Optimize in-memory XMLWriter

We're currently using a libxml buffer, which requires copying the buffer
to zend_strings every time we want to output the string. Furthermore,
its use of the system allocator instead of ZendMM makes it not count
towards the memory_limit and hinders performance.

This patch adds a custom writer such that the strings are written to a
smart_str instance, using ZendMM for improved performance, and giving
the ability to not copy the string in the common case where flush has
empty set to true.

Closes GH-16120.

NEWS

@@ -19,4 +19,7 @@ PHP                                                                        NEWS
   . Fixed bug #49169 (SoapServer calls wrong function, although "SOAP action"
     header is correct). (nielsdos)
 
+- XMLWriter:
+  . Improved performance and reduce memory consumption. (nielsdos)
+
 <<< NOTE: Insert NEWS from last stable release here prior to actual release! >>>

UPGRADING

@@ -95,3 +95,6 @@ PHP 8.5 UPGRADE NOTES
 ========================================
 14. Performance Improvements
 ========================================
+
+- XMLWriter:
+  . Improved performance and reduce memory consumption.

ext/xmlwriter/php_xmlwriter.c

@@ -24,6 +24,7 @@
 #include "ext/standard/info.h"
 #include "php_xmlwriter.h"
 #include "php_xmlwriter_arginfo.h"
+#include "zend_smart_str.h"
 
 static zend_class_entry *xmlwriter_class_entry_ce;
 
@@ -47,11 +48,9 @@ static zend_object_handlers xmlwriter_object_handlers;
 static zend_always_inline void xmlwriter_destroy_libxml_objects(ze_xmlwriter_object *intern)
 {
 	if (intern->ptr) {
+		/* Note: this call will also free the output pointer. */
 		xmlFreeTextWriter(intern->ptr);
 		intern->ptr = NULL;
-	}
-	if (intern->output) {
-		xmlBufferFree(intern->output);
 		intern->output = NULL;
 	}
 }
@@ -178,14 +177,14 @@ static char *_xmlwriter_get_valid_file_path(char *source, char *resolved_path, i
 }
 /* }}} */
 
-static void xml_writer_create_static(INTERNAL_FUNCTION_PARAMETERS, xmlTextWriterPtr writer, xmlBufferPtr output)
+static void xml_writer_create_static(INTERNAL_FUNCTION_PARAMETERS, xmlTextWriterPtr writer, smart_str *output)
 {
 	if (object_init_with_constructor(return_value, Z_CE_P(ZEND_THIS), 0, NULL, NULL) == SUCCESS) {
 		ze_xmlwriter_object *intern = Z_XMLWRITER_P(return_value);
 		intern->ptr = writer;
 		intern->output = output;
 	} else {
-		xmlBufferFree(output);
+		// output is freed by writer, so we don't need to free it here.
 		xmlFreeTextWriter(writer);
 	}
 }
@@ -877,11 +876,45 @@ PHP_METHOD(XMLWriter, toUri)
 	xml_writer_create_static(INTERNAL_FUNCTION_PARAM_PASSTHRU, writer, NULL);
 }
 
+static int xml_writer_stream_write_memory(void *context, const char *buffer, int len)
+{
+	smart_str *output = context;
+	smart_str_appendl(output, buffer, len);
+	return len;
+}
+
+static int xml_writer_stream_close_memory(void *context)
+{
+	smart_str *output = context;
+	smart_str_free_ex(output, false);
+	efree(output);
+	return 0;
+}
+
+static xmlTextWriterPtr xml_writer_create_in_memory(smart_str **output_ptr)
+{
+	smart_str *output = emalloc(sizeof(*output));
+	memset(output, 0, sizeof(*output));
+
+	xmlOutputBufferPtr output_buffer = xmlOutputBufferCreateIO(xml_writer_stream_write_memory, xml_writer_stream_close_memory, output, NULL);
+	if (output_buffer == NULL) {
+		efree(output);
+		return NULL;
+	}
+
+	xmlTextWriterPtr writer = xmlNewTextWriter(output_buffer);
+	if (!writer) {
+		/* This call will free output too. */
+		xmlOutputBufferClose(output_buffer);
+		return NULL;
+	}
+	*output_ptr = output;
+	return writer;
+}
+
 /* {{{ Create new xmlwriter using memory for string output */
 PHP_FUNCTION(xmlwriter_open_memory)
 {
-	xmlTextWriterPtr ptr;
-	xmlBufferPtr buffer;
 	zval *self = getThis();
 	ze_xmlwriter_object *ze_obj = NULL;
 
@@ -894,28 +927,21 @@ PHP_FUNCTION(xmlwriter_open_memory)
 		ze_obj = Z_XMLWRITER_P(self);
 	}
 
