Prevent memory leaks in our various bison parsers when an error occurs

during parsing.  Formerly the parser's stack was allocated with malloc
and so wouldn't be reclaimed; this patch makes it use palloc instead,
so that flushing the current context will reclaim the memory.  Per
Marko Kreen.

src/backend/parser/gram.y

@@ -11,7 +11,7 @@
  *
  *
  * IDENTIFICATION
- *	  $PostgreSQL: pgsql/src/backend/parser/gram.y,v 2.621 2008/09/01 20:42:44 tgl Exp $
+ *	  $PostgreSQL: pgsql/src/backend/parser/gram.y,v 2.622 2008/09/02 20:37:54 tgl Exp $
  *
  * HISTORY
  *	  AUTHOR			DATE			MAJOR EVENT
@@ -79,6 +79,17 @@
  */
 #define base_yylex filtered_base_yylex
 
+/*
+ * Bison doesn't allocate anything that needs to live across parser calls,
+ * so we can easily have it use palloc instead of malloc.  This prevents
+ * memory leaks if we error out during parsing.  Note this only works with
+ * bison >= 2.0.  However, in bison 1.875 the default is to use alloca()
+ * if possible, so there's not really much problem anyhow, at least if
+ * you're building with gcc.
+ */
+#define YYMALLOC palloc
+#define YYFREE   pfree
+
 extern List *parsetree;			/* final parse result is delivered here */
 
 static bool QueryIsRule = FALSE;