Only allow absolute paths in XDG environment variables

The XDG Base Directory specification does not allow using relative paths
(which broke things in Godot anyway). If a relative path is detected,
it should be ignored.

(cherry picked from commits 011a99316ab8c0e96971aebd9ba995313867fe17
and 0e1d45b210d0e5924b279419d9bba188f557e329)

platform/osx/os_osx.mm

@@ -2238,31 +2238,45 @@ MainLoop *OS_OSX::get_main_loop() const {
 }
 
 String OS_OSX::get_config_path() const {
+	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on macOS as well.
 	if (has_environment("XDG_CONFIG_HOME")) {
-		return get_environment("XDG_CONFIG_HOME");
-	} else if (has_environment("HOME")) {
+		if (get_environment("XDG_CONFIG_HOME").is_abs_path()) {
+			return get_environment("XDG_CONFIG_HOME");
+		} else {
+			WARN_PRINT_ONCE("`XDG_CONFIG_HOME` is a relative path. Ignoring its value and falling back to `$HOME/Library/Application Support` or `.` per the XDG Base Directory specification.");
+		}
+	}
+	if (has_environment("HOME")) {
 		return get_environment("HOME").plus_file("Library/Application Support");
-	} else {
-		return ".";
 	}
+	return ".";
 }
 
 String OS_OSX::get_data_path() const {
+	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on macOS as well.
 	if (has_environment("XDG_DATA_HOME")) {
-		return get_environment("XDG_DATA_HOME");
-	} else {
-		return get_config_path();
+		if (get_environment("XDG_DATA_HOME").is_abs_path()) {
+			return get_environment("XDG_DATA_HOME");
+		} else {
+			WARN_PRINT_ONCE("`XDG_DATA_HOME` is a relative path. Ignoring its value and falling back to `get_config_path()` per the XDG Base Directory specification.");
+		}
 	}
+	return get_config_path();
 }
 
 String OS_OSX::get_cache_path() const {
+	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on macOS as well.
 	if (has_environment("XDG_CACHE_HOME")) {
-		return get_environment("XDG_CACHE_HOME");
-	} else if (has_environment("HOME")) {
+		if (get_environment("XDG_CACHE_HOME").is_abs_path()) {
+			return get_environment("XDG_CACHE_HOME");
+		} else {
+			WARN_PRINT_ONCE("`XDG_CACHE_HOME` is a relative path. Ignoring its value and falling back to `$HOME/Libary/Caches` or `get_config_path()` per the XDG Base Directory specification.");
+		}
+	}
+	if (has_environment("HOME")) {
 		return get_environment("HOME").plus_file("Library/Caches");
-	} else {
-		return get_config_path();
 	}
+	return get_config_path();
 }
 
 String OS_OSX::get_bundle_resource_dir() const {

platform/windows/os_windows.cpp

@@ -3316,31 +3316,45 @@ MainLoop *OS_Windows::get_main_loop() const {
 }
 
 String OS_Windows::get_config_path() const {
-	if (has_environment("XDG_CONFIG_HOME")) { // unlikely, but after all why not?
-		return get_environment("XDG_CONFIG_HOME");
-	} else if (has_environment("APPDATA")) {
+	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on Windows as well.
+	if (has_environment("XDG_CONFIG_HOME")) {
+		if (get_environment("XDG_CONFIG_HOME").is_abs_path()) {
+			return get_environment("XDG_CONFIG_HOME");
+		} else {
+			WARN_PRINT_ONCE("`XDG_CONFIG_HOME` is a relative path. Ignoring its value and falling back to `%APPDATA%` or `.` per the XDG Base Directory specification.");
+		}
+	}
+	if (has_environment("APPDATA")) {
 		return get_environment("APPDATA");
-	} else {
-		return ".";
 	}
+	return ".";
 }
 
