Fix locale selection in multi-threaded Flask app

This fixes issue #117 by setting locale in `force_locale` instead of
replacing locale getter. Babel extension instance is shared across
threads which causes issues when `force_locale` is used in one request
while another starts. `forced_babel_locale` attribute of context is used
to override `babel_locale` in case `refresh` is called within
`force_locale` context.

Author: lautat

flask_babel/__init__.py

@@ -305,6 +305,9 @@ def refresh():
         if hasattr(ctx, key):
             delattr(ctx, key)
 
+    if hasattr(ctx, 'forced_babel_locale'):
+        ctx.babel_locale = ctx.forced_babel_locale
+
 
 @contextmanager
 def force_locale(locale):
@@ -325,20 +328,19 @@ def force_locale(locale):
         yield
         return
 
-    babel = current_app.extensions['babel']
-
-    orig_locale_selector_func = babel.locale_selector_func
     orig_attrs = {}
     for key in ('babel_translations', 'babel_locale'):
         orig_attrs[key] = getattr(ctx, key, None)
 
     try:
-        babel.locale_selector_func = lambda: locale
-        for key in orig_attrs:
-            setattr(ctx, key, None)
+        ctx.babel_locale = Locale.parse(locale)
+        ctx.forced_babel_locale = ctx.babel_locale
+        ctx.babel_translations = None
         yield
     finally:
-        babel.locale_selector_func = orig_locale_selector_func
+        if hasattr(ctx, 'forced_babel_locale'):
+            del ctx.forced_babel_locale
+
         for key, value in orig_attrs.items():
             setattr(ctx, key, value)
 

tests/tests.py

@@ -6,6 +6,7 @@
 sys.path.insert(0, os.path.join(os.path.dirname(__file__), '..'))
 
 import pickle
+from threading import Thread, Semaphore
 
 import unittest
 from decimal import Decimal
@@ -204,6 +205,46 @@ def select_locale():
                 assert str(babel.get_locale()) == 'en_US'
             assert str(babel.get_locale()) == 'de_DE'
 
+    def test_force_locale_with_threading(self):
+        app = flask.Flask(__name__)
+        b = babel.Babel(app)
+
+        @b.localeselector
+        def select_locale():
+            return 'de_DE'
+
+        semaphore = Semaphore(value=0)
+
+        def first_request():
+            with app.test_request_context():
+                with babel.force_locale('en_US'):
+                    assert str(babel.get_locale()) == 'en_US'
+                    semaphore.acquire()
+
+        thread = Thread(target=first_request)
+        thread.start()
+
+        try:
+            with app.test_request_context():
+                assert str(babel.get_locale()) == 'de_DE'
+        finally:
+            semaphore.release()
+            thread.join()
+
+    def test_refresh_during_force_locale(self):
+        app = flask.Flask(__name__)
+        b = babel.Babel(app)
+
+        @b.localeselector
+        def select_locale():
+            return 'de_DE'
+
+        with app.test_request_context():
+            with babel.force_locale('en_US'):
+                assert str(babel.get_locale()) == 'en_US'
+                babel.refresh()
+                assert str(babel.get_locale()) == 'en_US'
+
 
 class NumberFormattingTestCase(unittest.TestCase):