Validate Enum arguments before registering the collector

Enum.__init__ called super().__init__() -- which registers the collector
in the CollectorRegistry -- before validating that states is non-empty
and that the metric name does not overlap a label name. When either
check failed, the ValueError was raised as expected, but a half-built
Enum (whose _states was never assigned) had already been registered.

That left the registry in a broken state: the name was permanently
taken, so recreating the metric raised 'Duplicated timeseries', and any
subsequent scrape crashed with

    AttributeError: 'Enum' object has no attribute '_states'

when _child_samples iterated self._states. A realistic trigger is
building the states list from configuration that turns out to be empty.

Gauge and Histogram already validate before calling super().__init__();
this moves Enum's two guards ahead of registration to match, so a failed
constructor leaves the registry untouched.

Add test_failed_init_does_not_pollute_registry, which asserts that after
two failed Enum constructions the name is still free, the metric can be
created, and the registry scrapes cleanly.

Signed-off-by: Sean Kim <skim8705@gmail.com>

Author: sean-kim05

prometheus_client/metrics.py

@@ -769,6 +769,10 @@ def __init__(self,
                  _labelvalues: Optional[Sequence[str]] = None,
                  states: Optional[Sequence[str]] = None,
                  ):
+        if name in labelnames:
+            raise ValueError(f'Overlapping labels for Enum metric: {name}')
+        if not states:
+            raise ValueError(f'No states provided for Enum metric: {name}')
         super().__init__(
             name=name,
             documentation=documentation,
@@ -779,10 +783,6 @@ def __init__(self,
             registry=registry,
             _labelvalues=_labelvalues,
         )
-        if name in labelnames:
-            raise ValueError(f'Overlapping labels for Enum metric: {name}')
-        if not states:
-            raise ValueError(f'No states provided for Enum metric: {name}')
         self._kwargs['states'] = self._states = states
 
     def _metric_init(self) -> None:

tests/test_core.py

@@ -595,6 +595,19 @@ def test_overlapping_labels(self):
         with pytest.raises(ValueError):
             Enum('e', 'help', registry=None, labelnames=['e'])
 
+    def test_failed_init_does_not_pollute_registry(self):
+        registry = CollectorRegistry()
+        # A validation failure in __init__ must not leave a half-built collector
+        # registered: otherwise the name stays permanently taken and any later
+        # scrape of the registry crashes on the missing _states attribute.
+        with pytest.raises(ValueError):
+            Enum('task_state', 'help', states=None, registry=registry)
+        with pytest.raises(ValueError):
+            Enum('task_state', 'help', states=['a'], labelnames=['task_state'], registry=registry)
+        # The name is still free, so a correct definition registers and scrapes.
+        Enum('task_state', 'help', states=['a', 'b'], registry=registry)
+        self.assertEqual(1, registry.get_sample_value('task_state', {'task_state': 'a'}))
+
 
 class TestMetricWrapper(unittest.TestCase):
     def setUp(self):