Refuse to save if game state can't be represented as valid WML (#2375)

Such a save file can't be loaded anyway.
This way, at least the player can't overwrite an existing, working save
with one that cannot be loaded.

src/config.cpp

@@ -25,10 +25,12 @@
 #include "utils/const_clone.hpp"
 #include "utils/functional.hpp"
 
+#include <algorithm>
 #include <cstdlib>
 #include <cstring>
 #include <deque>
 #include <istream>
+#include <locale>
 
 #include <boost/variant/apply_visitor.hpp>
 #include <boost/variant/get.hpp>
@@ -1303,6 +1305,33 @@ void swap(config& lhs, config& rhs)
 	lhs.swap(rhs);
 }
 
+bool config::is_valid_wml_tag_name(config_key_type name)
+{
+	if(name == "") {
+		// Empty strings not allowed
+		return false;
+	} else if(name[0] == '_') {
+		// Underscore can't be the first character
+		return false;
+	} else {
+		return std::all_of(name.begin(), name.end(), [](const char& c)
+		{
+			// Only alphanumeric ASCII characters and underscores are allowed
+			return std::isalnum(static_cast<unsigned char>(c), std::locale::classic()) || c == '_';
+		});
+	}
+}
+
+bool config::validate_wml() const
+{
+	return std::all_of(children_.begin(), children_.end(), [](const child_map::value_type& pair)
+	{
+		return is_valid_wml_tag_name(pair.first) &&
+			std::all_of(pair.second.begin(), pair.second.end(),
+			[](const std::unique_ptr<config>& c) { return c->validate_wml(); });
+	});
+}
+
 bool operator==(const config& a, const config& b)
 {
 	a.check_valid(b);

src/config.hpp

@@ -761,6 +761,17 @@ public:
 	//this is a cheap O(1) operation
 	void swap(config& cfg);
 
+	/**
+	 * Returns true for valid WML tag names, false for all other strings.
+	 */
+	static bool is_valid_wml_tag_name(config_key_type name);
+
+	/**
+	 * Returns true if this object represents valid WML,
+	 * i.e. can be saved to disk and again loaded by the WML parser.
+	 */
+	bool validate_wml() const;
+
 private:
 	/**
 	 * Removes the child at position @a pos of @a l.

src/savegame.cpp

@@ -637,6 +637,10 @@ void ingame_savegame::create_filename()
 void ingame_savegame::write_game(config_writer &out) {
 	log_scope("write_game");
 
+	if(!gamestate().get_starting_pos().validate_wml()) {
+		throw game::save_game_failed(_("Game state is corrupted"));
+	}
+
 	savegame::write_game(out);
 
 	gamestate().write_carryover(out);