Preserve logs that LoggingPanel would previously overwrite

LoggingPanel upon its import, changes the Python interpreter's root
logger to log into a handler that collects its records for the
LoggingPanel's display.

However for users that don't explicitly configure the root logger this
looks like the Debug Toolbar suppresses their logs that previously
went to standard error.

With this commit, the logs will still keep going to standard error
even if the LoggingPanel is installed, but only if the root logger has
not been explicitly configured with a handler yet.

This behavior will make it so that installing DDT won't break logging
neither for users that already have logging customizations nor for
users that have an out-of-the-box django default logging setup.

Also: change LOGGING config in tests/settings.py because now DDT's own
test suite would show more logs without an explicit config.

Author: riwatt

debug_toolbar/panels/logging.py

@@ -45,6 +45,24 @@ def emit(self, record):
         self.collector.collect(record)
 
 
+# Preserve Python's fallback log mechanism before adding ThreadTrackingHandler.
+
+# If the root logger has no handlers attached then everything that reaches it goes
+# to the "handler of last resort".
+# So a Django app that doesn't explicitly configure the root logger actually logs
+# through logging.lastResort.
+# However, logging.lastResort is not used after ThreadTrackingHandler gets added to
+# the root logger below. This means that users who have LoggingPanel enabled might
+# find their logs are gone from their app as soon as they install DDT.
+# Explicitly adding logging.lastResort to logging.root's handler sidesteps this
+# potential confusion.
+# Note that if root has already been configured, or logging.lastResort has been
+# removed, then the configuration is unchanged, so users who configured their
+# logging aren't exposed to the opposite confusion of seeing extra log lines from
+# their app.
+if not logging.root.hasHandlers() and logging.lastResort is not None:
+    logging.root.addHandler(logging.lastResort)
+
 # We don't use enable/disable_instrumentation because logging is global.
 # We can't add thread-local logging handlers. Hopefully logging is cheap.