Kill off deprecated "config-on-the-fly" docker mode (#6918)

Lots of people seem to get confused by this mode, and it's been deprecated
since Synapse 1.1.0. It's time for it to go.

changelog.d/6918.docker

@@ -0,0 +1 @@
+The deprecated "generate-config-on-the-fly" mode is no longer supported.

docker/README.md

@@ -110,12 +110,12 @@ argument to `docker run`.
 
 ## Legacy dynamic configuration file support
 
-For backwards-compatibility only, the docker image supports creating a dynamic
-configuration file based on environment variables. This is now deprecated, but
-is enabled when the `SYNAPSE_SERVER_NAME` variable is set (and `generate` is
-not given).
+The docker image used to support creating a dynamic configuration file based
+on environment variables. This is no longer supported, and an error will be
+raised if you try to run synapse without a config file.
 
-To migrate from a dynamic configuration file to a static one, run the docker
+It is, however, possible to generate a static configuration file based on
+the environment variables that were previously used. To do this, run the docker
 container once with the environment variables set, and `migrate_config`
 command line option. For example:
 
@@ -127,9 +127,14 @@ docker run -it --rm \
     matrixdotorg/synapse:latest migrate_config
 ```
 
-This will generate the same configuration file as the legacy mode used, but
-will store it in `/data/homeserver.yaml` instead of a temporary location. You
-can then use it as shown above at [Running synapse](#running-synapse).
+This will generate the same configuration file as the legacy mode used, and
+will store it in `/data/homeserver.yaml`. You can then use it as shown above at
+[Running synapse](#running-synapse).
+
+Note that the defaults used in this configuration file may be different to
+those when generating a new config file with `generate`: for example, TLS is
+enabled by default in this mode. You are encouraged to inspect the generated
+configuration file and edit it to ensure it meets your needs.
 
 ## Building the image
 

docker/start.py

@@ -188,11 +188,6 @@ def main(args, environ):
     else:
         ownership = "{}:{}".format(desired_uid, desired_gid)
 
-    log(
-        "Container running as UserID %s:%s, ENV (or defaults) requests %s:%s"
-        % (os.getuid(), os.getgid(), desired_uid, desired_gid)
-    )
-
     if ownership is None:
         log("Will not perform chmod/su-exec as UserID already matches request")
 
@@ -213,30 +208,22 @@ def main(args, environ):
     if mode is not None:
         error("Unknown execution mode '%s'" % (mode,))
 
-    if "SYNAPSE_SERVER_NAME" in environ:
-        # backwards-compatibility generate-a-config-on-the-fly mode
-        if "SYNAPSE_CONFIG_PATH" in environ:
-            error(
-                "SYNAPSE_SERVER_NAME can only be combined with SYNAPSE_CONFIG_PATH "
-                "in `generate` or `migrate_config` mode. To start synapse using a "
-                "config file, unset the SYNAPSE_SERVER_NAME environment variable."
-            )
+    config_dir = environ.get("SYNAPSE_CONFIG_DIR", "/data")
+    config_path = environ.get("SYNAPSE_CONFIG_PATH", config_dir + "/homeserver.yaml")
 
-        config_path = "/compiled/homeserver.yaml"
-        log(
-            "Generating config file '%s' on-the-fly from environment variables.\n"
-            "Note that this mode is deprecated. You can migrate to a static config\n"
-            "file by running with 'migrate_config'. See the README for more details."
+    if not os.path.exists(config_path):
+        if "SYNAPSE_SERVER_NAME" in environ:
+            error(
+                """\
+Config file '%s' does not exist.
+
+The synapse docker image no longer supports generating a config file on-the-fly
+based on environment variables. You can migrate to a static config file by
+running with 'migrate_config'. See the README for more details.
+"""
                 % (config_path,)
             )
 
-        generate_config_from_template("/compiled", config_path, environ, ownership)
-    else:
-        config_dir = environ.get("SYNAPSE_CONFIG_DIR", "/data")
-        config_path = environ.get(
-            "SYNAPSE_CONFIG_PATH", config_dir + "/homeserver.yaml"
-        )
-        if not os.path.exists(config_path):
         error(
             "Config file '%s' does not exist. You should either create a new "
             "config file by running with the `generate` argument (and then edit "