XPath: Assume that every function can fail and return 0

Propagate the failure to the caller manually. This is a first step to
parser structure that does not depend on exceptions or longjmp for error
handling (and thus matches the XML parser). To preserve semantics we'll
have to convert error code to exception later.

src/pugixml.cpp

@@ -11222,6 +11222,7 @@ PUGI__NS_BEGIN
 				_lexer.next();
 
 				xpath_ast_node* n = parse_expression();
+				if (!n) return 0;
 
 				if (_lexer.current() != lex_close_brace)
 					throw_error("Unmatched braces");
@@ -11234,12 +11235,11 @@ PUGI__NS_BEGIN
 			case lex_quoted_string:
 			{
 				const char_t* value = alloc_string(_lexer.contents());
-
+				if (!value) return 0;
-				xpath_ast_node* n = new (alloc_node()) xpath_ast_node(ast_string_constant, xpath_type_string, value);
 
 				_lexer.next();
 
-				return n;
+				return new (alloc_node()) xpath_ast_node(ast_string_constant, xpath_type_string, value);
 			}
 
 			case lex_number:
@@ -11249,11 +11249,9 @@ PUGI__NS_BEGIN
 				if (!convert_string_to_number_scratch(_scratch, _lexer.contents().begin, _lexer.contents().end, &value))
 					throw_error_oom();
 
-				xpath_ast_node* n = new (alloc_node()) xpath_ast_node(ast_number_constant, xpath_type_number, value);
-
 				_lexer.next();
 
-				return n;
+				return new (alloc_node()) xpath_ast_node(ast_number_constant, xpath_type_number, value);
 			}
 
 			case lex_string:
@@ -11280,6 +11278,7 @@ PUGI__NS_BEGIN
 					}
 
 					xpath_ast_node* n = parse_expression();
+					if (!n) return 0;
 
 					if (argc < 2) args[argc] = n;
 					else last_arg->set_next(n);
@@ -11306,17 +11305,20 @@ PUGI__NS_BEGIN
 		xpath_ast_node* parse_filter_expression()
 		{
 			xpath_ast_node* n = parse_primary_expression();
+			if (!n) return 0;
 
 			while (_lexer.current() == lex_open_square_brace)
 			{
 				_lexer.next();
 
 				xpath_ast_node* expr = parse_expression();
+				if (!expr) return 0;
 
 				if (n->rettype() != xpath_type_node_set)
 					throw_error("Predicate has to be applied to node set");
 
 				n = new (alloc_node()) xpath_ast_node(ast_filter, n, expr, predicate_default);
+				if (!n) return 0;
 
 				if (_lexer.current() != lex_close_square_brace)
 					throw_error("Unmatched square brace");
@@ -11461,7 +11463,10 @@ PUGI__NS_BEGIN
 			}
 
 			const char_t* nt_name_copy = alloc_string(nt_name);
+			if (!nt_name_copy) return 0;
+
 			xpath_ast_node* n = new (alloc_node()) xpath_ast_node(ast_step, set, axis, nt_type, nt_name_copy);
+			if (!n) return 0;
 
 			xpath_ast_node* last = 0;
 
@@ -11470,8 +11475,10 @@ PUGI__NS_BEGIN
 				_lexer.next();
 
 				xpath_ast_node* expr = parse_expression();
+				if (!expr) return 0;
 
 				xpath_ast_node* pred = new (alloc_node()) xpath_ast_node(ast_predicate, 0, expr, predicate_default);
+				if (!pred) return 0;
 
 				if (_lexer.current() != lex_close_square_brace)
 					throw_error("Unmatched square brace");
@@ -11490,6 +11497,7 @@ PUGI__NS_BEGIN
 		xpath_ast_node* parse_relative_location_path(xpath_ast_node* set)
 		{
 			xpath_ast_node* n = parse_step(set);
+			if (!n) return 0;
 
 			while (_lexer.current() == lex_slash || _lexer.current() == lex_double_slash)
 			{
@@ -11497,9 +11505,13 @@ PUGI__NS_BEGIN
 				_lexer.next();
 
 				if (l == lex_double_slash)
+				{
 					n = new (alloc_node()) xpath_ast_node(ast_step, n, axis_descendant_or_self, nodetest_type_node, 0);
+					if (!n) return 0;
+				}
 
 				n = parse_step(n);
+				if (!n) return 0;
 			}
 
 			return n;
@@ -11514,6 +11526,7 @@ PUGI__NS_BEGIN
 				_lexer.next();
 
 				xpath_ast_node* n = new (alloc_node()) xpath_ast_node(ast_step_root, xpath_type_node_set);
+				if (!n) return 0;
 
