Fix GH-11404: DOMDocument::savexml and friends ommit xmlns="" declaration for null namespace, creating incorrect xml representation of the DOM

The NULL namespace is only correct when there is no default namespace
override. When there is, we need to manually set it to the empty string
namespace.

Author: nielsdos

ext/dom/document.c

@@ -878,6 +878,10 @@ PHP_METHOD(DOMDocument, createElementNS)
 
 	if (errorcode == 0) {
 		if (xmlValidateName((xmlChar *) localname, 0) == 0) {
+			/* https://dom.spec.whatwg.org/#validate-and-extract: demands us to set an empty string uri to NULL */
+			if (uri_len == 0) {
+				uri = NULL;
+			}
 			nodep = xmlNewDocNode(docp, NULL, (xmlChar *) localname, (xmlChar *) value);
 			if (nodep != NULL && uri != NULL) {
 				nsptr = xmlSearchNsByHref(nodep->doc, nodep, (xmlChar *) uri);

ext/dom/element.c

@@ -56,6 +56,10 @@ PHP_METHOD(DOMElement, __construct)
 	if (uri_len > 0) {
 		errorcode = dom_check_qname(name, &localname, &prefix, uri_len, name_len);
 		if (errorcode == 0) {
+			/* https://dom.spec.whatwg.org/#validate-and-extract: demands us to set an empty string uri to NULL */
+			if (uri_len == 0) {
+				uri = NULL;
+			}
 			nodep = xmlNewNode (NULL, (xmlChar *)localname);
 			if (nodep != NULL && uri != NULL) {
 				nsptr = dom_get_ns(nodep, uri, &errorcode, prefix);

ext/dom/node.c

@@ -531,7 +531,6 @@ Since: DOM Level 2
 int dom_node_namespace_uri_read(dom_object *obj, zval *retval)
 {
 	xmlNode *nodep = dom_object_get_node(obj);
-	char *str = NULL;
 
 	if (nodep == NULL) {
 		php_dom_throw_error(INVALID_STATE_ERR, 1);
@@ -543,20 +542,19 @@ int dom_node_namespace_uri_read(dom_object *obj, zval *retval)
 		case XML_ATTRIBUTE_NODE:
 		case XML_NAMESPACE_DECL:
 			if (nodep->ns != NULL) {
-				str = (char *) nodep->ns->href;
+				char *str = (char *) nodep->ns->href;
+				/* https://dom.spec.whatwg.org/#concept-attribute: namespaceUri is "null or a non-empty string" */
+				if (str != NULL && str[0] != '\0') {
+					ZVAL_STRING(retval, str);
+					return SUCCESS;
+				}
 			}
 			break;
 		default:
-			str = NULL;
 			break;
 	}
 
-	if (str != NULL) {
-		ZVAL_STRING(retval, str);
-	} else {
-		ZVAL_NULL(retval);
-	}
-
+	ZVAL_NULL(retval);
 	return SUCCESS;
 }
 

ext/dom/php_dom.c

@@ -1478,12 +1478,27 @@ static void dom_libxml_reconcile_ensure_namespaces_are_declared(xmlNodePtr nodep
 	xmlDOMWrapReconcileNamespaces(&dummy_ctxt, nodep, /* options */ 0);
 }
 
+static void dom_check_default_namespace(xmlDocPtr doc, xmlNodePtr nodep)
+{
+	if (nodep->ns == NULL) {
+		xmlNsPtr default_ns = xmlSearchNs(doc, nodep->parent, NULL);
+		if (default_ns != NULL && default_ns->href != NULL && default_ns->href[0] != '\0') {
+			/* The node uses the default empty namespace, but the current default namespace is non-empty.
+			 * We can't unconditionally do this because otherwise libxml2 creates an xmlns="" declaration.
+			 * Note: there's no point searching the oldNs list, because we haven't found it in the tree anyway.
+			 *       Ideally this would be pre-allocated but unfortunately libxml2 doesn't offer such a functionality. */
+			xmlSetNs(nodep, xmlNewNs(nodep, (const xmlChar *) "", NULL));
+		}
+	}
+}
+
 void dom_reconcile_ns(xmlDocPtr doc, xmlNodePtr nodep) /* {{{ */
 {
 	/* Although the node type will be checked by the libxml2 API,
 	 * we still want to do the internal reconciliation conditionally. */
 	if (nodep->type == XML_ELEMENT_NODE) {
 		dom_reconcile_ns_internal(doc, nodep, nodep->parent);
+		dom_check_default_namespace(doc, nodep);
 		dom_libxml_reconcile_ensure_namespaces_are_declared(nodep);
 	}
 }
@@ -1509,6 +1524,7 @@ static void dom_reconcile_ns_list_internal(xmlDocPtr doc, xmlNodePtr nodep, xmlN
 void dom_reconcile_ns_list(xmlDocPtr doc, xmlNodePtr nodep, xmlNodePtr last)
 {
 	dom_reconcile_ns_list_internal(doc, nodep, last, nodep->parent);
+	dom_check_default_namespace(doc, nodep);
 	/* The loop is outside of the recursion in the above call because
 	 * dom_libxml_reconcile_ensure_namespaces_are_declared() performs its own recursion. */
 	while (true) {

