fix: handle StopIteration in discovery (#2913)

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: 3 people

docs/changelog/2493.bugfix.rst

@@ -0,0 +1,3 @@
+When no discovery plugins are found, the application would crash with a StopIteration.
+This change catches the StopIteration and raises a RuntimeError with a more informative message.
+Contributed by :user:`esafak`.

src/virtualenv/run/plugin/discovery.py

@@ -16,10 +16,15 @@ def get_discover(parser, args):
     choices = _get_default_discovery(discover_types)
     # prefer the builtin if present, otherwise fallback to first defined type
     choices = sorted(choices, key=lambda a: 0 if a == "builtin" else 1)
+    try:
+        default_discovery = next(iter(choices))
+    except StopIteration as e:
+        msg = "No discovery plugin found. Try reinstalling virtualenv to fix this issue."
+        raise RuntimeError(msg) from e
     discovery_parser.add_argument(
         "--discovery",
         choices=choices,
-        default=next(iter(choices)),
+        default=default_discovery,
         required=False,
         help="interpreter discovery method",
     )

tests/unit/config/test___main__.py

@@ -52,6 +52,16 @@ def test_fail_no_traceback(raise_on_session_done, tmp_path, capsys):
     assert err == "err\n"
 
 
+def test_discovery_fails_no_discovery_plugin(mocker, tmp_path, capsys):
+    mocker.patch("virtualenv.run.plugin.discovery.Discovery.entry_points_for", return_value={})
+    with pytest.raises(SystemExit) as context:
+        run_with_catch([str(tmp_path)])
+    assert context.value.code == 1
+    out, err = capsys.readouterr()
+    assert "RuntimeError: No discovery plugin found. Try reinstalling virtualenv to fix this issue." in out
+    assert not err
+
+
 def test_fail_with_traceback(raise_on_session_done, tmp_path, capsys):
     raise_on_session_done(TypeError("something bad"))