Fix spin conversion error to avoid NaN

bilby/gw/conversion.py

@@ -232,19 +232,24 @@ def convert_to_lal_binary_black_hole_parameters(parameters):
         key = 'chi_{}'.format(idx)
         if key in original_keys:
             if "chi_{}_in_plane".format(idx) in original_keys:
-                converted_parameters["a_{}".format(idx)] = (
-                    converted_parameters[f"chi_{idx}"] ** 2
-                    + converted_parameters[f"chi_{idx}_in_plane"] ** 2
-                ) ** 0.5
-                converted_parameters[f"cos_tilt_{idx}"] = (
-                    converted_parameters[f"chi_{idx}"]
-                    / converted_parameters[f"a_{idx}"]
-                )
+                a = (converted_parameters[f"chi_{idx}"] ** 2
+                     + converted_parameters[f"chi_{idx}_in_plane"] ** 2) ** 0.5
+                converted_parameters[f"a_{idx}"] = a
+                with np.errstate(invalid="raise"):
+                    try:
+                        converted_parameters[f"cos_tilt_{idx}"] = (
+                            converted_parameters[f"chi_{idx}"] / a
+                        )
+                    except (FloatingPointError, ZeroDivisionError):
+                        logger.debug(
+                            "Error in conversion to spherical spin tilt. "
+                            "This is often due to the spin parameters being zero. "
+                            f"Setting cos_tilt_{idx} = 1."
+                        )
+                        converted_parameters[f"cos_tilt_{idx}"] = 1.0
             elif "a_{}".format(idx) not in original_keys:
-                converted_parameters['a_{}'.format(idx)] = abs(
-                    converted_parameters[key])
-                converted_parameters['cos_tilt_{}'.format(idx)] = \
-                    np.sign(converted_parameters[key])
+                converted_parameters['a_{}'.format(idx)] = abs(converted_parameters[key])
+                converted_parameters['cos_tilt_{}'.format(idx)] = np.sign(converted_parameters[key])
             else:
                 with np.errstate(invalid="raise"):
                     try:

test/gw/conversion_test.py

@@ -379,6 +379,51 @@ def test_bbh_cos_angle_to_angle_conversion(self):
         self.bbh_convert()
         self.assertEqual(self.parameters["tilt_1"], t1)
 
+    def test_bbh_chi_in_plane_zero_spin_both_components(self):
+        """
+        When chi_1 = chi_1_in_plane = 0 the magnitude a_1 = 0,
+        and the bare division chi_1 / a_1 = 0/0 would produce a NaN.
+        This tests whether the wrapped np.errstate(invalid="raise")
+        correctly catches the resulting error, and sets cos_tilt_1 = 1.0 instead.
+        Same for component labeled 2.
+        """
+        self.parameters["chi_1"] = 0.0
+        self.parameters["chi_1_in_plane"] = 0.0
+        self.parameters["chi_2"] = 0.0
+        self.parameters["chi_2_in_plane"] = 0.0
+        self.bbh_convert()
+        for idx in ["1", "2"]:
+            self.assertFalse(
+                np.isnan(self.parameters[f"cos_tilt_{idx}"]),
+                f"cos_tilt_{idx} must not be NaN for zero spins",
+            )
+            self.assertEqual(self.parameters[f"a_{idx}"], 0.0)
+            self.assertEqual(self.parameters[f"cos_tilt_{idx}"], 1.0)
+
+    def test_bbh_double_conversion_zero_spin_no_nan(self):
+        """
+        Verify that, when calling convert_to_lal_binary_black_hole_parameters
+        twice on a parameters dict that has chi_1 = chi_2 = 0, the np.errstate
+        is correctly triggered and avoids NaNs.
+        """
+        self.parameters["chi_1"] = 0.0
+        self.parameters["chi_2"] = 0.0
+        # First conversion
+        self.bbh_convert()
+        first_pass = self.parameters.copy()
+        self.assertFalse(np.isnan(first_pass.get("cos_tilt_1", 0.0)))
+
+        # Second conversion on the already-converted parameters (the crash scenario)
+        (second_pass, _) = conversion.convert_to_lal_binary_black_hole_parameters(first_pass)
+        self.assertFalse(
+            np.isnan(second_pass["cos_tilt_1"]),
+            "cos_tilt_1 must not be NaN after a second conversion of zero-spin parameters",
+        )
+        self.assertFalse(
+            np.isnan(second_pass["cos_tilt_2"]),
+            "cos_tilt_2 must not be NaN after a second conversion of zero-spin parameters",
+        )
+
     def _conversion_to_component_tidal(self, keys):
         for key in keys:
             self.parameters[key] = self.tidal_parameters[key]