Keep tracking modules that are still initializing

A module is in sys.modules while its body runs, so lazy_modules_add()
counted it as loaded and skipped recording the name. If the body then
raises, the module is removed from sys.modules again and the lazy
import is left pending under no name at all.

Check __spec__._initializing so such a module does not count as loaded.
A name added while a module initializes is still discarded once the
import completes, since _set_lazy_attributes() runs after the body.

Author: brittanyrey

Lib/test/test_lazy_import/__init__.py

@@ -1354,6 +1354,24 @@ def test_blocked_module_is_still_tracked(self):
         """)
         assert_python_ok("-c", code)
 
+    def test_initializing_module_is_still_tracked(self):
+        """A module that is still executing must not count as loaded."""
+        code = textwrap.dedent("""
+            import sys
+            name = "test.test_lazy_import.data.init_fails"
+            try:
+                import test.test_lazy_import.data.init_fails
+            except ValueError:
+                pass
+            else:
+                raise AssertionError("ValueError was not raised")
+            assert name not in sys.modules, "failed import left a module behind"
+            assert name in sys.lazy_modules, (
+                f"expected {name!r} tracked, got {sys.lazy_modules}"
+            )
+        """)
+        assert_python_ok("-c", code)
+
     def test_pending_submodule_is_still_tracked(self):
         """`lazy from` a submodule that is not loaded must stay tracked."""
         code = textwrap.dedent("""

Lib/test/test_lazy_import/data/init_fails.py

@@ -0,0 +1,4 @@
+# Imported by test_initializing_module_is_still_tracked.  The module it imports
+# lazily imports this one back while this one is still initializing.
+import test.test_lazy_import.data.lazy_on_init_fails
+raise ValueError("initialization failed")

Lib/test/test_lazy_import/data/lazy_on_init_fails.py

@@ -0,0 +1 @@
+lazy import test.test_lazy_import.data.init_fails

Python/import.c

@@ -4390,6 +4390,20 @@ lazy_modules_add(PyThreadState *tstate, PyObject *name)
     }
     // A None entry blocks the import rather than satisfying it.
     int loaded = (existing != NULL && existing != Py_None);
+    if (loaded) {
+        // Check if the module is still initializing.
+        PyObject *spec;
+        int rc = PyObject_GetOptionalAttr(existing, &_Py_ID(__spec__), &spec);
+        if (rc > 0) {
+            rc = _PyModuleSpec_IsInitializing(spec);
+            Py_DECREF(spec);
+        }
+        if (rc < 0) {
+            Py_DECREF(existing);
+            return -1;
+        }
+        loaded = !rc;
+    }
     Py_XDECREF(existing);
     if (loaded) {
         return 0;