BORG_EXIT_CODES=modern can be set to get more specific process exit codes

If not set, it will default to "legacy" (always return 2 for errors).

This commit only changes the Error exception class and its subclasses.

The more specific exit codes need to be defined via .exit_mcode in the subclasses.

src/borg/helpers/errors.py

@@ -1,3 +1,5 @@
+import os
+
 from ..constants import *  # NOQA
 
 from ..crypto.low_level import IntegrityError as IntegrityErrorBase
@@ -10,8 +12,9 @@ class Error(Exception):
 
     # if we raise such an Error and it is only caught by the uppermost
     # exception handler (that exits short after with the given exit_code),
-    # it is always a (fatal and abrupt) EXIT_ERROR, never just a warning.
-    exit_code = EXIT_ERROR
+    # it is always a (fatal and abrupt) error, never just a warning.
+    exit_mcode = EXIT_ERROR  # modern, more specific exit code (defaults to EXIT_ERROR)
+
     # show a traceback?
     traceback = False
 
@@ -24,6 +27,13 @@ class Error(Exception):
 
     __str__ = get_message
 
+    @property
+    def exit_code(self):
+        # legacy: borg used to always use rc 2 (EXIT_ERROR) for all errors.
+        # modern: users can opt in to more specific return codes, using BORG_RC_STYLE:
+        modern = os.environ.get("BORG_EXIT_CODES", "legacy") == "modern"
+        return self.exit_mcode if modern else EXIT_ERROR
+
 
 class ErrorWithTraceback(Error):
     """Error: {}"""