Xml_generator: fix exception handling in Node(...)

When the functor provided to the Node constructor throws an exception,
do revert all changes in reverse order. Previously, the changes made
to the parent node were not considered by the exception handler which
caused unnecessary characters to remain in the out buffer for each
reverted node.

Issue genodelabs#2967

repos/base/include/util/xml_generator.h

@@ -60,6 +60,9 @@ class Genode::Xml_generator
 					_used += len;
 				}
 
+				void undo_append(size_t const len) {
+					_used = len < _used ? _used - len : 0; }
+
 				/**
 				 * Append character
 				 */
@@ -162,6 +165,8 @@ class Genode::Xml_generator
 				unsigned const _indent_level;
 
 				Node * const _parent_node = 0;
+				bool   const _parent_was_indented;
+				bool   const _parent_had_content;
 
 				Out_buffer _out_buffer;
 
@@ -197,6 +202,20 @@ class Genode::Xml_generator
 					return _out_buffer.remainder();
 				}
 
+				void _undo_content_buffer(bool indented,
+				                          bool was_indented,
+				                          bool had_content)
+				{
+					_is_indented = was_indented;
+					_has_content = had_content;
+
+					if (indented)
+						_out_buffer.undo_append(1);
+
+					if (!_has_content)
+						_out_buffer.undo_append(1);
+				}
+
 				/**
 				 * Called by sub node
 				 */
@@ -217,7 +236,7 @@ class Genode::Xml_generator
 					dst.append(name);
 					dst.append("=\"");
 					dst.append(value, strlen(value));
-					dst.append("\"");
+					dst.append('\"');
 
 					_attr_offset += gap;
 				}
@@ -241,6 +260,8 @@ class Genode::Xml_generator
 				:
 					_indent_level(xml._curr_indent),
 					_parent_node(xml._curr_node),
+					_parent_was_indented(_parent_node ? _parent_node->is_indented() : false),
+					_parent_had_content (_parent_node ? _parent_node->has_content() : false),
 					_out_buffer(_parent_node ? _parent_node->_content_buffer(true)
 					                         : xml._out_buffer)
 				{
@@ -259,8 +280,10 @@ class Genode::Xml_generator
 						func();
 					} catch (...) {
 						/* reset and drop changes by not committing it */
-						xml._curr_node = _parent_node;
 						xml._curr_indent--;
+						xml._curr_node = _parent_node;
+						if (_parent_node) {
+							_parent_node->_undo_content_buffer(true, _parent_was_indented, _parent_had_content); }
 						throw;
 					}
 
@@ -286,6 +309,9 @@ class Genode::Xml_generator
 
 					_out_buffer.append('\0');
 				}
+
+				bool has_content() { return _has_content; }
+				bool is_indented() { return _is_indented; }
 		};
 
 		Out_buffer _out_buffer;

repos/os/run/xml_generator.run

@@ -53,20 +53,17 @@ compare_output_to {
 	[init -> test-xml_generator] exception on level3 (expected exception value=11)
 	[init -> test-xml_generator] exception on level3 (expected exception value=12)
 	[init -> test-xml_generator] 
-	[init -> test-xml_generator] used 183 bytes, result:
+	[init -> test-xml_generator] used 180 bytes, result:
 	[init -> test-xml_generator] 
 	[init -> test-xml_generator] <config>
 	[init -> test-xml_generator] 	<level1>
 	[init -> test-xml_generator] 		<level2>
-	[init -> test-xml_generator] 
 	[init -> test-xml_generator] 			<level3>
 	[init -> test-xml_generator] 				<level4/>
 	[init -> test-xml_generator] 			</level3>
-	[init -> test-xml_generator] 
 	[init -> test-xml_generator] 			<level3>
 	[init -> test-xml_generator] 				<level4/>
 	[init -> test-xml_generator] 			</level3>
-	[init -> test-xml_generator] 
 	[init -> test-xml_generator] 			<level3>
 	[init -> test-xml_generator] 				<level4/>
 	[init -> test-xml_generator] 			</level3>