Support UPDATE/DELETE WHERE CURRENT OF cursor_name, per SQL standard.

Along the way, allow FOR UPDATE in non-WITH-HOLD cursors; there may once
have been a reason to disallow that, but it seems to work now, and it's
really rather necessary if you want to select a row via a cursor and then
update it in a concurrent-safe fashion.

Original patch by Arul Shaji, rather heavily editorialized by Tom Lane.

src/backend/executor/execCurrent.c

@@ -0,0 +1,185 @@
+/*-------------------------------------------------------------------------
+ *
+ * execCurrent.c
+ *	  executor support for WHERE CURRENT OF cursor
+ *
+ * Portions Copyright (c) 1996-2007, PostgreSQL Global Development Group
+ * Portions Copyright (c) 1994, Regents of the University of California
+ *
+ *	$PostgreSQL: pgsql/src/backend/executor/execCurrent.c,v 1.1 2007/06/11 01:16:22 tgl Exp $
+ *
+ *-------------------------------------------------------------------------
+ */
+#include "postgres.h"
+
+#include "executor/executor.h"
+#include "utils/lsyscache.h"
+#include "utils/portal.h"
+
+
+static ScanState *search_plan_tree(PlanState *node, Oid table_oid);
+
+
+/*
+ * execCurrentOf
+ *
+ * Given the name of a cursor and the OID of a table, determine which row
+ * of the table is currently being scanned by the cursor, and return its
+ * TID into *current_tid.
+ *
+ * Returns TRUE if a row was identified.  Returns FALSE if the cursor is valid
+ * for the table but is not currently scanning a row of the table (this is a
+ * legal situation in inheritance cases).  Raises error if cursor is not a
+ * valid updatable scan of the specified table.
+ */
+bool
+execCurrentOf(char *cursor_name, Oid table_oid,
+			  ItemPointer current_tid)
+{
+	char	   *table_name;
+	Portal		portal;
+	QueryDesc *queryDesc;
+	ScanState  *scanstate;
+	HeapTuple tup;
+
+	/* Fetch table name for possible use in error messages */
+	table_name = get_rel_name(table_oid);
+	if (table_name == NULL)
+		elog(ERROR, "cache lookup failed for relation %u", table_oid);
+
+	/* Find the cursor's portal */
+	portal = GetPortalByName(cursor_name);
+	if (!PortalIsValid(portal))
+		ereport(ERROR,
+				(errcode(ERRCODE_UNDEFINED_CURSOR),
+				 errmsg("cursor \"%s\" does not exist", cursor_name)));
+
+	/*
+	 * We have to watch out for non-SELECT queries as well as held cursors,
+	 * both of which may have null queryDesc.
+	 */
+	if (portal->strategy != PORTAL_ONE_SELECT)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_CURSOR_STATE),
+				 errmsg("cursor \"%s\" is not a SELECT query",
+						cursor_name)));
+	queryDesc = PortalGetQueryDesc(portal);
+	if (queryDesc == NULL)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_CURSOR_STATE),
+				 errmsg("cursor \"%s\" is held from a previous transaction",
+						cursor_name)));
+
+	/*
+	 * Dig through the cursor's plan to find the scan node.  Fail if it's
+	 * not there or buried underneath aggregation.
+	 */
+	scanstate = search_plan_tree(ExecGetActivePlanTree(queryDesc),
+								 table_oid);
+	if (!scanstate)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_CURSOR_STATE),
+				 errmsg("cursor \"%s\" is not a simply updatable scan of table \"%s\"",
+						cursor_name, table_name)));
+
+	/*
+	 * The cursor must have a current result row: per the SQL spec, it's
+	 * an error if not.  We test this at the top level, rather than at
+	 * the scan node level, because in inheritance cases any one table
+	 * scan could easily not be on a row.  We want to return false, not
+	 * raise error, if the passed-in table OID is for one of the inactive
+	 * scans.
+	 */
+	if (portal->atStart || portal->atEnd)
+		ereport(ERROR,
+				(errcode(ERRCODE_INVALID_CURSOR_STATE),
+				 errmsg("cursor \"%s\" is not positioned on a row",
+						cursor_name)));
+
+	/* Now OK to return false if we found an inactive scan */
+	if (TupIsNull(scanstate->ss_ScanTupleSlot))
+		return false;
+
+	tup = scanstate->ss_ScanTupleSlot->tts_tuple;
+	if (tup == NULL)
+		elog(ERROR, "CURRENT OF applied to non-materialized tuple");
+	Assert(tup->t_tableOid == table_oid);
+
+	*current_tid = tup->t_self;
+
+	return true;
+}
+
+/*
+ * search_plan_tree
+ *
+ * Search through a PlanState tree for a scan node on the specified table.
+ * Return NULL if not found or multiple candidates.
+ */
+static ScanState *
+search_plan_tree(PlanState *node, Oid table_oid)
+{
+	if (node == NULL)
+		return NULL;
+	switch (nodeTag(node))
+	{
+			/*
+			 * scan nodes can all be treated alike
+			 */
+		case T_SeqScanState:
+		case T_IndexScanState:
+		case T_BitmapHeapScanState:
+		case T_TidScanState:
+		{
+			ScanState *sstate = (ScanState *) node;
+
+			if (RelationGetRelid(sstate->ss_currentRelation) == table_oid)
+				return sstate;
+			break;
+		}
+
+			/*
+			 * For Append, we must look through the members; watch out for
+			 * multiple matches (possible if it was from UNION ALL)
+			 */
+		case T_AppendState:
+		{
+			AppendState *astate = (AppendState *) node;
+			ScanState *result = NULL;
+			int		i;
+
+			for (i = 0; i < astate->as_nplans; i++)
+			{
+				ScanState *elem = search_plan_tree(astate->appendplans[i],
+												   table_oid);
+
+				if (!elem)
+					continue;
+				if (result)
+					return NULL;				/* multiple matches */
+				result = elem;
+			}
+			return result;
+		}
+
+			/*
+			 * Result and Limit can be descended through (these are safe
+			 * because they always return their input's current row)
+			 */
+		case T_ResultState:
+		case T_LimitState:
+			return search_plan_tree(node->lefttree, table_oid);
+
+			/*
+			 * SubqueryScan too, but it keeps the child in a different place
+			 */
+		case T_SubqueryScanState:
+			return search_plan_tree(((SubqueryScanState *) node)->subplan,
+									table_oid);
+
+		default:
+			/* Otherwise, assume we can't descend through it */
+			break;
+	}
+	return NULL;
+}