log: Introduce _iter_loggers

Ic58dafceefde1b109721a58631c223522bf4cc9c introduces this function
for Victor's rate_limit module. I also want it for my global_filter
and mutable logging config patches so it seems sensible to have it
centrally and reuse it.

Change-Id: I9571badcfc437fb7d565d8dc9632792862e20b3f
Co-Authored-By: Victor Stinner <vstinner@redhat.com>

oslo_log/log.py

@@ -79,6 +79,22 @@ def _get_log_file_path(conf, binary=None):
     return None
 
 
+def _iter_loggers():
+    """Iterate on existing loggers."""
+
+    # Sadly, Logger.manager and Manager.loggerDict are not documented,
+    # but there is no logging public function to iterate on all loggers.
+
+    # The root logger is not part of loggerDict.
+    yield logging.getLogger()
+
+    manager = logging.Logger.manager
+    for logger in manager.loggerDict.values():
+        if isinstance(logger, logging.PlaceHolder):
+            continue
+        yield logger
+
+
 class BaseLoggerAdapter(logging.LoggerAdapter):
 
     warn = logging.LoggerAdapter.warning

oslo_log/tests/unit/test_log.py

@@ -215,6 +215,12 @@ class LogHandlerTestCase(BaseTestCase):
                          binary=prefix),
                          expected)
 
+    def test_iter_loggers(self):
+        mylog = logging.getLogger("abc.cde")
+        loggers = list(log._iter_loggers())
+        self.assertIn(logging.getLogger(), loggers)
+        self.assertIn(mylog, loggers)
+
 
 class SysLogHandlersTestCase(BaseTestCase):
     """Test the standard Syslog handler."""