instrumentation/urllib3: fix urllib3 2.0.1+ crash with many args

In c1dd69e we changed the logic of
update_headers taking into account a new body arg before the one for
headers.
The problem is that HTTPConnectionPool.urlopen did not change at all,
only HTTPConnectionPool.request did so the old login update_headers was
fine.

Fix #1928

elasticapm/instrumentation/packages/urllib3.py

@@ -61,12 +61,7 @@ def update_headers(args, kwargs, instance, transaction, trace_parent):
     :param trace_parent: the TraceParent object
     :return: an (args, kwargs) tuple
     """
-    from urllib3._version import __version__ as urllib3_version
-
-    if urllib3_version.startswith("2") and len(args) >= 5 and args[4]:
-        headers = args[4].copy()
-        args = tuple(itertools.chain((args[:4]), (headers,), args[5:]))
-    elif len(args) >= 4 and args[3]:
+    if len(args) >= 4 and args[3]:
         headers = args[3].copy()
         args = tuple(itertools.chain((args[:3]), (headers,), args[4:]))
     elif "headers" in kwargs and kwargs["headers"]:

tests/instrumentation/urllib3_tests.py

@@ -294,3 +294,20 @@ def test_instance_headers_are_respected(
         assert "kwargs" in request_headers
     if instance_headers and not (header_arg or header_kwarg):
         assert "instance" in request_headers
+
+
+def test_connection_pool_urlopen_does_not_crash_with_many_args(instrument, elasticapm_client, waiting_httpserver):
+    """Mimics ConnectionPool.urlopen error path with broken connection, see #1928"""
+    waiting_httpserver.serve_content("")
+    url = waiting_httpserver.url + "/hello_world"
+    parsed_url = urllib.parse.urlparse(url)
+    pool = urllib3.HTTPConnectionPool(
+        parsed_url.hostname,
+        parsed_url.port,
+        maxsize=1,
+        block=True,
+    )
+    retry = urllib3.util.Retry(10)
+    elasticapm_client.begin_transaction("transaction")
+    r = pool.urlopen("GET", url, None, {"args": "true"}, retry, False, False)
+    assert r.status == 200