bpo-45020: Add -X frozen_modules=[on|off] to explicitly control use of frozen modules. (gh-28320)

Currently we freeze several modules into the runtime. For each of these modules it is essential to bootstrapping the runtime that they be frozen. Any other stdlib module that we later freeze into the runtime is not essential. We can just as well import from the .py file.  This PR lets users explicitly choose which should be used, with the new "-X frozen_modules=[on|off]" CLI flag. The default is "off" for now.

https://bugs.python.org/issue45020

Python/import.c

@@ -1050,17 +1050,58 @@ _imp_create_builtin(PyObject *module, PyObject *spec)
 
 /* Frozen modules */
 
+static bool
+is_essential_frozen_module(const char *name)
+{
+    /* These modules are necessary to bootstrap the import system. */
+    if (strcmp(name, "_frozen_importlib") == 0) {
+        return true;
+    }
+    if (strcmp(name, "_frozen_importlib_external") == 0) {
+        return true;
+    }
+    if (strcmp(name, "zipimport") == 0) {
+        return true;
+    }
+    /* This doesn't otherwise have anywhere to find the module.
+       See frozenmain.c. */
+    if (strcmp(name, "__main__") == 0) {
+        return true;
+    }
+    return false;
+}
+
+static bool
+use_frozen(void)
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    int override = interp->override_frozen_modules;
+    if (override > 0) {
+        return true;
+    }
+    else if (override < 0) {
+        return false;
+    }
+    else {
+        return interp->config.use_frozen_modules;
+    }
+}
+
 static PyObject *
-list_frozen_module_names(bool force)
+list_frozen_module_names()
 {
     PyObject *names = PyList_New(0);
     if (names == NULL) {
         return NULL;
     }
+    bool enabled = use_frozen();
     for (const struct _frozen *p = PyImport_FrozenModules; ; p++) {
         if (p->name == NULL) {
             break;
         }
+        if (!enabled && !is_essential_frozen_module(p->name)) {
+            continue;
+        }
         PyObject *name = PyUnicode_FromString(p->name);
         if (name == NULL) {
             Py_DECREF(names);
@@ -1077,18 +1118,27 @@ list_frozen_module_names(bool force)
 }
 
 static const struct _frozen *
-find_frozen(PyObject *name)
+find_frozen(PyObject *modname)
 {
-    const struct _frozen *p;
-
-    if (name == NULL)
+    if (modname == NULL) {
         return NULL;
-
+    }
+    const char *name = PyUnicode_AsUTF8(modname);
+    if (name == NULL) {
+        PyErr_Clear();
+        return NULL;
+    }
+    if (!use_frozen() && !is_essential_frozen_module(name)) {
+        return NULL;
+    }
+    const struct _frozen *p;
     for (p = PyImport_FrozenModules; ; p++) {
-        if (p->name == NULL)
+        if (p->name == NULL) {
             return NULL;
-        if (_PyUnicode_EqualToASCIIString(name, p->name))
+        }
+        if (strcmp(name, p->name) == 0) {
             break;
+        }
     }
     return p;
 }
@@ -1991,7 +2041,28 @@ static PyObject *
 _imp__frozen_module_names_impl(PyObject *module)
 /*[clinic end generated code: output=80609ef6256310a8 input=76237fbfa94460d2]*/
 {
-    return list_frozen_module_names(true);
+    return list_frozen_module_names();
+}
+
+/*[clinic input]
+_imp._override_frozen_modules_for_tests
+
+    override: int
+    /
+
+(internal-only) Override PyConfig.use_frozen_modules.
+
+(-1: "off", 1: "on", 0: no override)
+See frozen_modules() in Lib/test/support/import_helper.py.
+[clinic start generated code]*/
+
+static PyObject *
+_imp__override_frozen_modules_for_tests_impl(PyObject *module, int override)
+/*[clinic end generated code: output=36d5cb1594160811 input=8f1f95a3ef21aec3]*/
+{
+    PyInterpreterState *interp = _PyInterpreterState_GET();
+    interp->override_frozen_modules = override;
+    Py_RETURN_NONE;
 }
 
 /* Common implementation for _imp.exec_dynamic and _imp.exec_builtin */
@@ -2155,6 +2226,7 @@ static PyMethodDef imp_methods[] = {
     _IMP_IS_BUILTIN_METHODDEF
     _IMP_IS_FROZEN_METHODDEF
     _IMP__FROZEN_MODULE_NAMES_METHODDEF
+    _IMP__OVERRIDE_FROZEN_MODULES_FOR_TESTS_METHODDEF
     _IMP_CREATE_DYNAMIC_METHODDEF
     _IMP_EXEC_DYNAMIC_METHODDEF
     _IMP_EXEC_BUILTIN_METHODDEF