-	buffer = xmlBufferCreate();
-
-	if (buffer == NULL) {
-		php_error_docref(NULL, E_WARNING, "Unable to create output buffer");
-		RETURN_FALSE;
-	}
-
-	ptr = xmlNewTextWriterMemory(buffer, 0);
+	smart_str *output;
+	xmlTextWriterPtr ptr = xml_writer_create_in_memory(&output);
 	if (! ptr) {
-		xmlBufferFree(buffer);
 		RETURN_FALSE;
 	}
 
 	if (self) {
 		xmlwriter_destroy_libxml_objects(ze_obj);
 		ze_obj->ptr = ptr;
-		ze_obj->output = buffer;
+		ze_obj->output = output;
 		RETURN_TRUE;
 	} else {
 		ze_obj = php_xmlwriter_fetch_object(xmlwriter_object_new(xmlwriter_class_entry_ce));
 		ze_obj->ptr = ptr;
-		ze_obj->output = buffer;
+		ze_obj->output = output;
 		RETURN_OBJ(&ze_obj->std);
 	}
 
@@ -926,17 +952,16 @@ PHP_METHOD(XMLWriter, toMemory)
 {
 	ZEND_PARSE_PARAMETERS_NONE();
 
-	xmlBufferPtr buffer = xmlBufferCreate();
-	xmlTextWriterPtr writer = xmlNewTextWriterMemory(buffer, 0);
+	smart_str *output;
+	xmlTextWriterPtr writer = xml_writer_create_in_memory(&output);
 
 	/* No need for an explicit buffer check as this will fail on a NULL buffer. */
 	if (!writer) {
-		xmlBufferFree(buffer);
 		zend_throw_error(NULL, "Could not construct libxml writer");
 		RETURN_THROWS();
 	}
 
-	xml_writer_create_static(INTERNAL_FUNCTION_PARAM_PASSTHRU, writer, buffer);
+	xml_writer_create_static(INTERNAL_FUNCTION_PARAM_PASSTHRU, writer, output);
 }
 
 static int xml_writer_stream_write(void *context, const char *buffer, int len)
@@ -992,7 +1017,6 @@ PHP_METHOD(XMLWriter, toStream)
 /* {{{ php_xmlwriter_flush */
 static void php_xmlwriter_flush(INTERNAL_FUNCTION_PARAMETERS, int force_string) {
 	xmlTextWriterPtr ptr;
-	xmlBufferPtr buffer;
 	bool empty = 1;
 	int output_bytes;
 	zval *self;
@@ -1002,16 +1026,18 @@ static void php_xmlwriter_flush(INTERNAL_FUNCTION_PARAMETERS, int force_string)
 	}
 	XMLWRITER_FROM_OBJECT(ptr, self);
 
-	buffer = Z_XMLWRITER_P(self)->output;
-	if (force_string == 1 && buffer == NULL) {
+	smart_str *output = Z_XMLWRITER_P(self)->output;
+	if (force_string == 1 && output == NULL) {
 		RETURN_EMPTY_STRING();
 	}
 	output_bytes = xmlTextWriterFlush(ptr);
-	if (buffer) {
-		const xmlChar *content = xmlBufferContent(buffer);
-		RETVAL_STRING((const char *) content);
+	if (output) {
 		if (empty) {
-			xmlBufferEmpty(buffer);
+			RETURN_STR(smart_str_extract(output));
+		} else if (smart_str_get_len(output) > 0) {
+			RETURN_NEW_STR(zend_string_dup(output->s, false));
+		} else {
+			RETURN_EMPTY_STRING();
 		}
 	} else {
 		RETVAL_LONG(output_bytes);

ext/xmlwriter/php_xmlwriter.h

@@ -35,7 +35,7 @@ extern zend_module_entry xmlwriter_module_entry;
 /* Extends zend object */
 typedef struct _ze_xmlwriter_object {
 	xmlTextWriterPtr ptr;
-	xmlBufferPtr output;
+	smart_str *output;
 	zend_object std;
 } ze_xmlwriter_object;
 

ext/xmlwriter/tests/xmlwriter_toMemory_flush_combinations.phpt

@@ -0,0 +1,23 @@
+--TEST--
+XMLWriter::toMemory() with combinations of empty flush and non-empty flush
+--EXTENSIONS--
+xmlwriter
+--FILE--
+<?php
+
+$writer = XMLWriter::toMemory();
+var_dump($writer->flush(empty: false));
+$writer->startElement('foo');
+var_dump($writer->flush(empty: false));
+$writer->endElement();
+var_dump($writer->flush(empty: false));
+var_dump($writer->flush());
+var_dump($writer->flush());
+
+?>
+--EXPECT--
+string(0) ""
+string(4) "<foo"
+string(6) "<foo/>"
+string(6) "<foo/>"
+string(0) ""