fs: Add nearest_extant_parent() function and tests

src/filesystem.hpp

@@ -182,6 +182,19 @@ std::string base_name(const std::string& file);
  */
 std::string directory_name(const std::string& file);
 
+/**
+ * Finds the nearest parent in existence for a file or directory.
+ *
+ * @note    The file's own existence is not checked.
+ *
+ * @returns An absolute path to the closest parent of the given path, or an
+ *          empty string if none could be found. While on POSIX platforms this
+ *          cannot happen (unless the original path was already empty), on
+ *          Windows it might be the case that the original path refers to a
+ *          drive letter or network share that does not exist.
+ */
+std::string nearest_extant_parent(const std::string& file);
+
 /**
  * Returns the absolute path of a file.
  *

src/filesystem_boost.cpp

@@ -920,6 +920,26 @@ std::string directory_name(const std::string& file)
 	return path(file).parent_path().string();
 }
 
+std::string nearest_extant_parent(const std::string& file)
+{
+	if(file.empty()) {
+		return "";
+	}
+
+	bfs::path p{file};
+	error_code ec;
+
+	do {
+		p = p.parent_path();
+		bfs::path q = canonical(p, ec);
+		if (!ec) {
+			p = q;
+		}
+	} while(ec && !is_root(p.string()));
+
+	return ec ? "" : p.string();
+}
+
 bool is_path_sep(char c)
 {
 	static const path sep = path("/").make_preferred();

src/tests/test_filesystem.cpp

@@ -190,6 +190,27 @@ BOOST_AUTO_TEST_CASE( test_fs_wml_path )
 	BOOST_CHECK( get_wml_location("why_would_anyone_ever_name_a_file_like_this").empty() );
 }
 
+BOOST_AUTO_TEST_CASE( test_fs_search )
+{
+	const std::string& userdata = get_user_data_dir();
+
+	BOOST_CHECK_EQUAL( nearest_extant_parent(userdata + "/THIS_DOES_NOT_EXIST/foo/bar"), userdata );
+
+	BOOST_CHECK_EQUAL( nearest_extant_parent(gamedata + "/THIS_DOES_NOT_EXIST_EITHER/foo"), gamedata );
+	BOOST_CHECK_EQUAL( nearest_extant_parent(gamedata + "/data/_main.cfg"), gamedata + "/data" );
+	BOOST_CHECK_EQUAL( nearest_extant_parent(gamedata + "/data/core/THIS_DOES_NOT_EXIST/test"), gamedata + "/data/core" );
+
+	BOOST_CHECK_EQUAL( nearest_extant_parent("/THIS_HOPEFULLY_DOES_NOT_EXIST"), "/" );
+	BOOST_CHECK_EQUAL( nearest_extant_parent("/bin/THIS_HOPEFULLY_DOES_NOT_EXIST/foo/bar/."), "/bin" );
+	BOOST_CHECK_EQUAL( nearest_extant_parent("/bin/THIS_HOPEFULLY_DOES_NOT_EXIST/foo"), "/bin" );
+	BOOST_CHECK_EQUAL( nearest_extant_parent("/bin/THIS_HOPEFULLY_DOES_NOT_EXIST"), "/bin" );
+	// Directories that don't exist can't have a .. entry.
+	BOOST_CHECK_EQUAL( nearest_extant_parent("/bin/THIS_HOPEFULLY_DOES_NOT_EXIST/.."), "/bin" );
+	BOOST_CHECK_EQUAL( nearest_extant_parent("/bin/."), "/bin" );
+	BOOST_CHECK_EQUAL( nearest_extant_parent("/bin/./../bin/././"), "/bin" );
+	BOOST_CHECK_EQUAL( nearest_extant_parent("/bin/./../././"), "/" );
+}
+
 BOOST_AUTO_TEST_CASE( test_fs_fluff )
 {
 	BOOST_CHECK( ends_with("foobarbazbat", "bazbat") );