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

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
index 12861ac454f3231dbda755fc3908984336b9f860..83281a09aa4e34d9b02b96af1c3a7a58b94f5cdd 100644 (file)
--- a/src/backend/tcop/postgres.c
+++ b/src/backend/tcop/postgres.c
@@ -824,7 +824,7 @@ 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
index 54bf6895f99e10843bfed5062ab6c3b9094f2abd..26d0f42d12b8a5db35e6d90f5896b7077d145c56 100644 (file)
--- a/src/include/nodes/nodes.h
+++ b/src/include/nodes/nodes.h
@@ -480,6 +480,28 @@ 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
  * ----------------------------------------------------------------