Make 'import gdb.events' work

Pierre-Marie noticed that, while gdb.events is a Python module, it
can't be imported.  This patch changes how this module is created, so
that it can be imported, while also ensuring that the module is always
visible, just as it was in the past.

This new approach required one non-obvious change -- when running
gdb.base/warning.exp, where --data-directory is intentionally not
found, the event registries can now be nullptr.  Consequently, this
patch probably also requires

    https://sourceware.org/pipermail/gdb-patches/2022-June/189796.html

Note that this patch obsoletes

    https://sourceware.org/pipermail/gdb-patches/2022-June/189797.html

gdb/python/lib/gdb/__init__.py

@@ -27,6 +27,11 @@ else:
 
 from _gdb import *
 
+# Historically, gdb.events was always available, so ensure it's
+# still available without an explicit import.
+import _gdbevents as events
+sys.modules['gdb.events'] = events
+
 
 class _GdbFile(object):
     # These two are needed in Python 3

gdb/python/py-evtregistry.c

@@ -118,7 +118,9 @@ gdbpy_initialize_eventregistry (void)
 bool
 evregpy_no_listeners_p (eventregistry_object *registry)
 {
-  return PyList_Size (registry->callbacks) == 0;
+  /* REGISTRY can be nullptr if gdb failed to find the data directory
+     at startup.  */
+  return registry == nullptr || PyList_Size (registry->callbacks) == 0;
 }
 
 static PyMethodDef eventregistry_object_methods[] =

gdb/python/py-evts.c

@@ -23,7 +23,7 @@
 static struct PyModuleDef EventModuleDef =
 {
   PyModuleDef_HEAD_INIT,
-  "gdb.events",
+  "_gdbevents",
   NULL,
   -1,
   NULL,
@@ -33,7 +33,8 @@ static struct PyModuleDef EventModuleDef =
   NULL
 };
 
-/* Initialize python events.  */
+/* Helper function to add a single event registry to the events
+   module.  */
 
 static int CPYCHECKER_NEGATIVE_RESULT_SETS_EXCEPTION
 add_new_registry (eventregistry_object **registryp, const char *name)
@@ -48,24 +49,21 @@ add_new_registry (eventregistry_object **registryp, const char *name)
 				 (PyObject *)(*registryp));
 }
 
-int
-gdbpy_initialize_py_events (void)
+/* Create and populate the _gdbevents module.  Note that this is
+   always created, see the base gdb __init__.py.  */
+
+PyMODINIT_FUNC
+gdbpy_events_mod_func ()
 {
   gdb_py_events.module = PyModule_Create (&EventModuleDef);
-
-  if (!gdb_py_events.module)
-    return -1;
+  if (gdb_py_events.module == nullptr)
+    return nullptr;
 
 #define GDB_PY_DEFINE_EVENT(name)					\
   if (add_new_registry (&gdb_py_events.name, #name) < 0)	\
-    return -1;
+    return nullptr;
 #include "py-all-events.def"
 #undef GDB_PY_DEFINE_EVENT
 
-  if (gdb_pymodule_addobject (gdb_module,
-			      "events",
-			      (PyObject *) gdb_py_events.module) < 0)
-    return -1;
-
-  return 0;
+  return gdb_py_events.module;
 }

gdb/python/python-internal.h

@@ -521,8 +521,6 @@ int gdbpy_initialize_eventregistry (void)
   CPYCHECKER_NEGATIVE_RESULT_SETS_EXCEPTION;
 int gdbpy_initialize_event (void)
   CPYCHECKER_NEGATIVE_RESULT_SETS_EXCEPTION;
-int gdbpy_initialize_py_events (void)
-  CPYCHECKER_NEGATIVE_RESULT_SETS_EXCEPTION;
 int gdbpy_initialize_arch (void)
   CPYCHECKER_NEGATIVE_RESULT_SETS_EXCEPTION;
 int gdbpy_initialize_registers ()
@@ -543,6 +541,8 @@ void gdbpy_finalize_micommands ();
 int gdbpy_initialize_disasm ()
   CPYCHECKER_NEGATIVE_RESULT_SETS_EXCEPTION;
 
+PyMODINIT_FUNC gdbpy_events_mod_func ();
+
 /* A wrapper for PyErr_Fetch that handles reference counting for the
    caller.  */
 class gdbpy_err_fetch

gdb/python/python.c

@@ -2005,11 +2005,20 @@ do_start_initialization ()
      remain alive for the duration of the program's execution, so
      it is not freed after this call.  */
   Py_SetProgramName (progname_copy);
-
-  /* Define _gdb as a built-in module.  */
-  PyImport_AppendInittab ("_gdb", init__gdb_module);
 #endif
 
+  /* Define all internal modules.  These are all imported (and thus
+     created) during initialization.  */
+  struct _inittab mods[3] =
+  {
+    { "_gdb", init__gdb_module },
+    { "_gdbevents", gdbpy_events_mod_func },
+    { nullptr, nullptr }
+  };
+
+  if (PyImport_ExtendInittab (mods) < 0)
+    return false;
+
   Py_Initialize ();
 #if PY_VERSION_HEX < 0x03090000
   /* PyEval_InitThreads became deprecated in Python 3.9 and will
@@ -2077,7 +2086,6 @@ do_start_initialization ()
       || gdbpy_initialize_thread () < 0
       || gdbpy_initialize_inferior () < 0
       || gdbpy_initialize_eventregistry () < 0
-      || gdbpy_initialize_py_events () < 0
       || gdbpy_initialize_event () < 0
       || gdbpy_initialize_arch () < 0
       || gdbpy_initialize_registers () < 0

gdb/testsuite/gdb.python/py-events.exp

@@ -38,6 +38,8 @@ clean_restart ${testfile}
 
 if { [skip_python_tests] } { continue }
 
+gdb_test_no_output "python import gdb.events"
+
 set pyfile [gdb_remote_download host ${srcdir}/${subdir}/py-events.py]
 gdb_test_no_output "source ${pyfile}" "load python file"