[python] Ensure GIL is held during CPython API call

ROOT's CPython extension defines a custom error handler that tries to use Python standard warnings when possible. In such cases, the CPython API is called and thus the GIL must be held. If the calling Python function had previously released the GIL it will result in trouble. This is the case for the TH1.Fit method which has been changed to always releasing the GIL, under the assumption that everything happening during the call happens in C++, but that's not true. This commit proposes to restore the GIL for the call to the CPython API to raise the warning.

Author: vepadulano

bindings/pyroot/pythonizations/src/RPyROOTApplication.cxx

@@ -122,7 +122,9 @@ static void ErrMsgHandler(int level, Bool_t abort, const char *location, const c
       // the GIL.
       if (!gGlobalMutex) {
          // Either printout or raise exception, depending on user settings
+         auto state = PyGILState_Ensure();
          PyErr_WarnExplicit(NULL, (char *)msg, (char *)location, 0, (char *)"ROOT", NULL);
+         PyGILState_Release(state);
       } else {
          ::DefaultErrorHandler(level, abort, location, msg);
       }

bindings/pyroot/pythonizations/test/th1.py

@@ -65,6 +65,18 @@ def func(x, pars):
         self.assertTrue(r.IsValid())
         self.assertGreater(r.Parameter(0), 0)
 
+    def test_fit_with_warning(self):
+        """
+        Regression test for https://github.com/root-project/root/issues/22396
+        """
+        import ROOT
+
+        h = ROOT.TH1D("test", "test", 100, 0, 1)
+        # Trying to fit a empty histogram will result in a warning on the C++ side which is re-raised as a Python
+        # warning by the ROOT CPython extension
+        with self.assertWarnsRegex(RuntimeWarning, "Fit data is empty"):
+            h.Fit("gaus", "S")
+
 
 if __name__ == "__main__":
     unittest.main()