Log number of events fetched from DB

When we finish processing a request, log the number of events we fetched from
the database to handle it.

[I'm trying to figure out which requests are responsible for large amounts of
event cache churn. It may turn out to be more helpful to add counts to the
prometheus per-request/block metrics, but that is an extension to this code
anyway.]

synapse/http/site.py

@@ -99,16 +99,18 @@ class SynapseRequest(Request):
             db_txn_count = context.db_txn_count
             db_txn_duration_sec = context.db_txn_duration_sec
             db_sched_duration_sec = context.db_sched_duration_sec
+            evt_db_fetch_count = context.evt_db_fetch_count
         except Exception:
             ru_utime, ru_stime = (0, 0)
             db_txn_count, db_txn_duration_sec = (0, 0)
+            evt_db_fetch_count = 0
 
         end_time = time.time()
 
         self.site.access_logger.info(
             "%s - %s - {%s}"
             " Processed request: %.3fsec (%.3fsec, %.3fsec) (%.3fsec/%.3fsec/%d)"
-            " %sB %s \"%s %s %s\" \"%s\"",
+            " %sB %s \"%s %s %s\" \"%s\" [%d dbevts]",
             self.getClientIP(),
             self.site.site_tag,
             self.authenticated_entity,
@@ -124,6 +126,7 @@ class SynapseRequest(Request):
             self.get_redacted_uri(),
             self.clientproto,
             self.get_user_agent(),
+            evt_db_fetch_count,
         )
 
         try:

synapse/storage/events_worker.py

@@ -21,6 +21,7 @@ from synapse.events.utils import prune_event
 
 from synapse.util.logcontext import (
     PreserveLoggingContext, make_deferred_yieldable, run_in_background,
+    LoggingContext,
 )
 from synapse.util.metrics import Measure
 from synapse.api.errors import SynapseError
@@ -145,6 +146,9 @@ class EventsWorkerStore(SQLBaseStore):
         missing_events_ids = [e for e in event_ids if e not in event_entry_map]
 
         if missing_events_ids:
+            log_ctx = LoggingContext.current_context()
+            log_ctx.evt_db_fetch_count += len(missing_events_ids)
+
             missing_events = yield self._enqueue_events(
                 missing_events_ids,
                 check_redacted=check_redacted,

synapse/util/logcontext.py

@@ -60,6 +60,7 @@ class LoggingContext(object):
     __slots__ = [
         "previous_context", "name", "ru_stime", "ru_utime",
         "db_txn_count", "db_txn_duration_sec", "db_sched_duration_sec",
+        "evt_db_fetch_count",
         "usage_start",
         "main_thread", "alive",
         "request", "tag",
@@ -109,6 +110,9 @@ class LoggingContext(object):
         # sec spent waiting for db txns to be scheduled
         self.db_sched_duration_sec = 0
 
+        # number of events this thread has fetched from the db
+        self.evt_db_fetch_count = 0
+
         # If alive has the thread resource usage when the logcontext last
         # became active.
         self.usage_start = None