Backend: Cache localized GetLanguages/GetRegions responses per locale

Since #9173, GetLanguages localized all ~300 language names on every
request via babel.Locale.parse, costing ~38ms of CPU per call. The
responses are static per locale, so cache the built protos.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: aapeliv

app/backend/src/couchers/i18n/context.py

@@ -21,8 +21,6 @@
     localize_list,
     localize_time,
     localize_timezone,
-    try_localize_language_name_from_iso639,
-    try_localize_region_name_from_iso3166,
 )
 from couchers.models.users import User
 from couchers.utils import to_timezone
@@ -67,12 +65,6 @@ def __setattr__(self, name: str, value: Any) -> None:
     def localized_timezone(self) -> str:
         return localize_timezone(self.timezone, self.babel_locale)
 
-    def try_localize_language_name_from_iso639(self, code: str, standalone: bool = False) -> str | None:
-        return try_localize_language_name_from_iso639(code, self.babel_locale, standalone=standalone)
-
-    def try_localize_region_name_from_iso3166(self, code: str) -> str | None:
-        return try_localize_region_name_from_iso3166(code, self.babel_locale)
-
     def localize_string(
         self, key: str, *, i18next: I18Next | None = None, substitutions: SubstitutionDict | None = None
     ) -> str:

app/backend/src/couchers/i18n/localize.py

@@ -4,7 +4,7 @@
 """
 
 import re
-from collections.abc import Sequence
+from collections.abc import Mapping, Sequence
 from datetime import date, datetime, time, tzinfo
 from typing import cast
 
@@ -13,7 +13,7 @@
 from babel.dates import get_datetime_format, get_timezone_name, match_skeleton, parse_pattern
 from babel.lists import format_list
 
-from couchers.resources import get_region_code_iso3166_alpha3_to_alpha2
+from couchers.resources import get_language_dict, get_region_code_iso3166_alpha3_to_alpha2, get_region_dict
 
 
 def localize_list(items: Sequence[str], locale: babel.Locale) -> str:
@@ -57,6 +57,28 @@ def try_localize_region_name_from_iso3166(code: str, locale: babel.Locale) -> st
     return region_name
 
 
+def get_localized_language_names(locale: babel.Locale) -> Mapping[str, str]:
+    """
+    Maps every allowed language code to its standalone display name in the given locale
+    (falling back to English), ordered by code.
+    """
+    return {
+        code: try_localize_language_name_from_iso639(code, locale, standalone=True) or english_name
+        for code, english_name in sorted(get_language_dict().items())
+    }
+
+
+def get_localized_region_names(locale: babel.Locale) -> Mapping[str, str]:
+    """
+    Maps every region alpha3 code to its display name in the given locale
+    (falling back to English), ordered by code.
+    """
+    return {
+        alpha3: try_localize_region_name_from_iso3166(alpha3, locale) or english_name
+        for alpha3, english_name in sorted(get_region_dict().items())
+    }
+
+
 def localize_date(
     value: date, locale: babel.Locale, *, abbrev: bool = False, with_year: bool = True, with_day_of_week: bool = False
 ) -> str:

app/backend/src/couchers/servicers/resources.py

@@ -1,17 +1,14 @@
+import functools
 import logging
 
+import babel
 from google.protobuf import empty_pb2
 from sqlalchemy.orm import Session
 
 from couchers.context import CouchersContext
+from couchers.i18n.localize import get_localized_language_names, get_localized_region_names
 from couchers.proto import resources_pb2, resources_pb2_grpc
-from couchers.resources import (
-    get_badge_dict,
-    get_icon,
-    get_language_dict,
-    get_region_dict,
-    get_terms_of_service,
-)
+from couchers.resources import get_badge_dict, get_icon, get_terms_of_service
 
 logger = logging.getLogger(__name__)
 
@@ -23,6 +20,29 @@
 ]
 
 
+# These responses are static per locale, and localizing ~300 names costs tens of ms of CPU (mostly
+# babel.Locale.parse), so cache the built protos. Sharing one message across requests is safe as
+# long as nothing mutates responses after return (the tracing interceptor deepcopies before
+# sanitizing).
+@functools.lru_cache
+def _get_regions_res(locale: babel.Locale) -> resources_pb2.GetRegionsRes:
+    return resources_pb2.GetRegionsRes(
+        regions=[
+            resources_pb2.Region(alpha3=alpha3, name=name)
+            for alpha3, name in get_localized_region_names(locale).items()
+        ]
+    )
+
+
+@functools.lru_cache
+def _get_languages_res(locale: babel.Locale) -> resources_pb2.GetLanguagesRes:
+    return resources_pb2.GetLanguagesRes(
+        languages=[
+            resources_pb2.Language(code=code, name=name) for code, name in get_localized_language_names(locale).items()
+        ]
+    )
+
+
 class Resources(resources_pb2_grpc.ResourcesServicer):
     def GetTermsOfService(
         self, request: empty_pb2.Empty, context: CouchersContext, session: Session
@@ -46,29 +66,12 @@ def GetCommunityGuidelines(
     def GetRegions(
         self, request: empty_pb2.Empty, context: CouchersContext, session: Session
     ) -> resources_pb2.GetRegionsRes:
-        return resources_pb2.GetRegionsRes(
-            regions=[
-                resources_pb2.Region(
-                    alpha3=alpha3,
-                    name=context.localization.try_localize_region_name_from_iso3166(alpha3) or english_name,
-                )
-                for alpha3, english_name in sorted(get_region_dict().items())
-            ]
-        )
+        return _get_regions_res(context.localization.babel_locale)
 
     def GetLanguages(
         self, request: empty_pb2.Empty, context: CouchersContext, session: Session
     ) -> resources_pb2.GetLanguagesRes:
-        return resources_pb2.GetLanguagesRes(
-            languages=[
-                resources_pb2.Language(
-                    code=code,
-                    name=context.localization.try_localize_language_name_from_iso639(code, standalone=True)
-                    or english_name,
-                )
-                for code, english_name in sorted(get_language_dict().items())
-            ]
-        )
+        return _get_languages_res(context.localization.babel_locale)
 
     def GetBadges(
         self, request: empty_pb2.Empty, context: CouchersContext, session: Session