Optimize and reduce memory usage of XML serialization (#14204)

The serialization process uses the system allocator and requires a copy
to request allocated memory once finished. This patch improves this by
using smart_str to build the resulting string, reducing the number of
copies and reducing total peak memory usage.

UPGRADING

@@ -693,6 +693,7 @@ PHP 8.4 UPGRADE NOTES
   . The performance of DOMNode::C14N() is greatly improved for the case without
     an xpath query. This can give a time improvement of easily two order of
     magnitude for documents with tens of thousands of nodes.
+  . Improved performance and reduce memory consumption of XML serialization.
 
 - FTP:
   . Improved the performance of FTP uploads up to a factor of 10x for large
@@ -708,6 +709,9 @@ PHP 8.4 UPGRADE NOTES
 - MySQLnd:
   . Improved the performance of MySQLnd quoting.
 
+- SimpleXML:
+  . Improved performance and reduce memory consumption of XML serialization.
+
 - Standard:
   . Improved the performance of strpbrk().
   . get_browser() is much faster now, up to 1.5x - 2.5x for some test cases.

ext/dom/xml_document.c

@@ -250,42 +250,38 @@ PHP_METHOD(Dom_XMLDocument, createFromFile)
 	load_from_helper(INTERNAL_FUNCTION_PARAM_PASSTHRU, DOM_LOAD_FILE);
 }
 
+static int php_new_dom_write_smart_str(void *context, const char *buffer, int len)
+{
+	smart_str *str = context;
+	smart_str_appendl(str, buffer, len);
+	return len;
+}
+
 static zend_string *php_new_dom_dump_node_to_str(xmlDocPtr doc, xmlNodePtr node, bool format, const char *encoding)
 {
-	xmlBufferPtr buf = xmlBufferCreate();
-	if (!buf) {
-		return NULL;
-	}
+	smart_str str = {0};
 
 	int status = -1;
-	xmlSaveCtxtPtr ctxt = xmlSaveToBuffer(buf, encoding, XML_SAVE_AS_XML);
+	xmlSaveCtxtPtr ctxt = xmlSaveToIO(php_new_dom_write_smart_str, NULL, &str, encoding, XML_SAVE_AS_XML);
 	if (EXPECTED(ctxt != NULL)) {
 		xmlCharEncodingHandlerPtr handler = xmlFindCharEncodingHandler(encoding);
-		xmlOutputBufferPtr out = xmlOutputBufferCreateBuffer(buf, handler);
+		xmlOutputBufferPtr out = xmlOutputBufferCreateIO(php_new_dom_write_smart_str, NULL, &str, handler);
 		if (EXPECTED(out != NULL)) {
 			status = dom_xml_serialize(ctxt, out, node, format);
 			status |= xmlOutputBufferFlush(out);
 			status |= xmlOutputBufferClose(out);
+		} else {
+			xmlCharEncCloseFunc(handler);
 		}
 		(void) xmlSaveClose(ctxt);
-		xmlCharEncCloseFunc(handler);
 	}
 
 	if (UNEXPECTED(status < 0)) {
-		xmlBufferFree(buf);
+		smart_str_free_ex(&str, false);
 		return NULL;
 	}
 
-	const xmlChar *content = xmlBufferContent(buf);
-	if (!content) {
-		xmlBufferFree(buf);
-		return NULL;
-	}
-
-	int size = xmlBufferLength(buf);
-	zend_string *res = zend_string_init((const char *) content, size, false);
-	xmlBufferFree(buf);
-	return res;
+	return smart_str_extract(&str);
 }
 
 static zend_string *php_new_dom_dump_doc_to_str(xmlDocPtr doc, int options, const char *encoding)

ext/libxml/libxml.c

@@ -1481,43 +1481,38 @@ PHP_LIBXML_API xmlChar *php_libxml_attr_value(const xmlAttr *attr, bool *free)
 	return value;
 }
 
+static int php_libxml_write_smart_str(void *context, const char *buffer, int len)
+{
+	smart_str *str = context;
+	smart_str_appendl(str, buffer, len);
+	return len;
+}
+
 static zend_string *php_libxml_default_dump_doc_to_str(xmlDocPtr doc, int options, const char *encoding)
 {
-	xmlBufferPtr buf = xmlBufferCreate();
-	if (!buf) {
-		return NULL;
-	}
+	smart_str str = {0};
 
 	/* Encoding is handled from the encoding property set on the document */
-	xmlSaveCtxtPtr ctxt = xmlSaveToBuffer(buf, encoding, options);
+	xmlSaveCtxtPtr ctxt = xmlSaveToIO(php_libxml_write_smart_str, NULL, &str, encoding, options);
 	if (!ctxt) {
-		xmlBufferFree(buf);
 		return NULL;
 	}
 
 	long status = xmlSaveDoc(ctxt, doc);
 	(void) xmlSaveClose(ctxt);
 	if (status < 0) {
-		xmlBufferFree(buf);
+		smart_str_free_ex(&str, false);
 		return NULL;
 	}
 
-	const xmlChar *content = xmlBufferContent(buf);
-	if (!content) {
-		xmlBufferFree(buf);
-		return NULL;
-	}
-
-	int size = xmlBufferLength(buf);
-	zend_string *str = zend_string_init((const char *) content, size, false);
-	xmlBufferFree(buf);
-	return str;
+	return smart_str_extract(&str);
 }
 
 static zend_string *php_libxml_default_dump_node_to_str(xmlDocPtr doc, xmlNodePtr node, bool format, const char *encoding)
 {
-	// TODO: should this alloc take an encoding? For now keep it NULL for BC.
-	xmlOutputBufferPtr buf = xmlAllocOutputBuffer(NULL);
+	smart_str str = {0};
+	// TODO: should this buffer take an encoding? For now keep it NULL for BC.
+	xmlOutputBufferPtr buf = xmlOutputBufferCreateIO(php_libxml_write_smart_str, NULL, &str, NULL);
 	if (!buf) {
 		return NULL;
 	}
@@ -1525,16 +1520,14 @@ static zend_string *php_libxml_default_dump_node_to_str(xmlDocPtr doc, xmlNodePt
 	xmlNodeDumpOutput(buf, doc, node, 0, format, encoding);
 
 	if (xmlOutputBufferFlush(buf) < 0) {
+		smart_str_free_ex(&str, false);
 		xmlOutputBufferClose(buf);
 		return NULL;
 	}
 
-	const xmlChar *content = xmlOutputBufferGetContent(buf);
-	size_t size = xmlOutputBufferGetSize(buf);
-
-	zend_string *str = zend_string_init((const char *) content, size, false);
 	xmlOutputBufferClose(buf);
-	return str;
+
+	return smart_str_extract(&str);
 }
 
 static zend_long php_libxml_default_dump_doc_to_file(const char *filename, xmlDocPtr doc, bool format, const char *encoding)