xml-support.c: Use std::string for growing string buffer

This main idea behind this patch is this change to xml-support.c:scope_level

  -  /* Body text accumulation.  This is an owning pointer.  */
  -  struct obstack *body;
  +  /* Body text accumulation.  */
  +  std::string body;

... which allows simplifying other parts of the code.

In target_fetch_description_xml, we want to distinguish between
returning "success + empty std::string" and "no success", and
gdb::optional is a natural fit for that.

gdb/ChangeLog:
2017-04-18  Pedro Alves  <palves@redhat.com>

	* tracefile-tfile.c (tfile_write_tdesc): Adjust to use
	gdb::optional<std::string>.
	* xml-support.c: Include <string>.
	(scope_level::scope_level(scope_level &&))
	(scope_level::~scope_level): Delete.
	(scope_level::body): Now a std::string.
	(gdb_xml_body_text, gdb_xml_end_element): Adjust.
	(xinclude_parsing_data::xinclude_parsing_data): Add 'output'
	parameter.
	(xinclude_parsing_data::~xinclude_parsing_data): Delete.
	(xinclude_parsing_data::output): Now a std::string reference.
	(xinclude_start_include): Adjust.
	(xml_xinclude_default): Adjust.
	(xml_process_xincludes): Add 'output' parameter, and return bool.
	* xml-support.h (xml_process_xincludes): Add 'output' parameter,
	and return bool.
	* xml-tdesc.c: Include <unordered_map> and <string>.
	(tdesc_xml_cache): Delete.
	(tdesc_xml_cache_s): Delete.
	(xml_cache): Now an std::unordered_map.
	(tdesc_parse_xml): Adjust to use std::string and unordered_map.
	(target_fetch_description_xml): Change return type to
	gdb::optional<std::string>, and adjust.
	* xml-tdesc.h: Include "common/gdb_optional.h" and <string>.
	(target_fetch_description_xml): Change return type to
	gdb::optional<std::string>.

1 files changed, 13 insertions, 4 deletions

diff --git a/gdb/xml-tdesc.h b/gdb/xml-tdesc.h
index 1637a89..2375395 100644
--- a/gdb/xml-tdesc.h
+++ b/gdb/xml-tdesc.h
@@ -19,6 +19,12 @@
    You should have received a copy of the GNU General Public License
    along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
+#ifndef XML_TDESC_H
+#define XML_TDESC_H
+
+#include "common/gdb_optional.h"
+#include <string>
+
 struct target_ops;
 struct target_desc;
 
@@ -32,8 +38,11 @@ const struct target_desc *file_read_description_xml (const char *filename);
 
 const struct target_desc *target_read_description_xml (struct target_ops *);
 
-/* Fetches an XML target description using OPS,  processing
-   includes, but not parsing it.  Used to dump whole tdesc
-   as a single XML file.  */
+/* Fetches an XML target description using OPS, processing includes,
+   but not parsing it.  Used to dump whole tdesc as a single XML file.
+   Returns the description on success, and a disengaged optional
+   otherwise.  */
+gdb::optional<std::string> target_fetch_description_xml (target_ops *ops);
+
+#endif /* XML_TDESC_H */
 
-char *target_fetch_description_xml (struct target_ops *ops);