utils: combine MITOGEN_LOG_LEVEL and MITOGEN_LOG_IO.

Saves lots of readline fiddling.

docs/getting_started.rst

@@ -115,15 +115,15 @@ Logging Environment Variables
     Overrides the :py:mod:`logging` package log level set by any call to
     :py:func:`mitogen.utils.log_to_file`. Defaults to ``INFO``.
 
+    If set to ``IO``, equivalent to ``DEBUG`` but additionally enabled IO
+    logging for any call to :py:func:`mitogen.utils.log_to_file`. IO logging
+    produces verbose records of any IO interaction, which is useful for
+    debugging hangs and deadlocks.
+
 ``MITOGEN_LOG_USEC``
     If present, forces microsecond-level timestamps for any call to
     :py:func:`mitogen.utils.log_to_file`.
 
-``MITOGEN_LOG_IO``
-    If present, forces IO logging for any call to
-    :py:func:`mitogen.utils.log_to_file`. IO logging produces extremely verbose
-    logs of any IO interaction, which is useful when debugging deadlocks.
-
 
 
 Logging Records

mitogen/utils.py

@@ -71,13 +71,14 @@ def log_to_file(path=None, io=False, usec=False, level='INFO'):
         fp = sys.stderr
 
     level = os.environ.get('MITOGEN_LOG_LEVEL', level).upper()
-    level = getattr(logging, level, logging.INFO)
-    log.setLevel(level)
-
-    io = ('MITOGEN_LOG_IO' in os.environ) or io
+    io = level == 'IO'
     if io:
+        level = 'DEBUG'
         logging.getLogger('mitogen.io').setLevel(level)
 
+    level = getattr(logging, level, logging.INFO)
+    log.setLevel(level)
+
     handler = logging.StreamHandler(fp)
     handler.formatter = log_get_formatter(usec=usec)
     log.handlers.insert(0, handler)