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/backend/access/table/Makefile

@@ -0,0 +1,17 @@
+#-------------------------------------------------------------------------
+#
+# Makefile--
+#    Makefile for access/table
+#
+# IDENTIFICATION
+#    src/backend/access/table/Makefile
+#
+#-------------------------------------------------------------------------
+
+subdir = src/backend/access/table
+top_builddir = ../../../..
+include $(top_builddir)/src/Makefile.global
+
+OBJS = table.o
+
+include $(top_srcdir)/src/backend/common.mk

src/backend/access/table/table.c

@@ -0,0 +1,136 @@
+/*-------------------------------------------------------------------------
+ *
+ * table.c
+ *	  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
+ *
+ *
+ * IDENTIFICATION
+ *	  src/backend/access/table/table.c
+ *
+ *
+ * NOTES
+ *	  This file contains table_ routines that implement access to tables (in
+ *	  contrast to other relation types like indexes) that are independent of
+ *	  individual table access methods.
+ *
+ *-------------------------------------------------------------------------
+ */
+
+#include "postgres.h"
+
+#include "access/relation.h"
+#include "access/table.h"
+#include "storage/lmgr.h"
+
+
+/* ----------------
+ *		table_open - open a table relation by relation OID
+ *
+ *		This is essentially relation_open plus check that the relation
+ *		is not an index nor a composite type.  (The caller should also
+ *		check that it's not a view or foreign table before assuming it has
+ *		storage.)
+ * ----------------
+ */
+Relation
+table_open(Oid relationId, LOCKMODE lockmode)
+{
+	Relation	r;
+
+	r = relation_open(relationId, lockmode);
+
+	if (r->rd_rel->relkind == RELKIND_INDEX ||
+		r->rd_rel->relkind == RELKIND_PARTITIONED_INDEX)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("\"%s\" is an index",
+						RelationGetRelationName(r))));
+	else if (r->rd_rel->relkind == RELKIND_COMPOSITE_TYPE)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("\"%s\" is a composite type",
+						RelationGetRelationName(r))));
+
+	return r;
+}
+
+/* ----------------
+ *		table_openrv - open a table relation specified
+ *		by a RangeVar node
+ *
+ *		As above, but relation is specified by a RangeVar.
+ * ----------------
+ */
+Relation
+table_openrv(const RangeVar *relation, LOCKMODE lockmode)
+{
+	Relation	r;
+
+	r = relation_openrv(relation, lockmode);
+
+	if (r->rd_rel->relkind == RELKIND_INDEX ||
+		r->rd_rel->relkind == RELKIND_PARTITIONED_INDEX)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("\"%s\" is an index",
+						RelationGetRelationName(r))));
+	else if (r->rd_rel->relkind == RELKIND_COMPOSITE_TYPE)
+		ereport(ERROR,
+				(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+				 errmsg("\"%s\" is a composite type",
+						RelationGetRelationName(r))));
+
+	return r;
+}
+
+/* ----------------
+ *		table_openrv_extended - open a table relation specified
+ *		by a RangeVar node
+ *
+ *		As above, but optionally return NULL instead of failing for
+ *		relation-not-found.
+ * ----------------
+ */
+Relation
+table_openrv_extended(const RangeVar *relation, LOCKMODE lockmode,
+					  bool missing_ok)
+{
+	Relation	r;
+
+	r = relation_openrv_extended(relation, lockmode, missing_ok);
+
+	if (r)
+	{
+		if (r->rd_rel->relkind == RELKIND_INDEX ||
+			r->rd_rel->relkind == RELKIND_PARTITIONED_INDEX)
+			ereport(ERROR,
+					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+					 errmsg("\"%s\" is an index",
+							RelationGetRelationName(r))));
+		else if (r->rd_rel->relkind == RELKIND_COMPOSITE_TYPE)
+			ereport(ERROR,
+					(errcode(ERRCODE_WRONG_OBJECT_TYPE),
+					 errmsg("\"%s\" is a composite type",
+							RelationGetRelationName(r))));
+	}
+
+	return r;
+}
+
+/* ----------------
+ *		table_close - close a table
+ *
+ *		If lockmode is not "NoLock", we then release the specified lock.
+ *
+ *		Note that it is often sensible to hold a lock beyond relation_close;
+ *		in that case, the lock is released automatically at xact end.
+ *		----------------
+ */
+void
+table_close(Relation relation, LOCKMODE lockmode)
+{
+	relation_close(relation, lockmode);
+}