cli.py: Allow filtering log record on their domains

This patch allows filtering the log records that get printed to stdout
and stderr based on their domain name using the BGD_MESSAGE_DEBUG
environment variable. A comma separated list of domains is expected:

  BGD_MESSAGE_DEBUG="buildgrid.server.cas,buildgrid.client.cas"

buildgrid/_app/cli.py

@@ -139,6 +139,31 @@ class BuildGridCLI(click.MultiCommand):
         return mod.cli
 
 
+class DebugFilter(logging.Filter):
+
+    def __init__(self, debug_domains, name=''):
+        super().__init__(name=name)
+        self.__domains_tree = {}
+
+        for domain in debug_domains.split(','):
+            domains_tree = self.__domains_tree
+            for label in domain.split('.'):
+                if label not in domains_tree:
+                    domains_tree[label] = {}
+                domains_tree = domains_tree[label]
+
+    def filter(self, record):
+        domains_tree = self.__domains_tree
+        for label in record.name.split('.'):
+            if '*' in domains_tree:
+                return True
+            if label not in domains_tree:
+                return False
+            domains_tree = domains_tree[label]
+
+        return True
+
+
 @click.command(cls=BuildGridCLI, context_settings=CONTEXT_SETTINGS)
 @click.option('--no-print', is_flag=True, show_default=True,
               help="Do not print log records to stdout/stderr.")
@@ -157,10 +182,19 @@ def cli(context, no_print, verbose):
 
     # Do not register a stream handler if no-print is requested:
     if not no_print:
-        logging.basicConfig(format=LOG_RECORD_FORMAT)
+        log_handler = logging.StreamHandler()
+
+        # Filter debug messages using BGD_MESSAGE_DEBUG value:
+        debug_domains = os.environ.get('BGD_MESSAGE_DEBUG', None)
+        if debug_domains:
+            log_handler.addFilter(DebugFilter(debug_domains))
+
     else:
-        logging.basicConfig(format=LOG_RECORD_FORMAT,
-                            handlers=[logging.NullHandler()])
+        log_handler = logging.NullHandler()
+
+    logging.basicConfig(format=LOG_RECORD_FORMAT,
+                        handlers=[log_handler])
+
     if verbose == 1:
         logger.setLevel(logging.WARNING)
     elif verbose == 2: