grass.gunittest: Add --config parameter (#2182)

A configuration file can now be specified explicitly in the command line which is useful for providing custom configuration file, e.g., with more or less tests excluded or with different timeout for different platforms or hardware setups.

Removes a useless return from the config function. Reports OSErrors from the function without traceback.

python/grass/gunittest/main.py

@@ -129,18 +129,28 @@ def discovery():
 CONFIG_FILENAME = ".gunittest.cfg"
 
 
-def get_config(start_directory):
-    """Read configuration if available, return empty dict if not"""
+def get_config(start_directory, config_file):
+    """Read configuration if available, return empty section proxy if not
+
+    If file is explicitly specified, it must exist.
+
+    Raises OSError if file is not accessible, e.g., if it exists,
+    but there is an issue with permissions.
+    """
     config_parser = configparser.ConfigParser()
-    config_file = Path(start_directory) / CONFIG_FILENAME
-    if config_file.is_file():
+    if config_file:
+        with open(config_file, encoding="utf-8") as file:
+            config_parser.read_file(file)
+    elif start_directory:
+        config_file = Path(start_directory) / CONFIG_FILENAME
+        # Does not check presence of the file
         config_parser.read(config_file)
     else:
-        # Create an empty section if file is not available.
+        raise ValueError("Either start_directory or config_file must be set")
+    if "gunittest" not in config_parser:
+        # Create an empty section if file is not available or section is not present.
         config_parser.read_dict({"gunittest": {}})
-    if "gunittest" in config_parser:
-        return config_parser["gunittest"]
-    return config_parser
+    return config_parser["gunittest"]
 
 
 def main():
@@ -188,6 +198,13 @@ def main():
             " than this will result in a non-zero return code; values 0-100)"
         ),
     )
+    parser.add_argument(
+        "--config",
+        dest="config",
+        action="store",
+        type=str,
+        help=f"Path to a configuration file (default: {CONFIG_FILENAME})",
+    )
     args = parser.parse_args()
     gisdbase = args.gisdbase
     if gisdbase is None:
@@ -212,7 +229,10 @@ def main():
     start_dir = "."
     abs_start_dir = os.path.abspath(start_dir)
 
-    config = get_config(start_dir)
+    try:
+        config = get_config(start_directory=start_dir, config_file=args.config)
+    except OSError as error:
+        return f"Error reading configuration: {error}"
 
     invoker = GrassTestFilesInvoker(
         start_dir=start_dir,