Speedup tests by caching HomeServerConfig instances

These two lines:

```
config_obj = HomeServerConfig()
config_obj.parse_config_dict(config, "", "")
```

are called many times with the exact same value for `config`.

As the test suite is CPU-bound and non-negligeably time is spent in
`parse_config_dict`, this saves ~5% on the overall runtime of the Trial
test suite (tested with both `-j2` and `-j12` on a 12t CPU).

This is sadly rather limited, as the cache cannot be shared between
processes (it contains at least jinja2.Template and RLock objects which
aren't pickleable), and Trial tends to run close tests in different
processes.

changelog.d/15283.misc

@@ -0,0 +1 @@
+Speedup tests by caching HomeServerConfig instances

tests/unittest.py

@@ -16,6 +16,7 @@
 import gc
 import hashlib
 import hmac
+import json
 import logging
 import secrets
 import time
@@ -28,12 +29,14 @@
     Generic,
     Iterable,
     List,
+    Literal,
     NoReturn,
     Optional,
     Tuple,
     Type,
     TypeVar,
     Union,
+    overload,
 )
 from unittest.mock import Mock, patch
 
@@ -53,6 +56,7 @@
 from synapse import events
 from synapse.api.constants import EventTypes
 from synapse.api.room_versions import KNOWN_ROOM_VERSIONS, RoomVersion
+from synapse.config._base import Config, RootConfig
 from synapse.config.homeserver import HomeServerConfig
 from synapse.config.server import DEFAULT_ROOM_VERSION
 from synapse.crypto.event_signing import add_hashes_and_signatures
@@ -124,6 +128,68 @@ def new(*args: P.args, **kwargs: P.kwargs) -> R:
     return _around
 
 
+@overload
+def deepcopy_config(config: RootConfig, root: Literal[True]) -> RootConfig:
+    ...
+
+
+@overload
+def deepcopy_config(config: Config, root: Literal[False]) -> Config:
+    ...
+
+
+def deepcopy_config(config, root):
+    if root:
+        new_config = config.__class__(config.config_files)
+    else:
+        new_config = config.__class__(config.root)
+
+    for attr_name in config.__dict__:
+        if attr_name.startswith("__") or attr_name == "root":
+            continue
+        attr = getattr(config, attr_name)
+        if isinstance(attr, Config):
+            new_attr = deepcopy_config(attr, root=False)
+        else:
+            new_attr = attr
+
+        setattr(new_config, attr_name, new_attr)
+
+    return new_config
+
+
+_make_homeserver_config_obj_cache: Dict[str, Union[RootConfig, Config]] = {}
+
+
+def make_homeserver_config_obj(config: Dict[str, Any]) -> RootConfig:
+    """Creates a :class:`HomeServerConfig` instance with the given configuration dict.
+
+    This is equivalent to::
+
+        config_obj = HomeServerConfig()
+        config_obj.parse_config_dict(config, "", "")
+
+    but it keeps a cache of `HomeServerConfig` instances and deepcopies them as needed,
+    to avoid validating the whole configuration every time.
+    """
+    cache_key = json.dumps(config)
+
+    if cache_key in _make_homeserver_config_obj_cache:
+        # Cache hit: reuse the existing instance
+        config_obj = _make_homeserver_config_obj_cache[cache_key]
+    else:
+        # Cache miss; create the actual instance
+        config_obj = HomeServerConfig()
+        config_obj.parse_config_dict(config, "", "")
+
+        # Add to the cache
+        _make_homeserver_config_obj_cache[cache_key] = config_obj
+
+    assert isinstance(config_obj, RootConfig)
+
+    return deepcopy_config(config_obj, root=True)
+
+
 class TestCase(unittest.TestCase):
     """A subclass of twisted.trial's TestCase which looks for 'loglevel'
     attributes on both itself and its individual test methods, to override the
@@ -518,8 +584,7 @@ def setup_test_homeserver(self, *args: Any, **kwargs: Any) -> HomeServer:
             config = kwargs["config"]
 
         # Parse the config from a config dict into a HomeServerConfig
-        config_obj = HomeServerConfig()
-        config_obj.parse_config_dict(config, "", "")
+        config_obj = make_homeserver_config_obj(config)
         kwargs["config"] = config_obj
 
         async def run_bg_updates() -> None: