l)				table_openrv(r, l)
+#define heap_openrv_extended(r, l, m)	table_openrv_extended(r, l, m)
+#define heap_close(r, l)				table_close(r, l)
+
+#endif							/* TABLE_H */