Add stack depth checks to key recursive functions in backend/nodes/*.c.

Although copyfuncs.c has a check_stack_depth call in its recursion,
equalfuncs.c, outfuncs.c, and readfuncs.c lacked one.  This seems
unwise.

Likewise fix planstate_tree_walker(), in branches where that exists.

Discussion: https://postgr.es/m/30253.1544286631@sss.pgh.pa.us

src/backend/nodes/equalfuncs.c

@@ -29,6 +29,7 @@
 
 #include "postgres.h"
 
+#include "miscadmin.h"
 #include "nodes/relation.h"
 #include "utils/datum.h"
 
@@ -2514,6 +2515,9 @@ equal(const void *a, const void *b)
 	if (nodeTag(a) != nodeTag(b))
 		return false;
 
+	/* Guard against stack overflow due to overly complex expressions */
+	check_stack_depth();
+
 	switch (nodeTag(a))
 	{
 			/*

src/backend/nodes/outfuncs.c

@@ -24,6 +24,7 @@
 #include <ctype.h>
 
 #include "lib/stringinfo.h"
+#include "miscadmin.h"
 #include "nodes/plannodes.h"
 #include "nodes/relation.h"
 #include "utils/datum.h"
@@ -2757,6 +2758,9 @@ _outConstraint(StringInfo str, const Constraint *node)
 static void
 _outNode(StringInfo str, const void *obj)
 {
+	/* Guard against stack overflow due to overly complex expressions */
+	check_stack_depth();
+
 	if (obj == NULL)
 		appendStringInfoString(str, "<>");
 	else if (IsA(obj, List) ||IsA(obj, IntList) || IsA(obj, OidList))

src/backend/nodes/readfuncs.c

@@ -28,6 +28,7 @@
 
 #include <math.h>
 
+#include "miscadmin.h"
 #include "nodes/parsenodes.h"
 #include "nodes/readfuncs.h"
 
@@ -1292,6 +1293,9 @@ parseNodeString(void)
 
 	READ_TEMP_LOCALS();
 
+	/* Guard against stack overflow due to overly complex expressions */
+	check_stack_depth();
+
 	token = pg_strtok(&length);
 
 #define MATCH(tokname, namelen) \