Improve handling of configuration file

* Allow MBS_CONFIG_FILE="" to entirely suppress loading any configuration
   file (useful for running tests and avoiding loading a system-wide
   configuration file.)
 * When loading the configuration file:
   * If the default configuration file path doesn't exist, silently fall back
     to the default configuration
   * For any other OSError, print the exact error
   * Let any other exception throw through, to allow people to debug their
     configuration file

README.rst

@@ -903,7 +903,9 @@ the following rules (all of them are evaluated from top to bottom):
   recognized:
 
     - ``MBS_CONFIG_FILE``: Overrides default configuration file location,
-      typically ``/etc/module-build-service/config.py``.
+      typically ``/etc/module-build-service/config.py``. If set to the
+      empty string, no configuration file will be read and default
+      values will be used.
     - ``MBS_CONFIG_SECTION``: Overrides configuration section.
 
   It is possible to set these values in httpd using ``SetEnv``,

module_build_service/common/config.py

@@ -1,6 +1,7 @@
 # -*- coding: utf-8 -*-
 # SPDX-License-Identifier: MIT
 from __future__ import absolute_import
+import errno
 import imp
 import logging
 import os
@@ -108,15 +109,27 @@ def init_config():
         to configure Flask with.
     :rtype: tuple(Config, object)
     """
-    config_file = os.environ.get("MBS_CONFIG_FILE", "/etc/module-build-service/config.py")
+    env_config_file = os.environ.get("MBS_CONFIG_FILE")
+    if env_config_file is None:
+        config_file = "/etc/module-build-service/config.py"
+    else:
+        config_file = env_config_file
 
-    try:
-        config_module = imp.load_source("mbs_runtime_config", config_file)
-        log.info("Using the configuration file at %s", config_file)
-    except Exception:
-        log.warning("The configuration file at %s was not present", config_file)
+    config_module = None
+
+    # MBS_CONFIG_FILE="" entirely suppresses looking a config file
+    if config_file != "":
+        try:
+            config_module = imp.load_source("mbs_runtime_config", config_file)
+            log.info("Using the configuration file at %s", config_file)
+        except OSError as e:
+            if e.errno != errno.ENOENT or env_config_file:
+                log.error("Can't open config file: %s", e)
+
+    if config_module is None:
         # Default to this file as the configuration module
         config_module = sys.modules[__name__]
+        log.debug("Using default configuration")
 
     true_options = ("1", "on", "true", "y", "yes")
     if any(["py.test" in arg or "pytest" in arg for arg in sys.argv]):