Add a test for PyInterpreterRef_Main().

Author: ZeroIntensity

Lib/test/test_embed.py

@@ -1920,6 +1920,9 @@ def test_gilstate_after_finalization(self):
     def test_thread_state_ensure(self):
         self.run_embedded_interpreter("test_thread_state_ensure")
 
+    def test_main_interpreter_ref(self):
+        self.run_embedded_interpreter("test_main_interpreter_ref")
+
 
 class MiscTests(EmbeddingTestsMixin, unittest.TestCase):
     def test_unicode_id_init(self):

Programs/_testembed.c

@@ -2400,6 +2400,31 @@ test_gilstate_after_finalization(void)
     return PyThread_detach_thread(handle);
 }
 
+static int
+test_main_interpreter_ref(void)
+{
+    // It should not work before the runtime has started.
+    PyInterpreterRef ref;
+    int res = PyInterpreterRef_Main(&ref);
+    (void)res;
+    assert(res == -1);
+
+    _testembed_initialize();
+
+    // Main interpreter is initialized and ready.
+    res = PyInterpreterRef_Main(&ref);
+    assert(res == 0);
+    assert(PyInterpreterRef_AsInterpreter(ref) == PyInterpreterState_Main());
+    PyInterpreterRef_Close(ref);
+
+    Py_Finalize();
+
+    // Main interpreter is dead, we can no longer acquire references to it.
+    res = PyInterpreterRef_Main(&ref);
+    assert(res == -1);
+    return 0;
+}
+
 /* *********************************************************
  * List of test cases and the function that implements it.
  *
@@ -2491,6 +2516,7 @@ static struct TestCase TestCases[] = {
     {"test_get_incomplete_frame", test_get_incomplete_frame},
     {"test_thread_state_ensure", test_thread_state_ensure},
     {"test_gilstate_after_finalization", test_gilstate_after_finalization},
+    {"test_main_interpreter_ref", test_main_interpreter_ref},
     {NULL, NULL}
 };
 

Python/pystate.c

@@ -3305,6 +3305,12 @@ PyInterpreterRef_Main(PyInterpreterRef *strong_ptr)
     assert(strong_ptr != NULL);
     _PyRuntimeState *runtime = &_PyRuntime;
     HEAD_LOCK(runtime);
+    if (runtime->initialized == 0) {
+        // Main interpreter is not initialized.
+        // This can be the case before Py_Initialize(), or after Py_Finalize().
+        HEAD_UNLOCK(runtime);
+        return -1;
+    }
     int res = try_acquire_strong_ref(&runtime->_main_interpreter, strong_ptr);
     HEAD_UNLOCK(runtime);