refactor(get_locale): resolve locale from single list of preferences

As of 9c11950, get_locale() handled user/agent-provided locale
preferences and deferred to resolve_fallback_locale() if none
could be satisfied.  Here we build a single list of locales,
beginning with user/agent preferences and ending with the server-side
{DEFAULT_FALLBACK}_LOCALEs, and call babel.core.negotiate_locale()
once to negotiate one (if possible).

securedrop/i18n.py

@@ -202,28 +202,6 @@ def configure(config: SDConfig, app: Flask) -> None:
     map_locale_display_names(config)
 
 
-def resolve_fallback_locale(config: SDConfig) -> str:
-    """
-    Return a *usable* fallback locale.  Namely:
-
-    1. Don't fall back to the configured `DEFAULT_LOCALE` if it isn't available.
-
-    2. Don't fall back to the hard-coded `FALLBACK_LOCALE` (`en_US`) if it isn't configured.
-
-    NB. If neither the default nor the fallback locale is usable, then we should have crashed
-    already in `validate_locale_configuration()`.
-    """
-
-    if Locale.parse(config.DEFAULT_LOCALE) in USABLE_LOCALES:
-        return config.DEFAULT_LOCALE
-
-    elif Locale.parse(FALLBACK_LOCALE) in USABLE_LOCALES:
-        return FALLBACK_LOCALE
-
-    else:
-        raise ValueError('No usable fallback locale')
-
-
 def get_locale(config: SDConfig) -> str:
     """
     Return the best supported locale for a request.
@@ -232,25 +210,24 @@ def get_locale(config: SDConfig) -> str:
     - l request argument or session['locale']
     - browser suggested locale, from the Accept-Languages header
     - config.DEFAULT_LOCALE
+    - config.FALLBACK_LOCALE
     """
-    # Default to any usable locale set in the session.
-    locale = session.get("locale")
-    if locale:
-        locale = negotiate_locale([locale], LOCALES.keys())
-
-    # A usable locale specified in request.args takes precedence.
+    preferences = []
+    if session.get("locale"):
+        preferences.append(session.get("locale"))
     if request.args.get("l"):
-        negotiated = negotiate_locale([request.args["l"]], LOCALES.keys())
-        if negotiated:
-            locale = negotiated
+        preferences.insert(0, request.args.get("l"))
+    if not preferences:
+        preferences.extend(get_accepted_languages())
+    preferences.append(config.DEFAULT_LOCALE)
+    preferences.append(FALLBACK_LOCALE)
+
+    negotiated = negotiate_locale(preferences, LOCALES.keys())
 
-    # If the locale is not in the session or request.args, negotiate
-    # the best supported option from the browser's accepted languages.
-    if not locale:
-        locale = negotiate_locale(get_accepted_languages(), LOCALES.keys())
+    if not negotiated:
+        raise ValueError("No usable locale")
 
-    # Finally, if we can't negotiate a requested locale, resolve a fallback.
-    return locale or resolve_fallback_locale(config)
+    return negotiated
 
 
 def get_accepted_languages() -> List[str]:

securedrop/tests/test_i18n.py

@@ -32,7 +32,7 @@
 from flask import request
 from flask import session
 from flask_babel import gettext
-from i18n import parse_locale_set, resolve_fallback_locale
+from i18n import parse_locale_set
 from sdconfig import FALLBACK_LOCALE, SDConfig
 from sh import pybabel
 from sh import sed
@@ -233,32 +233,6 @@ def test_parse_locale_set():
     assert parse_locale_set([FALLBACK_LOCALE]) == set([Locale.parse(FALLBACK_LOCALE)])
 
 
-def test_resolve_fallback_locale(config):
-    """
-    Only a usable default or fallback locale is returned.
-    """
-    i18n.USABLE_LOCALES = parse_locale_set([FALLBACK_LOCALE, 'es_ES'])
-    fake_config = SDConfig()
-
-    # The default locale is neither configured nor available.
-    fake_config.DEFAULT_LOCALE = NEVER_LOCALE
-    assert resolve_fallback_locale(fake_config) == FALLBACK_LOCALE
-
-    # The default locale is configured but not available.
-    fake_config.SUPPORTED_LOCALES = [FALLBACK_LOCALE, NEVER_LOCALE]
-    assert resolve_fallback_locale(fake_config) == FALLBACK_LOCALE
-
-    # The default locale is available but not configured.
-    fake_config.SUPPORTED_LOCALES = [FALLBACK_LOCALE]
-    fake_config.DEFAULT_LOCALE = NEVER_LOCALE
-    assert resolve_fallback_locale(fake_config) == FALLBACK_LOCALE
-
-    # Happy path: a non-fallback default locale is both available and configured.
-    fake_config.SUPPORTED_LOCALES = [FALLBACK_LOCALE, 'es_ES']
-    fake_config.DEFAULT_LOCALE = 'es_ES'
-    assert resolve_fallback_locale(fake_config) == 'es_ES'
-
-
 def test_no_usable_fallback_locale(journalist_app, config):
     """
     The apps fail if neither the default nor the fallback locale is usable.
@@ -269,8 +243,6 @@ def test_no_usable_fallback_locale(journalist_app, config):
     fake_config.TRANSLATION_DIRS = Path(config.TEMP_DIR)
 
     i18n.USABLE_LOCALES = set()
-    with pytest.raises(ValueError, match='No usable fallback locale'):
-        resolve_fallback_locale(fake_config)
 
     with pytest.raises(ValueError, match='in the set of usable locales'):
         journalist_app_module.create_app(fake_config)