 String OS_Windows::get_data_path() const {
+	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on Windows as well.
 	if (has_environment("XDG_DATA_HOME")) {
-		return get_environment("XDG_DATA_HOME");
-	} else {
-		return get_config_path();
+		if (get_environment("XDG_DATA_HOME").is_abs_path()) {
+			return get_environment("XDG_DATA_HOME");
+		} else {
+			WARN_PRINT_ONCE("`XDG_DATA_HOME` is a relative path. Ignoring its value and falling back to `get_config_path()` per the XDG Base Directory specification.");
+		}
 	}
+	return get_config_path();
 }
 
 String OS_Windows::get_cache_path() const {
+	// The XDG Base Directory specification technically only applies on Linux/*BSD, but it doesn't hurt to support it on Windows as well.
 	if (has_environment("XDG_CACHE_HOME")) {
-		return get_environment("XDG_CACHE_HOME");
-	} else if (has_environment("TEMP")) {
+		if (get_environment("XDG_CACHE_HOME").is_abs_path()) {
+			return get_environment("XDG_CACHE_HOME");
+		} else {
+			WARN_PRINT_ONCE("`XDG_CACHE_HOME` is a relative path. Ignoring its value and falling back to `%TEMP%` or `get_config_path()` per the XDG Base Directory specification.");
+		}
+	}
+	if (has_environment("TEMP")) {
 		return get_environment("TEMP");
-	} else {
-		return get_config_path();
 	}
+	return get_config_path();
 }
 
 // Get properly capitalized engine name for system paths

platform/x11/os_x11.cpp

@@ -3208,32 +3208,44 @@ bool OS_X11::_check_internal_feature_support(const String &p_feature) {
 
 String OS_X11::get_config_path() const {
 	if (has_environment("XDG_CONFIG_HOME")) {
-		return get_environment("XDG_CONFIG_HOME");
-	} else if (has_environment("HOME")) {
+		if (get_environment("XDG_CONFIG_HOME").is_abs_path()) {
+			return get_environment("XDG_CONFIG_HOME");
+		} else {
+			WARN_PRINT_ONCE("`XDG_CONFIG_HOME` is a relative path. Ignoring its value and falling back to `$HOME/.config` or `.` per the XDG Base Directory specification.");
+		}
+	}
+	if (has_environment("HOME")) {
 		return get_environment("HOME").plus_file(".config");
-	} else {
-		return ".";
 	}
+	return ".";
 }
 
 String OS_X11::get_data_path() const {
 	if (has_environment("XDG_DATA_HOME")) {
-		return get_environment("XDG_DATA_HOME");
-	} else if (has_environment("HOME")) {
+		if (get_environment("XDG_DATA_HOME").is_abs_path()) {
+			return get_environment("XDG_DATA_HOME");
+		} else {
+			WARN_PRINT_ONCE("`XDG_DATA_HOME` is a relative path. Ignoring its value and falling back to `$HOME/.local/share` or `get_config_path()` per the XDG Base Directory specification.");
+		}
+	}
+	if (has_environment("HOME")) {
 		return get_environment("HOME").plus_file(".local/share");
-	} else {
-		return get_config_path();
 	}
+	return get_config_path();
 }
 
 String OS_X11::get_cache_path() const {
 	if (has_environment("XDG_CACHE_HOME")) {
-		return get_environment("XDG_CACHE_HOME");
-	} else if (has_environment("HOME")) {
+		if (get_environment("XDG_CACHE_HOME").is_abs_path()) {
+			return get_environment("XDG_CACHE_HOME");
+		} else {
+			WARN_PRINT_ONCE("`XDG_CACHE_HOME` is a relative path. Ignoring its value and falling back to `$HOME/.cache` or `get_config_path()` per the XDG Base Directory specification.");
+		}
+	}
+	if (has_environment("HOME")) {
 		return get_environment("HOME").plus_file(".cache");
-	} else {
-		return get_config_path();
 	}
+	return get_config_path();
 }
 
 String OS_X11::get_system_dir(SystemDir p_dir) const {