fix issue #879 and add a test for it

Signed-off-by: Armin Ronacher <armin.ronacher@active-4.com>

flask/app.py

@@ -1492,9 +1492,9 @@ class Flask(_PackageBoundObject):
         with self._before_request_lock:
             if self._got_first_request:
                 return
-            self._got_first_request = True
             for func in self.before_first_request_funcs:
                 func()
+            self._got_first_request = True
 
     def make_default_options_response(self):
         """This method is called to create the default `OPTIONS` response.

flask/testsuite/basic.py

@@ -16,6 +16,7 @@ import pickle
 import unittest
 from datetime import datetime
 from threading import Thread
+from time import sleep
 from flask.testsuite import FlaskTestCase, emits_module_deprecation_warning
 from flask._compat import text_type
 from werkzeug.exceptions import BadRequest, NotFound
@@ -1015,6 +1016,23 @@ class BasicFunctionalityTestCase(FlaskTestCase):
         self.assert_equal(got, [42])
         self.assert_true(app.got_first_request)
 
+    def test_before_first_request_functions_concurrent(self):
+        got = []
+        app = flask.Flask(__name__)
+        @app.before_first_request
+        def foo():
+            sleep(1)
+            got.append(42)
+        c = app.test_client()
+        def get_and_assert():
+            c.get("/")
+            self.assert_equal(got, [42])
+        t = Thread(target=get_and_assert)
+        t.start()
+        get_and_assert()
+        t.join()
+        self.assert_true(app.got_first_request)
+
     def test_routing_redirect_debugging(self):
         app = flask.Flask(__name__)
         app.debug = True