Log when events are (unexpectedly) filtered out of responses in tests (#14213)

See https://github.com/matrix-org/synapse/pull/14095#discussion_r990335492

This is useful because when see that a relevant event is an `outlier` or `soft-failed`, then that's a good unexpected indicator explaining why it's not showing up. `filter_events_for_client` is used in `/sync`, `/messages`, `/context` which are all common end-to-end assertion touch points (also notifications, relations).

3 files changed, 29 insertions, 7 deletions

diff --git a/docker/README.md b/docker/README.md
index eda3221c23..08372e95c6 100644
--- a/docker/README.md
+++ b/docker/README.md
@@ -73,7 +73,8 @@ The following environment variables are supported in `generate` mode:
   will log sensitive information such as access tokens.
   This should not be needed unless you are a developer attempting to debug something
   particularly tricky.
-
+* `SYNAPSE_LOG_TESTING`: if set, Synapse will log additional information useful
+  for testing.
 
 ## Postgres
 
diff --git a/docker/conf/log.config b/docker/conf/log.config
index 90b5179838..5772321202 100644
--- a/docker/conf/log.config
+++ b/docker/conf/log.config
@@ -49,17 +49,35 @@ handlers:
     class: logging.StreamHandler
     formatter: precise
 
-{% if not SYNAPSE_LOG_SENSITIVE %}
-{#
-  If SYNAPSE_LOG_SENSITIVE is unset, then override synapse.storage.SQL to INFO
-  so that DEBUG entries (containing sensitive information) are not emitted.
-#}
 loggers:
+    # This is just here so we can leave `loggers` in the config regardless of whether
+    # we configure other loggers below (avoid empty yaml dict error).
+    _placeholder:
+        level: "INFO"
+
+    {% if not SYNAPSE_LOG_SENSITIVE %}
+    {#
+      If SYNAPSE_LOG_SENSITIVE is unset, then override synapse.storage.SQL to INFO
+      so that DEBUG entries (containing sensitive information) are not emitted.
+    #}
     synapse.storage.SQL:
         # beware: increasing this to DEBUG will make synapse log sensitive
         # information such as access tokens.
         level: INFO
-{% endif %}
+    {% endif %}
+
+    {% if SYNAPSE_LOG_TESTING %}
+    {#
+      If Synapse is under test, log a few more useful things for a developer
+      attempting to debug something particularly tricky.
+
+      With `synapse.visibility.filtered_event_debug`, it logs when events are (maybe
+      unexpectedly) filtered out of responses in tests. It's just nice to be able to
+      look at the CI log and figure out why an event isn't being returned.
+    #}
+    synapse.visibility.filtered_event_debug:
+        level: DEBUG
+    {% endif %}
 
 root:
     level: {{ SYNAPSE_LOG_LEVEL or "INFO" }}
diff --git a/docker/configure_workers_and_start.py b/docker/configure_workers_and_start.py
index 79b5b87397..87a740e3d4 100755
--- a/docker/configure_workers_and_start.py
+++ b/docker/configure_workers_and_start.py
@@ -40,6 +40,8 @@
 #         log level. INFO is the default.
 #   * SYNAPSE_LOG_SENSITIVE: If unset, SQL and SQL values won't be logged,
 #         regardless of the SYNAPSE_LOG_LEVEL setting.
+#   * SYNAPSE_LOG_TESTING: if set, Synapse will log additional information useful
+#     for testing.
 #
 # NOTE: According to Complement's ENTRYPOINT expectations for a homeserver image (as defined
 # in the project's README), this script may be run multiple times, and functionality should
@@ -947,6 +949,7 @@ def generate_worker_log_config(
     extra_log_template_args["SYNAPSE_LOG_SENSITIVE"] = environ.get(
         "SYNAPSE_LOG_SENSITIVE"
     )
+    extra_log_template_args["SYNAPSE_LOG_TESTING"] = environ.get("SYNAPSE_LOG_TESTING")
 
     # Render and write the file
     log_config_filepath = f"/conf/workers/{worker_name}.log.config"