swift-ring-builder: exit ERROR (2) on uncaught exceptions

swift-ring-builder has three exit statuses: 0 (OK), 1 (WARNING),
2 (ERROR). Uncaught exceptions in python result in an exit code of 1,
so for example problems writing a builder file to disk will result in
an exit of 1 (warning) rather than 2 (error).

This addresses that by overriding sys.excepthook to produce the usual
backtrace and then exit 2 (error); excepthook is called when an
exception is unhandled, unless that is SystemExit.

Closes-Bug: 1960657
Change-Id: I7cfeff4f436ade319cf21d0d29853931aef6d20f

bin/swift-ring-builder

@@ -16,9 +16,22 @@
 
 
 import sys
+import traceback
+
+# We exit code 1 on WARNING statuses, 2 on ERROR. This means we need
+# to handle any uncaught exceptions by printing the usual backtrace,
+# but then exiting 2 (not 1 as is usual for a python
+# exception).
+
+
+def exit_with_status_two(tp, val, tb):
+    traceback.print_exception(tp, val, tb)
+    sys.exit(2)
+
+
+sys.excepthook = exit_with_status_two
 
 from swift.cli.ringbuilder import main
 
-
 if __name__ == "__main__":
     sys.exit(main())