Add castNode(type, ptr) for safe casting between NodeTag based types.

anarazel · anarazel · commit 1ad163ef0130 · 2017-01-26T16:47:04.000-08:00
The new function allows to cast from one NodeTag based type to another, while asserting that the conversion is valid. This replaces the common pattern of doing a cast and a Assert(IsA(ptr, type)) close-by. As this seems likely to be used pervasively, we decided to backpatch this change the addition of this macro. Otherwise backpatched fixes are more likely not to work on back-branches. On branches before 9.6, where we do not yet rely on inline functions being available, the type assertion is only performed if PG_USE_INLINE support is detected. The cast obviously is performed regardless. For the benefit of verifying the macro compiles in the back-branches, this commit contains a single use of the new macro. On master, a somewhat larger conversion will be committed separately. Author: Peter Eisentraut and Andres Freund Reviewed-By: Tom Lane Discussion: https://postgr.es/m/c5d387d9-3440-f5e0-f9d4-71d53b9fbe52@2ndquadrant.com Backpatch: 9.2-
diff --git a/src/backend/tcop/postgres.c b/src/backend/tcop/postgres.c
@@ -826,7 +826,7 @@ pg_plan_queries(List *querytrees, int cursorOptions, ParamListInfo boundParams)
 
 	foreach(query_list, querytrees)
 	{
-		Query	   *query = (Query *) lfirst(query_list);
+		Query	   *query = castNode(Query, lfirst(query_list));
 		Node	   *stmt;
 
 		if (query->commandType == CMD_UTILITY)
diff --git a/src/include/nodes/nodes.h b/src/include/nodes/nodes.h
@@ -486,6 +486,28 @@ extern PGDLLIMPORT Node *newNodeMacroHolder;
 
 #define IsA(nodeptr,_type_)		(nodeTag(nodeptr) == T_##_type_)
 
+/*
+ * castNode(type, ptr) casts ptr to type and, if cassert is enabled, verifies
+ * that the the c actually has the appropriate type (using it's nodeTag()).
+ *
+ * Use an inline function when assertions are enabled, to avoid multiple
+ * evaluations of the ptr argument (which could e.g. be a function call). If
+ * inline functions are not available - only a small number of platforms -
+ * don't Assert, but use the non-checking version.
+ */
+#if defined(USE_ASSERT_CHECKING) && defined(PG_USE_INLINE)
+static inline Node*
+castNodeImpl(enum NodeTag type, void *ptr)
+{
+	Assert(ptr == NULL || nodeTag(ptr) == type);
+	return ptr;
+}
+#define castNode(_type_, nodeptr) ((_type_ *) castNodeImpl(T_##_type_, nodeptr))
+#else
+#define castNode(_type_,nodeptr)  ((_type_ *)(nodeptr))
+#endif
+
+
 /* ----------------------------------------------------------------
  *					  extern declarations follow
  * ----------------------------------------------------------------

Original file line number	Diff line number	Diff line change
`@@ -826,7 +826,7 @@ pg_plan_queries(List *querytrees, int cursorOptions, ParamListInfo boundParams)`
`826`	`826`
`827`	`827`	`foreach(query_list, querytrees)`
`828`	`828`	`{`
`829`		`- Query query = (Query ) lfirst(query_list);`
	`829`	`+ Query *query = castNode(Query, lfirst(query_list));`
`830`	`830`	`Node *stmt;`
`831`	`831`
`832`	`832`	`if (query->commandType == CMD_UTILITY)`