gdb/python: add a new gdb_exiting event

Add a new event, gdb.events.gdb_exiting, which is called once GDB
decides it is going to exit.

This event is not triggered in the case that GDB performs a hard
abort, for example, when handling an internal error and the user
decides to quit the debug session, or if GDB hits an unexpected,
fatal, signal.

This event is triggered if the user just types 'quit' at the command
prompt, or if GDB is run with '-batch' and has processed all of the
required commands.

The new event type is gdb.GdbExitingEvent, and it has a single
attribute exit_code, which is the value that GDB is about to exit
with.

The event is triggered before GDB starts dismantling any of its own
internal state, so, my expectation is that most Python calls should
work just fine at this point.

When considering this functionality I wondered about using the
'atexit' Python module.  However, this is triggered when the Python
environment is shut down, which is done from a final cleanup.  At
this point we don't know for sure what other GDB state has already
been cleaned up.

1 files changed, 35 insertions, 0 deletions

diff --git a/gdb/python/python.c b/gdb/python/python.c
index fcd367f..451c5bd 100644
--- a/gdb/python/python.c
+++ b/gdb/python/python.c
@@ -36,6 +36,7 @@
 #include "location.h"
 #include "run-on-main-thread.h"
 #include "gdbsupport/selftest.h"
+#include "observable.h"
 
 /* Declared constants and enum for python stack printing.  */
 static const char python_excp_none[] = "none";
@@ -1720,6 +1721,38 @@ init__gdb_module (void)
 }
 #endif
 
+/* Emit a gdb.GdbExitingEvent, return a negative value if there are any
+   errors, otherwise, return 0.  */
+
+static int
+emit_exiting_event (int exit_code)
+{
+  gdbpy_ref<> event_obj = create_event_object (&gdb_exiting_event_object_type);
+  if (event_obj == nullptr)
+    return -1;
+
+  gdbpy_ref<> code = gdb_py_object_from_longest (exit_code);
+  if (evpy_add_attribute (event_obj.get (), "exit_code", code.get ()) < 0)
+    return -1;
+
+  return evpy_emit_event (event_obj.get (), gdb_py_events.gdb_exiting);
+}
+
+/* Callback for the gdb_exiting observable.  EXIT_CODE is the value GDB
+   will exit with.  */
+
+static void
+gdbpy_gdb_exiting (int exit_code)
+{
+  if (!gdb_python_initialized)
+    return;
+
+  gdbpy_enter enter_py (python_gdbarch, python_language);
+
+  if (emit_exiting_event (exit_code) < 0)
+    gdbpy_print_stack ();
+}
+
 static bool
 do_start_initialization ()
 {
@@ -1871,6 +1904,8 @@ do_start_initialization ()
   if (gdbpy_value_cst == NULL)
     return false;
 
+  gdb::observers::gdb_exiting.attach (gdbpy_gdb_exiting, "python");
+
   /* Release the GIL while gdb runs.  */
   PyEval_SaveThread ();