Fix parsetree representation of XMLTABLE(XMLNAMESPACES(DEFAULT ...)).

The original coding for XMLTABLE thought it could represent a default
namespace by a T_String Value node with a null string pointer.  That's
not okay, though; in particular outfuncs.c/readfuncs.c are not on board
with such a representation, meaning you'll get a null pointer crash
if you try to store a view or rule containing this construct.

To fix, change the parsetree representation so that we have a NULL
list element, instead of a bogus Value node.

This isn't really a functional limitation since default XML namespaces
aren't yet implemented in the executor; you'd just get "DEFAULT
namespace is not supported" anyway.  But crashes are not nice, so
back-patch to v10 where this syntax was added.  Ordinarily we'd consider
a parsetree representation change to be un-backpatchable; but since
existing releases would crash on the way to storing such constructs,
there can't be any existing views/rules to be incompatible with.

Per report from Andrey Lepikhov.

Discussion: https://postgr.es/m/3690074f-abd2-56a9-144a-aa5545d7a291@postgrespro.ru

src/backend/executor/nodeTableFuncscan.c

@@ -364,8 +364,9 @@ tfuncInitialize(TableFuncScanState *tstate, ExprContext *econtext, Datum doc)
 	forboth(lc1, tstate->ns_uris, lc2, tstate->ns_names)
 	{
 		ExprState  *expr = (ExprState *) lfirst(lc1);
-		char	   *ns_name = strVal(lfirst(lc2));
+		Value	   *ns_node = (Value *) lfirst(lc2);
 		char	   *ns_uri;
+		char	   *ns_name;
 
 		value = ExecEvalExpr((ExprState *) expr, econtext, &isnull);
 		if (isnull)
@@ -374,6 +375,9 @@ tfuncInitialize(TableFuncScanState *tstate, ExprContext *econtext, Datum doc)
 					 errmsg("namespace URI must not be null")));
 		ns_uri = TextDatumGetCString(value);
 
+		/* DEFAULT is passed down to SetNamespace as NULL */
+		ns_name = ns_node ? strVal(ns_node) : NULL;
+
 		routine->SetNamespace(tstate, ns_name, ns_uri);
 	}
 

src/backend/parser/parse_clause.c

@@ -779,7 +779,7 @@ transformRangeTableFunc(ParseState *pstate, RangeTableFunc *rtf)
 	/* undef ordinality column number */
 	tf->ordinalitycol = -1;
 
-
+	/* Process column specs */
 	names = palloc(sizeof(char *) * list_length(rtf->columns));
 
 	colno = 0;
@@ -900,15 +900,15 @@ transformRangeTableFunc(ParseState *pstate, RangeTableFunc *rtf)
 			{
 				foreach(lc2, ns_names)
 				{
-					char	   *name = strVal(lfirst(lc2));
+					Value	   *ns_node = (Value *) lfirst(lc2);
 
-					if (name == NULL)
+					if (ns_node == NULL)
 						continue;
-					if (strcmp(name, r->name) == 0)
+					if (strcmp(strVal(ns_node), r->name) == 0)
 						ereport(ERROR,
 								(errcode(ERRCODE_SYNTAX_ERROR),
 								 errmsg("namespace name \"%s\" is not unique",
-										name),
+										r->name),
 								 parser_errposition(pstate, r->location)));
 				}
 			}
@@ -922,8 +922,9 @@ transformRangeTableFunc(ParseState *pstate, RangeTableFunc *rtf)
 				default_ns_seen = true;
 			}
 
-			/* Note the string may be NULL */
+			/* We represent DEFAULT by a null pointer */
-			ns_names = lappend(ns_names, makeString(r->name));
+			ns_names = lappend(ns_names,
+							   r->name ? makeString(r->name) : NULL);
 		}
 
 		tf->ns_uris = ns_uris;

src/backend/utils/adt/ruleutils.c

@@ -9739,17 +9739,17 @@ get_tablefunc(TableFunc *tf, deparse_context *context, bool showimplicit)
 		forboth(lc1, tf->ns_uris, lc2, tf->ns_names)
 		{
 			Node	   *expr = (Node *) lfirst(lc1);
-			char	   *name = strVal(lfirst(lc2));
+			Value	   *ns_node = (Value *) lfirst(lc2);
 
 			if (!first)
 				appendStringInfoString(buf, ", ");
 			else
 				first = false;
 
-			if (name != NULL)
+			if (ns_node != NULL)
 			{
 				get_rule_expr(expr, context, showimplicit);
-				appendStringInfo(buf, " AS %s", name);
+				appendStringInfo(buf, " AS %s", strVal(ns_node));
 			}
 			else
 			{

src/include/nodes/execnodes.h

@@ -1573,8 +1573,8 @@ typedef struct TableFuncScanState
 	ExprState  *rowexpr;		/* state for row-generating expression */
 	List	   *colexprs;		/* state for column-generating expression */
 	List	   *coldefexprs;	/* state for column default expressions */
-	List	   *ns_names;		/* list of str nodes with namespace names */
+	List	   *ns_names;		/* same as TableFunc.ns_names */
-	List	   *ns_uris;		/* list of states of namespace uri exprs */
+	List	   *ns_uris;		/* list of states of namespace URI exprs */
 	Bitmapset  *notnulls;		/* nullability flag for each output column */
 	void	   *opaque;			/* table builder private space */
 	const struct TableFuncRoutine *routine; /* table builder methods */

src/include/nodes/primnodes.h

@@ -75,12 +75,15 @@ typedef struct RangeVar
 
 /*
  * TableFunc - node for a table function, such as XMLTABLE.
+ *
+ * Entries in the ns_names list are either string Value nodes containing
+ * literal namespace names, or NULL pointers to represent DEFAULT.
  */
 typedef struct TableFunc
 {
 	NodeTag		type;
-	List	   *ns_uris;		/* list of namespace uri */
+	List	   *ns_uris;		/* list of namespace URI expressions */
-	List	   *ns_names;		/* list of namespace names */
+	List	   *ns_names;		/* list of namespace names or NULL */
 	Node	   *docexpr;		/* input document expression */
 	Node	   *rowexpr;		/* row filter expression */
 	List	   *colnames;		/* column names (list of String) */