document WTF import issue

urllib3/_backends/_loader.py

@@ -0,0 +1,54 @@
+import logging
+
+from urllib3.packages import six
+
+
+def load_trio_backend():
+    if not six.PY3:
+        raise ValueError("trio backend requires Python 3")
+    from .trio_backend import TrioBackend
+    return TrioBackend
+
+
+def load_twisted_backend():
+    if not six.PY3:
+        raise ValueError("twisted backend requires Python 3")
+    from .twisted_backend import TwistedBackend
+    return TwistedBackend
+
+
+def load_sync_backend():
+    from .sync_backend import SyncBackend
+    return SyncBackend
+
+
+BACKENDS = {
+    "sync": load_sync_backend,
+    "trio": load_trio_backend,
+    "twisted": load_twisted_backend,
+}
+
+
+def available_backends():
+    """
+    Returns a list of backend names that we are able to successfully import.
+    """
+    output = []
+    for backend_name, load_backend_fn in sorted(BACKENDS.items()):
+        try:
+            load_backend_fn()
+            output.append(backend_name)
+        except:  # noqa
+            logging.exception("{} not available".format(backend_name))
+    return output
+
+
+def load_backend(backend_name, backend_args):
+    _backend_args = backend_args or {}
+    if backend_name is None:
+        return load_sync_backend()(**_backend_args)
+    elif backend_name in BACKENDS:
+        return BACKENDS[backend_name]()(**_backend_args)
+    else:
+        valid_backends = ", ".join(sorted(BACKENDS.keys()))
+        raise ValueError("Backend must be one of: {}".format(valid_backends))

urllib3/_backends/sync_backend.py

@@ -136,8 +136,18 @@ def send_and_receive_for_a_while(
 
                 if not made_progress:
                     self._wait(want_read, want_write, read_timeout)
-        except LoopAbort:
-            pass
+        except Exception as e:
+            # XX Somehow, LoopAbort != the LoopAbort that we have raised!
+            if not isinstance(e, LoopAbort):
+                raise ValueError("""
+                    WTF1: {} != {}
+                    WTF2: {} != {}
+                """.format(
+                    LoopAbort, type(e),
+                    id(LoopAbort), id(type(e)),
+                ))
+            else:
+                pass
 
     def forceful_close(self):
         self._sock.close()