FastHttpUser: Bump default max concurrency to 10, inspired by discuss…

…ion in #1810 Add a test (which also serves as an example for how to use it)

locust/contrib/fasthttp.py

@@ -313,9 +313,10 @@ class by using the :py:func:`@task decorator <locust.task>` on the methods, or b
     insecure: bool = True
     """Parameter passed to FastHttpSession. Default True, meaning no SSL verification."""
 
-    concurrency: int = 1
-    """Parameter passed to FastHttpSession. Describes number of concurrent requests allowed by the FastHttpSession. Default 1.
-    Note that setting this value has no effect when custom client_pool was given."""
+    concurrency: int = 10
+    """Parameter passed to FastHttpSession. Describes number of concurrent requests allowed by the FastHttpSession. Default 10.
+    Note that setting this value has no effect when custom client_pool was given, and you need to spawn a your own gevent pool
+    to use it (as Users only have one greenlet). See test_fasthttp.py / test_client_pool_concurrency for an example."""
 
     client_pool: Optional[HTTPClientPool] = None
     """HTTP client pool to use. If not given, a new pool is created per single user."""

locust/test/test_fasthttp.py

@@ -554,6 +554,29 @@ class MyUser(FastHttpUser):
         self.assertEqual(2, self.connections_count)
         self.assertEqual(4, self.requests_count)
 
+    def test_client_pool_concurrency(self):
+        class MyUser(FastHttpUser):
+            host = "http://127.0.0.1:%i" % self.port
+
+            @task
+            def t(self):
+                def concurrent_request(url):
+                    response = self.client.get(url)
+                    assert response.status_code == 200
+
+                pool = gevent.pool.Pool(20)
+                urls = ["/slow?delay=0.2"] * 20  # these urls are all the same, but they could be different
+                for url in urls:
+                    pool.spawn(concurrent_request, url)
+                pool.join()
+
+        user = MyUser(self.environment)
+        before_requests = time.time()
+        user.t()
+        after_requests = time.time()
+        expected_delta = 0.4  # 20 requests with concurrency 10 and response time 0.2
+        self.assertAlmostEqual(before_requests + expected_delta, after_requests, delta=0.1)
+
 
 class TestFastHttpCatchResponse(WebserverTestCase):
     def setUp(self):