Skip the final GC on shutdown to improve restart times (#10712)

Use `gc.freeze()` on exit to exclude all existing objects from the final GC.
In testing, this sped up shutdown by up to a few seconds.

`gc.freeze()` runs in constant time, so there is little chance of performance
regression.

Signed-off-by: Sean Quah <seanq@element.io>

changelog.d/10712.feature

@@ -0,0 +1 @@
+Skip final GC at shutdown to improve restart performance.

synapse/app/_base.py

@@ -12,6 +12,7 @@
 # WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 # See the License for the specific language governing permissions and
 # limitations under the License.
+import atexit
 import gc
 import logging
 import os
@@ -403,6 +404,12 @@ def run_sighup(*args, **kwargs):
         gc.collect()
         gc.freeze()
 
+    # Speed up shutdowns by freezing all allocated objects. This moves everything
+    # into the permanent generation and excludes them from the final GC.
+    # Unfortunately only works on Python 3.7
+    if platform.python_implementation() == "CPython" and sys.version_info >= (3, 7):
+        atexit.register(gc.freeze)
+
 
 def setup_sentry(hs):
     """Enable sentry integration, if enabled in configuration