automatic: Check availability of config file

Upstream commit: 13ecc39
RHEL issue: https://issues.redhat.com/browse/RHEL-49743

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`.

The most natural approach to handle a non-existing config file would be
by catching the exception thrown by the `read()` method of the
`libdnf.conf.ConfigParser` class. Unfortunately, the Python bindings
override the `read()` method at the SWIG level, causing it to suppress any
potentially raised IOError.
For details see this section of the commit
rpm-software-management/libdnf@8f1fedf

def ConfigParser__newRead(self, filenames):
    parsedFNames = []
    try:
        if isinstance(filenames, str) or isinstance(filenames, unicode):
            filenames = [filenames]
    except NameError:
        pass
    for fname in filenames:
        try:
            self.readFileName(fname)
            parsedFNames.append(fname)
        except IOError:
            pass
        except Exception as e:
            raise RuntimeError("Parsing file '%s' failed: %s" % (fname, str(e)))
    return parsedFNames
ConfigParser.read = ConfigParser__newRead

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()