gh-161996: Fix crash in Interpreter.exec() when the uncaught exception's __str__ raises

Lib/test/test_interpreters/test_api.py

@@ -947,6 +947,16 @@ def test_failure(self):
         with self.assertRaises(ExecutionFailed):
             interp.exec('raise Exception')
 
+    def test_failure_with_broken_exception_str(self):
+        interp = interpreters.create()
+        with self.assertRaises(interpreters.InterpreterError):
+            interp.exec(dedent("""
+                class BrokenError(Exception):
+                    def __str__(self):
+                        raise ValueError
+                raise BrokenError
+                """))
+
     @force_not_colorized
     def test_display_preserved_exception(self):
         tempdir = self.temp_dir()

Misc/NEWS.d/next/Library/2026-11-14-21-04-56.gh-issue-161996.35uL2d.rst

@@ -0,0 +1,2 @@
+Fix a crash in :meth:`concurrent.interpreters.Interpreter.exec` when the
+uncaught exception's ``__str__`` raises.

Python/crossinterp.c

@@ -2018,6 +2018,14 @@ _PyXI_ApplyError(_PyXI_error *error, const char *failure)
     PyThreadState *tstate = PyThreadState_Get();
 
     if (failure != NULL) {
+        if (error->override != NULL
+            && error->override->code == _PyXI_ERR_NO_MEMORY)
+        {
+            PyErr_NoMemory();
+        }
+        else {
+            PyErr_SetString(PyExc_InterpreterError, failure);
+        }
         xi_error_clear(error);
         return NULL;
     }