Clear leaked Python errors in converter collection and label paths

Several Converter failure paths could leave a Python error pending even
when called with setError: false, relying on the caller to clean up:

- ToList did not handle a failed PyObject_GetIter: the raised TypeError
  survived the call (and the iterator reference leaked). Handle it like
  ToArray does, disposing the reference and clearing when not setting
  errors.
- MakeList treated a null PyIter_Next result as normal exhaustion, but
  null also means the iterator raised: the conversion now fails instead
  of returning a silently truncated list, clearing the error when not
  setting errors and leaving it pending for the caller when setting
  errors. The swallowed CLR-exception catch and the element-conversion
  failure path also clear when not setting errors, since some element
  conversion paths (failed implicit operators, deleted types) raise
  regardless of setError to enrich MethodBinder's no-match message.
- The type_error and overflow labels now clear pending errors when not
  setting errors, mirroring what convert_error already did, so a
  probing sub-path that raised before jumping cannot leak.

The unconditional raises in the implicit-conversion catches are kept:
MethodBinder.Invoke deliberately surfaces a pending error as the
binding failure reason instead of its generic no-match message, and
Converter.TryAsManagedObject clears at the API boundary.

Author: jhonabreul

src/embed_tests/TestConverter.cs

@@ -513,6 +513,39 @@ class TestPythonModel(TestCSharpModel):
             Assert.AreEqual(shouldConvert, Converter.ToManaged(testPythonModelClass, type, out var result, setError: false));
             Assert.IsFalse(Exceptions.ErrorOccurred());
         }
+
+        [TestCase(true)]
+        [TestCase(false)]
+        public void RaisingIteratorFailsArrayConversion(bool setError)
+        {
+            using var _ = Py.GIL();
+
+            var module = PyModule.FromString("RaisingIteratorModule", @"
+def gen():
+    yield 1
+    raise ValueError('mid-iteration failure')
+");
+            using var generator = module.GetAttr("gen").Invoke();
+
+            // the conversion must fail rather than return a silently truncated array,
+            // and the raised error must only be left pending when setError is true
+            Assert.IsFalse(Converter.ToManaged(generator, typeof(int[]), out var result, setError));
+            Assert.IsNull(result);
+            Assert.AreEqual(setError, Exceptions.ErrorOccurred());
+            Exceptions.Clear();
+        }
+
+        [TestCase(true)]
+        [TestCase(false)]
+        public void OverflowConversionOnlyLeavesErrorWhenSettingErrors(bool setError)
+        {
+            using var _ = Py.GIL();
+
+            using var pyValue = new PyInt(300);
+            Assert.IsFalse(Converter.ToManaged(pyValue, typeof(byte), out var _, setError));
+            Assert.AreEqual(setError, Exceptions.ErrorOccurred());
+            Exceptions.Clear();
+        }
     }
 
     public interface IGetList

src/runtime/Converter.cs

@@ -507,6 +507,8 @@ internal static bool ToManagedValue(BorrowedReference value, Type obType,
                             catch
                             {
                                 // Failed to convert using implicit conversion,  must catch the error to stop program from exploding on Linux
+                                // Raised even when setError is false: MethodBinder.Invoke surfaces a pending
+                                // error as the binding failure reason instead of its generic no-match message
                                 Exceptions.RaiseTypeError($"Failed to implicitly convert {type} to {obType}");
                                 return false;
                             }
@@ -711,6 +713,8 @@ internal static bool ToManagedValue(BorrowedReference value, Type obType,
                         catch
                         {
                             // Failed to convert using implicit conversion,  must catch the error to stop program from exploding on Linux
+                            // Raised even when setError is false: MethodBinder.Invoke surfaces a pending
+                            // error as the binding failure reason instead of its generic no-match message
                             Exceptions.RaiseTypeError($"Failed to implicitly convert {result.GetType()} to {obType}");
                             return false;
                         }
@@ -1371,6 +1375,11 @@ internal static bool ToPrimitive(BorrowedReference value, Type obType, out objec
                 string tpName = Runtime.PyObject_GetTypeName(value);
                 Exceptions.SetError(Exceptions.TypeError, $"'{tpName}' value cannot be converted to {obType}");
             }
+            else
+            {
+                // a probing sub-path may have raised before jumping here
+                Exceptions.Clear();
+            }
             return false;
 
         overflow:
@@ -1379,6 +1388,10 @@ internal static bool ToPrimitive(BorrowedReference value, Type obType, out objec
             {
                 Exceptions.SetError(Exceptions.OverflowError, "value too large to convert");
             }
+            else
+            {
+                Exceptions.Clear();
+            }
             return false;
         }
 
@@ -1456,7 +1469,23 @@ private static bool ToArray(BorrowedReference value, Type obType, out object res
         private static bool ToList(BorrowedReference value, Type obType, out object result, bool setError)
         {
             var elementType = obType.GetGenericArguments()[0];
-            var IterObject = Runtime.PyObject_GetIter(value);
+            result = null;
+
+            using var IterObject = Runtime.PyObject_GetIter(value);
+            if (IterObject.IsNull())
+            {
+                if (setError)
+                {
+                    SetConversionError(value, obType);
+                }
+                else
+                {
+                    // PyObject_GetIter will have set an error
+                    Exceptions.Clear();
+                }
+                return false;
+            }
+
             result = MakeList(value, IterObject, obType, elementType, setError);
             return result != null;
         }
@@ -1499,17 +1528,41 @@ private static IList MakeList(BorrowedReference value, NewReference IterObject,
                     Exceptions.SetError(e);
                     SetConversionError(value, obType);
                 }
+                else
+                {
+                    // PySequence_Size may have raised (e.g. a broken __len__)
+                    Exceptions.Clear();
+                }
 
                 return null;
             }
 
             while (true)
             {
                 using var item = Runtime.PyIter_Next(IterObject.Borrow());
-                if (item.IsNull()) break;
+                if (item.IsNull())
+                {
+                    if (Exceptions.ErrorOccurred())
+                    {
+                        // the iterator raised mid-iteration: the conversion failed,
+                        // don't return a silently truncated list
+                        if (!setError)
+                        {
+                            Exceptions.Clear();
+                        }
+                        return null;
+                    }
+                    break;
+                }
 
                 if (!Converter.ToManaged(item.Borrow(), elementType, out var obj, setError))
                 {
+                    // some element conversion paths raise even when setError is false
+                    // (e.g. failed implicit operators, deleted types)
+                    if (!setError)
+                    {
+                        Exceptions.Clear();
+                    }
                     return null;
                 }