Fix `HomeServer`s leaking during `trial` test runs (#15630)

This change fixes two memory leaks during `trial` test runs.

Garbage collection is disabled during each test case and a gen-0 GC is
run at the end of each test. However, when the gen-0 GC is run, the
`TestCase` object usually still holds references to the `HomeServer`
used during the test. As a result, the `HomeServer` gets promoted to
gen-1 and then never garbage collected.

Fix this by periodically running full GCs.

Additionally, fix `HomeServer`s leaking after tests that touch inbound
federation due to `FederationRateLimiter`s adding themselves to a global
set, by turning the set into a `WeakSet`.

Resolves #15622.

Signed-off-by: Sean Quah <seanq@matrix.org>

1 files changed, 5 insertions, 1 deletions

diff --git a/synapse/util/ratelimitutils.py b/synapse/util/ratelimitutils.py
index f262bf95a0..2ad55ac13e 100644
--- a/synapse/util/ratelimitutils.py
+++ b/synapse/util/ratelimitutils.py
@@ -25,10 +25,12 @@ from typing import (
     Iterator,
     List,
     Mapping,
+    MutableSet,
     Optional,
     Set,
     Tuple,
 )
+from weakref import WeakSet
 
 from prometheus_client.core import Counter
 from typing_extensions import ContextManager
@@ -86,7 +88,9 @@ queue_wait_timer = Histogram(
 )
 
 
-_rate_limiter_instances: Set["FederationRateLimiter"] = set()
+# This must be a `WeakSet`, otherwise we indirectly hold on to entire `HomeServer`s
+# during trial test runs and leak a lot of memory.
+_rate_limiter_instances: MutableSet["FederationRateLimiter"] = WeakSet()
 # Protects the _rate_limiter_instances set from concurrent access
 _rate_limiter_instances_lock = threading.Lock()