fix wrong logging behavior

1. Handler configuration based on the result of "get_config" in main function,
but if something wrong happens in "get_config", the "logger.handlers" will
be empty, and result in the error 'No handlers could be found for logger "altsrc"'.

Add logging.basicConfig in main function for root logger's default handler creation,
this is used to catch log before "setup_logging" is called.

Update logic in setup_logging: only the root logger has handlers.

2. Add an autouse pytest fixture "reset_loggers" in conftest.py. It saves the
handlers before each test, then puts them back after end of the test. It keeps the
loggers and handlers in every test consistent.

alt_src/alt_src.py

@@ -2120,8 +2120,7 @@ def die(msg):
 
 
 def setup_logging(options):
-    logger = logging.getLogger("altsrc")
-    logger.setLevel(logging.DEBUG)
+    logger = logging.getLogger()
 
     #determine log levels
     output_log_level = logging.WARN #default
@@ -2141,6 +2140,9 @@ def setup_logging(options):
     file_log_level = min(file_log_level, output_log_level)
     options.file_log_level = file_log_level
 
+    # remove the default handler of root logger
+    logger.handlers = []
+
     # set up handlers
     handler = logging.StreamHandler(sys.stdout)
     handler.setFormatter(logging.Formatter(options.config['log_format']))
@@ -2199,6 +2201,7 @@ def main(args):
     options.branch = args[0]
     options.source = args[1]
 
+    logging.basicConfig(level=logging.DEBUG)
     options.config = get_config(options.cfile, options.copts)
 
     logger = setup_logging(options)

tests/conftest.py

@@ -0,0 +1,12 @@
+import logging
+
+import pytest
+
+
+@pytest.fixture(autouse=True)
+def reset_loggers():
+    root_handlers = logging.getLogger().handlers[:]
+    altsrc_handlers = logging.getLogger("altsrc").handlers[:]
+    yield
+    logging.getLogger().handlers = root_handlers
+    logging.getLogger("altsrc").handlers = altsrc_handlers