Add discovery of loggers

Author: ikretz

scfw/loggers/__init__.py

@@ -1,16 +1,51 @@
 """
-Exports a set of client loggers implementing the firewall's logging protocol.
+Exports the currently discoverable set of client loggers implementing the
+firewall's logging protocol.
+
+One logger ships with the supply chain firewall by default: `DDLogger`,
+which sends logs to Datadog. Firewall users may additionally provide
+custom loggers according to their own logging needs.
+
+The firewall discovers loggers at runtime via the following simple protocol.
+The module implementing the custom logger must contain a function with the
+following name and signature:
+
+```
+def load_logger() -> FirewallLogger
+```
+
+This `load_logger` function should return an instance of the custom logger
+for the firewall's use. The module may then be placed in the same directory as
+this source file for runtime import.
 """
 
+import importlib
+import logging
+import os
+import pkgutil
+
 from scfw.logger import FirewallLogger
-from scfw.loggers.dd_logger import DDLogger
+
+_log = logging.getLogger(__name__)
 
 
 def get_firewall_loggers() -> list[FirewallLogger]:
     """
-    Return the current set of available client loggers.
+    Return the currently discoverable set of client loggers.
 
     Returns:
-        A `list` of the currently available `FirewallLogger`s.
+        A `list` of the discovered `FirewallLogger`s.
     """
-    return [DDLogger()]
+    loggers = []
+
+    # TODO(ikretz): Allow the user to configure this directory
+    for _, module, _ in pkgutil.iter_modules([os.path.dirname(__file__)]):
+        try:
+            logger = importlib.import_module(module).load_logger()
+            loggers.append(logger)
+        except ModuleNotFoundError:
+            _log.warning(f"Failed to load module {module} while collecting loggers")
+        except AttributeError:
+            _log.warning(f"Module {module} does not export a logger")
+
+    return loggers

scfw/loggers/dd_logger.py

@@ -118,3 +118,13 @@ def emit(self, record):
         with ApiClient(configuration) as api_client:
             api_instance = LogsApi(api_client)
             api_instance.submit_log(content_encoding=ContentEncoding.DEFLATE, body=body)
+
+
+def load_logger() -> FirewallLogger:
+    """
+    Export `DDLogger` for discovery by the firewall.
+
+    Returns:
+        A `DDLogger` for use in a run of the supply chain firewall.
+    """
+    return DDLogger()