 				// relative location path can start from axis_attribute, dot, double_dot, multiply and string lexemes; any other lexeme means standalone root path
 				lexeme_t l = _lexer.current();
@@ -11528,7 +11541,10 @@ PUGI__NS_BEGIN
 				_lexer.next();
 
 				xpath_ast_node* n = new (alloc_node()) xpath_ast_node(ast_step_root, xpath_type_node_set);
+				if (!n) return 0;
+
 				n = new (alloc_node()) xpath_ast_node(ast_step, n, axis_descendant_or_self, nodetest_type_node, 0);
+				if (!n) return 0;
 
 				return parse_relative_location_path(n);
 			}
@@ -11551,7 +11567,6 @@ PUGI__NS_BEGIN
 			// PrimaryExpr begins with '$' in case of it being a variable reference,
 			// '(' in case of it being an expression, string literal, number constant or
 			// function call.
-
 			if (_lexer.current() == lex_var_ref || _lexer.current() == lex_open_brace ||
 				_lexer.current() == lex_quoted_string || _lexer.current() == lex_number ||
 				_lexer.current() == lex_string)
@@ -11572,6 +11587,7 @@ PUGI__NS_BEGIN
 				}
 
 				xpath_ast_node* n = parse_filter_expression();
+				if (!n) return 0;
 
 				if (_lexer.current() == lex_slash || _lexer.current() == lex_double_slash)
 				{
@@ -11584,6 +11600,7 @@ PUGI__NS_BEGIN
 							throw_error("Step has to be applied to node set");
 
 						n = new (alloc_node()) xpath_ast_node(ast_step, n, axis_descendant_or_self, nodetest_type_node, 0);
+						if (!n) return 0;
 					}
 
 					// select from location path
@@ -11598,6 +11615,7 @@ PUGI__NS_BEGIN
 
 				// precedence 7+ - only parses union expressions
 				xpath_ast_node* n = parse_expression(7);
+				if (!n) return 0;
 
 				return new (alloc_node()) xpath_ast_node(ast_op_negate, xpath_type_number, n);
 			}
@@ -11682,12 +11700,14 @@ PUGI__NS_BEGIN
 				_lexer.next();
 
 				xpath_ast_node* rhs = parse_path_or_unary_expression();
+				if (!rhs) return 0;
 
 				binary_op_t nextop = binary_op_t::parse(_lexer);
 
 				while (nextop.asttype != ast_unknown && nextop.precedence > op.precedence)
 				{
 					rhs = parse_expression_rec(rhs, nextop.precedence);
+					if (!rhs) return 0;
 
 					nextop = binary_op_t::parse(_lexer);
 				}
@@ -11696,6 +11716,7 @@ PUGI__NS_BEGIN
 					throw_error("Union operator has to be applied to node sets");
 
 				lhs = new (alloc_node()) xpath_ast_node(op.asttype, op.rettype, lhs, rhs);
+				if (!lhs) return 0;
 
 				op = binary_op_t::parse(_lexer);
 			}
@@ -11724,6 +11745,7 @@ PUGI__NS_BEGIN
 		xpath_ast_node* parse_expression(int limit = 0)
 		{
 			xpath_ast_node* n = parse_path_or_unary_expression();
+			if (!n) return 0;
 
 			return parse_expression_rec(n, limit);
 		}
@@ -11734,13 +11756,14 @@ PUGI__NS_BEGIN
 
 		xpath_ast_node* parse()
 		{
-			xpath_ast_node* result = parse_expression();
+			xpath_ast_node* n = parse_expression();
+			if (!n) return 0;
 
 			// check if there are unparsed tokens left
 			if (_lexer.current() != lex_eof)
 				throw_error("Incorrect query");
 
-			return result;
+			return n;
 		}
 
 		static xpath_ast_node* parse(const char_t* query, xpath_variable_set* variables, xpath_allocator* alloc, xpath_parse_result* result)