Fail if only one of SSH host and SSH user is set

Remote backup needs both, but if only one was set the program silently
ran a local backup instead, and a missing ssh_user in the config also
dropped ssh_host. Exit with an error (code 6) instead.

man/simple_backup.1

@@ -207,7 +207,7 @@ rsync error (rsync returned a value other than 0 or 24).
 SSH connection failed.
 .TP
 .B 6
-Bad configuration file.
+Bad configuration file or options (for example, only one of SSH host and SSH user specified).
 .SH SEE ALSO
 .BR rsync (1)
 .SH AUTHORS

simple_backup/simple_backup.py

@@ -806,15 +806,10 @@ def _read_config(config_file, user: Optional[str] = None):
 
     config_args['keep'] = keep
 
-    try:
-        ssh_host = config.get('server', 'ssh_host')
-        ssh_user = config.get('server', 'ssh_user')
-    except (configparser.NoSectionError, configparser.NoOptionError):
-        ssh_host = None
-        ssh_user = None
-
-    config_args['ssh_host'] = ssh_host
-    config_args['ssh_user'] = ssh_user
+    # Read host and user separately, so that a missing one is reported instead of silently
+    # disabling remote backup. Empty values count as not set
+    config_args['ssh_host'] = config.get('server', 'ssh_host', fallback='').strip() or None
+    config_args['ssh_user'] = config.get('server', 'ssh_user', fallback='').strip() or None
 
     try:
         ssh_keyfile = config.get('server', 'ssh_keyfile')
@@ -947,6 +942,11 @@ def simple_backup() -> int:
     ssh_host = args.ssh_host if args.ssh_host is not None else config_args['ssh_host']
     ssh_user = args.ssh_user if args.ssh_user is not None else config_args['ssh_user']
 
+    if (ssh_host is None) != (ssh_user is None):
+        logger.critical('Both SSH host and SSH user are needed for remote backup')
+
+        return 6
+
     if args.output is not None:
         output = args.output
     else: