config: Remove confusing magic in path handling

1. When passed a .yaml path (or any path with a non-whitelisted extension),
   don't read .yml instead when the .yaml exists.
   It's an extremely surprising behavior.

2. If the .yaml file does not exist, it will still try alternative extensions
   in order not to break existing deployments which may rely on it, but it
   raises a warning now.

3. When given a non-existing path, show an error log, but keep parsing
   it as an empty config, in order not to break existing deployments.

swh/core/config.py

@@ -70,34 +70,35 @@ def exists_accessible(filepath: str) -> bool:
             raise PermissionError("Permission denied: {filepath!r}")
 
 
-def config_basepath(config_path: str) -> str:
-    """Return the base path of a configuration file"""
-    if config_path.endswith(".yml"):
-        return config_path[:-4]
-
-    return config_path
-
-
 def read_raw_config(base_config_path: str) -> Dict[str, Any]:
     """Read the raw config corresponding to base_config_path.
 
     Can read yml files.
     """
-    yml_file = f"{base_config_path}.yml"
-    if exists_accessible(yml_file):
+    yml_file = config_path(base_config_path)
+    if yml_file is None:
+        logging.error("Config file %s does not exist, ignoring it.", base_config_path)
+        return {}
+    else:
         logger.debug("Loading config file %s", yml_file)
         with open(yml_file) as f:
             return yaml.safe_load(f)
 
-    return {}
 
-
-def config_exists(config_path):
+def config_path(config_path):
     """Check whether the given config exists"""
-    basepath = config_basepath(config_path)
-    return any(
-        exists_accessible(basepath + extension) for extension in SWH_CONFIG_EXTENSIONS
-    )
+    if exists_accessible(config_path):
+        return config_path
+    for extension in SWH_CONFIG_EXTENSIONS:
+        if exists_accessible(config_path + extension):
+            logger.warning(
+                "%s does not exist, using %s instead",
+                config_path,
+                config_path + extension,
+            )
+            return config_path + extension
+
+    return None
 
 
 def read(
@@ -122,7 +123,7 @@ def read(
     conf: Dict[str, Any] = {}
 
     if conf_file:
-        base_config_path = config_basepath(os.path.expanduser(conf_file))
+        base_config_path = os.path.expanduser(conf_file)
         conf = read_raw_config(base_config_path) or {}
 
     if not default_conf:
@@ -152,8 +153,8 @@ def priority_read(
 
     # Try all the files in order
     for filename in conf_filenames:
-        full_filename = os.path.expanduser(filename)
-        if config_exists(full_filename):
+        full_filename = config_path(os.path.expanduser(filename))
+        if full_filename is not None:
             return read(full_filename, default_conf)
 
     # Else, return the default configuration
@@ -303,6 +304,6 @@ def load_from_envvar(default_config: Optional[Dict[str, Any]] = None) -> Dict[st
     ), "SWH_CONFIG_FILENAME environment variable is undefined."
 
     cfg_path = os.environ["SWH_CONFIG_FILENAME"]
-    cfg = read_raw_config(config_basepath(cfg_path))
+    cfg = read_raw_config(cfg_path)
     cfg = merge_configs(default_config or {}, cfg)
     return cfg

swh/core/tests/test_config.py

-# Copyright (C) 2015-2020  The Software Heritage developers
+# Copyright (C) 2015-2023  The Software Heritage developers
 # See the AUTHORS file at the top-level directory of this distribution
 # License: GNU General Public License version 3, or any later version
 # See top-level LICENSE file for more information
 
 import os
+from pathlib import Path
 import shutil
 
 import pkg_resources.extern.packaging.version
@@ -115,6 +116,16 @@ def test_read(swh_config):
     assert res == parsed_conffile
 
 
+def test_read_dotyaml(swh_config):
+    swh_config2 = Path(str(swh_config).replace(".yml", ".yaml"))
+    swh_config.rename(swh_config2)
+    # when
+    res = config.read(swh_config2, default_conf)
+
+    # then
+    assert res == parsed_conffile
+
+
 def test_read_no_default_conf(swh_config):
     """If no default config if provided to read, this should directly parse the config file
     yaml