ext/dom/tests/bug47530.phpt

@@ -121,7 +121,7 @@ test_appendChild_with_shadowing();
 <html xmlns="https://php.net/something" xmlns:ns="https://php.net/whatever"><element ns:foo="https://php.net/bar"/></html>
 -- Test document fragment without import --
 <?xml version="1.0"?>
-<html xmlns=""><element xmlns:foo="https://php.net/bar"><foo:bar/><bar xmlns=""/></element></html>
+<html xmlns=""><element xmlns:foo="https://php.net/bar"><foo:bar/><bar/></element></html>
 string(7) "foo:bar"
 string(19) "https://php.net/bar"
 -- Test document import --

ext/dom/tests/gh11404.phpt

@@ -99,19 +99,19 @@ var_dump($dom1->firstElementChild->firstElementChild->namespaceURI);
 -- Test append and attributes: with default namespace variation --
 NULL
 NULL
-string(0) ""
+NULL
 NULL
 string(7) "some:ns"
 <?xml version="1.0"?>
-<with xmlns="some:ns"><a/><b/><c xmlns=""/><d xmlns:x="some:ns" x:attrib="val"/><e attrib1="val" attrib2="val"/><subtree><subtreechild1 xmlns="some:ns"><subtreechild2/></subtreechild1></subtree></with>
+<with xmlns="some:ns"><a xmlns=""/><b xmlns=""/><c xmlns=""/><d xmlns:x="some:ns" xmlns="" x:attrib="val"/><e attrib1="val" attrib2="val"/><subtree xmlns=""><subtreechild1 xmlns="some:ns"><subtreechild2 xmlns=""/></subtreechild1></subtree></with>
 -- Test append and attributes: without default namespace variation --
 NULL
 NULL
-string(0) ""
+NULL
 NULL
 string(7) "some:ns"
 <?xml version="1.0"?>
-<with><a/><b/><c xmlns=""/><d xmlns:x="some:ns" x:attrib="val"/><e xmlns="some:ns" attrib1="val" attrib2="val"/><subtree><subtreechild1 xmlns="some:ns"><subtreechild2/></subtreechild1></subtree></with>
+<with><a/><b/><c/><d xmlns:x="some:ns" x:attrib="val"/><e xmlns="some:ns" attrib1="val" attrib2="val"/><subtree><subtreechild1 xmlns="some:ns"><subtreechild2 xmlns=""/></subtreechild1></subtree></with>
 -- Test import --
 NULL
 <?xml version="1.0"?>
@@ -121,7 +121,7 @@ NULL
 <with xmlns=""><none/></with>
 NULL
 <?xml version="1.0"?>
-<with xmlns="some:ns"><none/></with>
+<with xmlns="some:ns"><none xmlns=""/></with>
 NULL
 <?xml version="1.0"?>
 <with xmlns=""><none><div xmlns="some:ns"/></none></with>
@@ -134,8 +134,8 @@ string(3) "a:b"
 string(3) "a:b"
 string(3) "a:b"
 NULL
-string(70) "<?xml version="1.0"?>
-<parent xmlns="a:b"><child1/><child2/></parent>
+string(79) "<?xml version="1.0"?>
+<parent xmlns="a:b"><child1/><child2 xmlns=""/></parent>
 "
 string(3) "a:b"
 string(3) "a:b"