automatic: Check availability of config file

If a configuration file is explicitly specified on the command line,
ensure that it exists and is readable. If the file is not found, notify
the user immediately and terminate the process.

This resolves issues where users may run dnf-automatic with unrecognized
positional arguments, such as `dnf-automatic install`.

Resolves: https://issues.redhat.com/browse/RHEL-46030

dnf/automatic/main.py

@@ -74,7 +74,7 @@ def build_emitters(conf):
 
 def parse_arguments(args):
     parser = argparse.ArgumentParser()
-    parser.add_argument('conf_path', nargs='?', default=dnf.const.CONF_AUTOMATIC_FILENAME)
+    parser.add_argument('conf_path', nargs='?')
     parser.add_argument('--timer', action='store_true')
     parser.add_argument('--installupdates', dest='installupdates', action='store_true')
     parser.add_argument('--downloadupdates', dest='downloadupdates', action='store_true')
@@ -89,7 +89,17 @@ def parse_arguments(args):
 class AutomaticConfig(object):
     def __init__(self, filename=None, downloadupdates=None,
                  installupdates=None):
-        if not filename:
+        if filename:
+            # Specific config file was explicitely requested. Check that it exists
+            # and is readable.
+            if os.access(filename, os.F_OK):
+                if not os.access(filename, os.R_OK):
+                    raise dnf.exceptions.Error(
+                            "Configuration file \"{}\" is not readable.".format(filename))
+            else:
+                raise dnf.exceptions.Error(
+                        "Configuration file \"{}\" not found.".format(filename))
+        else:
             filename = dnf.const.CONF_AUTOMATIC_FILENAME
         self.commands = CommandsConfig()
         self.email = EmailConfig()
@@ -304,9 +314,10 @@ def main(args):
     (opts, parser) = parse_arguments(args)
 
     try:
+        conf = None
+        emitters = None
         conf = AutomaticConfig(opts.conf_path, opts.downloadupdates,
                                opts.installupdates)
-        emitters = None
         with dnf.Base() as base:
             cli = dnf.cli.Cli(base)
             cli._read_conf_file()
@@ -371,7 +382,7 @@ def main(args):
                     raise dnf.exceptions.Error('reboot command returned nonzero exit code: %d', exit_code)
     except dnf.exceptions.Error as exc:
         logger.error(_('Error: %s'), ucd(exc))
-        if conf.emitters.send_error_messages and emitters != None:
+        if conf != None and conf.emitters.send_error_messages and emitters != None:
             emitters.notify_error(_('Error: %s') % str(exc))
             emitters.commit()
         return 1