Use single NullHandler for whole library

I was trying to get a feel for what the rest of the python ecosystem
does with its logging, so I looked into the top few libraries on pypi:

urllib3 maintains a logger for not quite every module, but for each
one that does heavy lifting. The logger name is `__name__`, and no
handlers are registered for any module-level loggers, including
NullHandler. Their documentation spells out how to configure logging
for the library.

They explicitly configure a library root-level logger called `urllib3`
to which they attach a `NullHandler`. This addresses the "no handlers
could be found" problem.

Their tests explicitly configure handlers, just like ours do.

scrapy is more hands-on. It provides a configuration module for its
logging and a whole document on how to handle logging with scrapy. It
looks like log.py's whole reason for existence is making sure that a
handler is attached to to the scrapy handler at startup.

I think the extra complexity here is because scrapy also offers a CLI,
so there has to be some way to configure logging without resorting to
writing python, so I felt we didn't need to resort to this added
complexity.

---

Based on all of the libraries I've looked at, I think our current
approach is reasonable. The one change I would make is to explicitly
configure a `grpc` logger and to only attach a `NullHandler` to it
instead of putting the burden on the author of each new module to
configure it there.

With this change, I have

- Configured a logger in each module that cares about logging
- Removed all NullHandlers attached to module-level loggers
- Explicitly configured a `grpc` logger with a `NullHandler` attached

Resolves: #16572
Related: #17064

src/python/grpcio/grpc/__init__.py

@@ -15,12 +15,14 @@
 
 import abc
 import enum
+import logging
 import sys
-
 import six
 
 from grpc._cython import cygrpc as _cygrpc
 
+logging.getLogger(__name__).addHandler(logging.NullHandler())
+
 ############################## Future Interface  ###############################
 
 

src/python/grpcio/grpc/_channel.py

@@ -25,7 +25,6 @@ from grpc._cython import cygrpc
 from grpc.framework.foundation import callable_util
 
 _LOGGER = logging.getLogger(__name__)
-_LOGGER.addHandler(logging.NullHandler())
 
 _USER_AGENT = 'grpc-python/{}'.format(_grpcio_metadata.__version__)
 

src/python/grpcio/grpc/_common.py

@@ -21,7 +21,6 @@ import grpc
 from grpc._cython import cygrpc
 
 _LOGGER = logging.getLogger(__name__)
-_LOGGER.addHandler(logging.NullHandler())
 
 CYGRPC_CONNECTIVITY_STATE_TO_CHANNEL_CONNECTIVITY = {
     cygrpc.ConnectivityState.idle:

src/python/grpcio/grpc/_cython/_cygrpc/grpc_string.pyx.pxi

@@ -15,7 +15,6 @@
 import logging
 
 _LOGGER = logging.getLogger(__name__)
-_LOGGER.addHandler(logging.NullHandler())
 
 # This function will ascii encode unicode string inputs if neccesary.
 # In Python3, unicode strings are the default str type.

src/python/grpcio/grpc/_cython/_cygrpc/server.pyx.pxi

@@ -19,7 +19,6 @@ import time
 import grpc
 
 _LOGGER = logging.getLogger(__name__)
-_LOGGER.addHandler(logging.NullHandler())
 
 cdef class Server:
 

src/python/grpcio/grpc/_plugin_wrapping.py

@@ -21,7 +21,6 @@ from grpc import _common
 from grpc._cython import cygrpc
 
 _LOGGER = logging.getLogger(__name__)
-_LOGGER.addHandler(logging.NullHandler())
 
 
 class _AuthMetadataContext(

src/python/grpcio/grpc/framework/foundation/callable_util.py

@@ -22,7 +22,6 @@ import logging
 import six
 
 _LOGGER = logging.getLogger(__name__)
-_LOGGER.addHandler(logging.NullHandler())
 
 
 class Outcome(six.with_metaclass(abc.ABCMeta)):

src/python/grpcio/grpc/framework/foundation/logging_pool.py

@@ -18,7 +18,6 @@ import logging
 from concurrent import futures
 
 _LOGGER = logging.getLogger(__name__)
-_LOGGER.addHandler(logging.NullHandler())
 
 
 def _wrap(behavior):

src/python/grpcio/grpc/framework/foundation/stream_util.py

@@ -20,7 +20,6 @@ from grpc.framework.foundation import stream
 
 _NO_VALUE = object()
 _LOGGER = logging.getLogger(__name__)
-_LOGGER.addHandler(logging.NullHandler())
 
 
 class TransformingConsumer(stream.Consumer):

src/python/grpcio_tests/tests/unit/_logging_test.py

@@ -68,6 +68,13 @@ class LoggingTest(unittest.TestCase):
         self.assertEqual(1, len(logging.getLogger().handlers))
         self.assertIs(logging.getLogger().handlers[0].stream, intended_stream)
 
+    @isolated_logging
+    def test_grpc_logger(self):
+        self.assertIn("grpc", logging.Logger.manager.loggerDict)
+        root_logger = logging.getLogger("grpc")
+        self.assertEqual(1, len(root_logger.handlers))
+        self.assertIsInstance(root_logger.handlers[0], logging.NullHandler)
+
 
 if __name__ == '__main__':
     unittest.main(verbosity=2)