Dockerfile-workers: give the master its own log config (#12466)

When we run a worker-mode synapse under docker, everything gets logged to stdout. Currently, output from the workers is tacked with a worker name, for example:

```
2022-04-13 15:27:56,810 - worker:frontend_proxy1 - synapse.util.caches.lrucache - 154 - INFO - LruCache._expire_old_entries-0 - Dropped 0 items from caches
```

- note `worker:frontend_proxy1`. No such tag is applied to log lines from the master, which makes somewhat confusing reading.

To fix this, we generate a dedicated log config file for the master in the same way that we do for the workers, and use that.

changelog.d/12466.misc

@@ -0,0 +1 @@
+Dockerfile-workers: give the master its own log config.

docker/configure_workers_and_start.py

@@ -29,7 +29,7 @@
 import os
 import subprocess
 import sys
-from typing import Any, Dict, Set
+from typing import Any, Dict, Mapping, Set
 
 import jinja2
 import yaml
@@ -341,7 +341,7 @@ def generate_worker_files(environ, config_path: str, data_dir: str):
     # base shared worker jinja2 template.
     #
     # This config file will be passed to all workers, included Synapse's main process.
-    shared_config = {"listeners": listeners}
+    shared_config: Dict[str, Any] = {"listeners": listeners}
 
     # The supervisord config. The contents of which will be inserted into the
     # base supervisord jinja2 template.
@@ -446,21 +446,7 @@ def generate_worker_files(environ, config_path: str, data_dir: str):
 
         # Write out the worker's logging config file
 
-        # Check whether we should write worker logs to disk, in addition to the console
+        log_config_filepath = generate_worker_log_config(environ, worker_name, data_dir)
-        extra_log_template_args = {}
-        if environ.get("SYNAPSE_WORKERS_WRITE_LOGS_TO_DISK"):
-            extra_log_template_args["LOG_FILE_PATH"] = "{dir}/logs/{name}.log".format(
-                dir=data_dir, name=worker_name
-            )
-
-        # Render and write the file
-        log_config_filepath = "/conf/workers/{name}.log.config".format(name=worker_name)
-        convert(
-            "/conf/log.config",
-            log_config_filepath,
-            worker_name=worker_name,
-            **extra_log_template_args,
-        )
 
         # Then a worker config file
         convert(
@@ -496,6 +482,10 @@ def generate_worker_files(environ, config_path: str, data_dir: str):
 
     # Finally, we'll write out the config files.
 
+    # log config for the master process
+    master_log_config = generate_worker_log_config(environ, "master", data_dir)
+    shared_config["log_config"] = master_log_config
+
     # Shared homeserver config
     convert(
         "/conf/shared.yaml.j2",
@@ -532,6 +522,30 @@ def generate_worker_files(environ, config_path: str, data_dir: str):
         os.mkdir(log_dir)
 
 
+def generate_worker_log_config(
+    environ: Mapping[str, str], worker_name: str, data_dir: str
+) -> str:
+    """Generate a log.config file for the given worker.
+
+    Returns: the path to the generated file
+    """
+    # Check whether we should write worker logs to disk, in addition to the console
+    extra_log_template_args = {}
+    if environ.get("SYNAPSE_WORKERS_WRITE_LOGS_TO_DISK"):
+        extra_log_template_args["LOG_FILE_PATH"] = "{dir}/logs/{name}.log".format(
+            dir=data_dir, name=worker_name
+        )
+    # Render and write the file
+    log_config_filepath = "/conf/workers/{name}.log.config".format(name=worker_name)
+    convert(
+        "/conf/log.config",
+        log_config_filepath,
+        worker_name=worker_name,
+        **extra_log_template_args,
+    )
+    return log_config_filepath
+
+
 def start_supervisord():
     """Starts up supervisord which then starts and monitors all other necessary processes