Raise an error if someone tries to use the log_file config option (#6626)

This has caused some confusion for people who didn't notice it going away.

changelog.d/6626.feature

@@ -0,0 +1 @@
+Raise an error if someone tries to use the log_file config option.

synapse/app/homeserver.py

@@ -310,7 +310,7 @@ def setup(config_options):
             "Synapse Homeserver", config_options
         )
     except ConfigError as e:
-        sys.stderr.write("\n" + str(e) + "\n")
+        sys.stderr.write("\nERROR: %s\n" % (e,))
         sys.exit(1)
 
     if not config:

synapse/config/logger.py

@@ -12,7 +12,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
-
+import argparse
 import logging
 import logging.config
 import os
@@ -37,7 +37,7 @@ from synapse.logging._structured import (
 from synapse.logging.context import LoggingContextFilter
 from synapse.util.versionstring import get_version_string
 
-from ._base import Config
+from ._base import Config, ConfigError
 
 DEFAULT_LOG_CONFIG = Template(
     """
@@ -81,11 +81,18 @@ disable_existing_loggers: false
 """
 )
 
+LOG_FILE_ERROR = """\
+Support for the log_file configuration option and --log-file command-line option was
+removed in Synapse 1.3.0. You should instead set up a separate log configuration file.
+"""
+
 
 class LoggingConfig(Config):
     section = "logging"
 
     def read_config(self, config, **kwargs):
+        if config.get("log_file"):
+            raise ConfigError(LOG_FILE_ERROR)
         self.log_config = self.abspath(config.get("log_config"))
         self.no_redirect_stdio = config.get("no_redirect_stdio", False)
 
@@ -106,6 +113,8 @@ class LoggingConfig(Config):
     def read_arguments(self, args):
         if args.no_redirect_stdio is not None:
             self.no_redirect_stdio = args.no_redirect_stdio
+        if args.log_file is not None:
+            raise ConfigError(LOG_FILE_ERROR)
 
     @staticmethod
     def add_arguments(parser):
@@ -118,6 +127,10 @@ class LoggingConfig(Config):
             help="Do not redirect stdout/stderr to the log",
         )
 
+        logging_group.add_argument(
+            "-f", "--log-file", dest="log_file", help=argparse.SUPPRESS,
+        )
+
     def generate_files(self, config, config_dir_path):
         log_config = config.get("log_config")
         if log_config and not os.path.exists(log_config):