Fix modified logger in add_file_handler, add tests

It appears that the `logger` parameter was previously unused, and that
the function just used `colcon_logger` directly. As it happens, the only
use of `add_file_handler` passes `colcon_logger`, so the mistake had no
effect.

This fixes the bug and adds a simple test for that code.

colcon_core/logging.py

@@ -95,15 +95,15 @@ def filter(self, record):  # noqa: A003
         if isinstance(handler, logging.StreamHandler):
             formatter = handler.formatter
             # filter colcon specific log messages from default stream handler
-            handler.addFilter(Filter(colcon_logger.name))
+            handler.addFilter(Filter(logger.name))
 
     # add a stream handler replacing the one filtered on the root logger
     handler = logging.StreamHandler()
     if formatter:
         # use same formatter as for stream handler
         handler.setFormatter(formatter)
-    handler.setLevel(colcon_logger.getEffectiveLevel())
-    colcon_logger.addHandler(handler)
+    handler.setLevel(logger.getEffectiveLevel())
+    logger.addHandler(handler)
 
     # add a file handler writing all log levels
     handler = logging.FileHandler(str(path))
@@ -121,9 +121,9 @@ def format_message_with_relative_time(record):
         # use same formatter as for stream handler
         handler.setFormatter(formatter)
     handler.setLevel(1)
-    colcon_logger.addHandler(handler)
+    logger.addHandler(handler)
 
     # change the logger to handle all levels
-    colcon_logger.setLevel(1)
+    logger.setLevel(1)
 
     return handler

test/spell_check.words

@@ -130,6 +130,7 @@ testcase
 testsfailed
 testsuite
 thomas
+tmpdir
 todo
 traceback
 tryfirst

test/test_logging.py

@@ -2,8 +2,10 @@
 # Licensed under the Apache License, Version 2.0
 
 import logging
+from pathlib import Path
 from unittest.mock import Mock
 
+from colcon_core.logging import add_file_handler
 from colcon_core.logging import get_numeric_log_level
 from colcon_core.logging import set_logger_level_from_env
 import pytest
@@ -56,3 +58,21 @@ def test_get_numeric_log_level():
     with pytest.raises(ValueError) as e:
         get_numeric_log_level('-1')
     assert str(e.value).endswith('numeric log levels must be positive')
+
+
+def test_add_file_handler(tmpdir):
+    log_path = Path(tmpdir) / 'test_add_file_handler.log'
+    log_path.touch()
+    logger = logging.getLogger('test_add_file_handler')
+    try:
+        logger.setLevel(logging.WARN)
+        add_file_handler(logger, log_path)
+        assert logger.getEffectiveLevel() != logging.WARN
+        logger.info('test_add_file_handler')
+    finally:
+        for handler in logger.handlers:
+            logger.removeHandler(handler)
+            handler.close()
+
+    # check only that we logged SOMETHING to the file
+    assert log_path.stat().st_size > 10