Early out in load_buffer for empty inputs to avoid allocations

Previously, calling xml_document::load_string on an empty buffer
resulted in a single byte allocation for the buffer copy; this
allocation was redundant as no data was parsed, so we now add the same
early-out that already exists in xml_parser::parse to load_buffer_impl.

Author: zeux

src/pugixml.cpp

@@ -264,7 +264,7 @@ PUGI_IMPL_NS_BEGIN
 
 		while (srclen && *dst && *src == *dst)
 		{
-			--srclen; ++dst; ++src; 
+			--srclen; ++dst; ++src;
 		}
 		return srclen == 0 && *dst == 0;
 	}
@@ -4780,6 +4780,9 @@ PUGI_IMPL_NS_BEGIN
 		// if convert_buffer below throws bad_alloc, we still need to deallocate contents if we own it
 		auto_deleter<void> contents_guard(own ? contents : NULL, xml_memory::deallocate);
 
+		// early-out for empty documents to avoid buffer allocation overhead
+		if (size == 0) return make_parse_result((options & parse_fragment) ? status_ok : status_no_document_element);
+
 		// get private buffer
 		char_t* buffer = NULL;
 		size_t length = 0;

tests/test_document.cpp

@@ -473,6 +473,17 @@ TEST(document_load_string)
 	CHECK_NODE(doc, STR("<node/>"));
 }
 
+TEST(document_load_string_empty)
+{
+	xml_document doc;
+
+	CHECK(doc.load_string(STR("")).status == status_no_document_element);
+	CHECK(!doc.first_child());
+
+	CHECK(doc.load_string(STR(""), parse_fragment));
+	CHECK(!doc.first_child());
+}
+
 TEST(document_load_file)
 {
 	xml_document doc;
@@ -864,6 +875,19 @@ TEST(document_load_buffer_inplace_own)
 	CHECK_NODE(doc, STR("<node/>"));
 }
 
+TEST(document_load_buffer_inplace_own_empty)
+{
+	allocation_function alloc = get_memory_allocation_function();
+
+	void* text1 = alloc(1);
+	void* text2 = alloc(1);
+	CHECK(text1 && text2);
+
+	xml_document doc;
+	CHECK(doc.load_buffer_inplace_own(text1, 0, parse_fragment));
+	CHECK(doc.load_buffer_inplace_own(text2, 0).status == status_no_document_element);
+}
+
 TEST(document_parse_result_bool)
 {
 	xml_parse_result result;