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>

3 files changed, 15 insertions, 3 deletions

diff --git a/changelog.d/15630.misc b/changelog.d/15630.misc
new file mode 100644
index 0000000000..a30304bfd6
--- /dev/null
+++ b/changelog.d/15630.misc
@@ -0,0 +1 @@
+Fix two memory leaks in `trial` test runs.
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()
 
diff --git a/tests/unittest.py b/tests/unittest.py
index b6fdf69635..623c5a75a2 100644
--- a/tests/unittest.py
+++ b/tests/unittest.py
@@ -229,13 +229,20 @@ class TestCase(unittest.TestCase):
         #
         # The easiest way to do this would be to do a full GC after each test
         # run, but that is very expensive. Instead, we disable GC (above) for
-        # the duration of the test so that we only need to run a gen-0 GC, which
-        # is a lot quicker.
+        # the duration of the test and only run a gen-0 GC, which is a lot
+        # quicker. This doesn't clean up everything, since the TestCase
+        # instance still holds references to objects created during the test,
+        # such as HomeServers, so we do a full GC every so often.
 
         @around(self)
         def tearDown(orig: Callable[[], R]) -> R:
             ret = orig()
             gc.collect(0)
+            # Run a full GC every 50 gen-0 GCs.
+            gen0_stats = gc.get_stats()[0]
+            gen0_collections = gen0_stats["collections"]
+            if gen0_collections % 50 == 0:
+                gc.collect()
             gc.enable()
             set_current_context(SENTINEL_CONTEXT)