Add gdb.free_objfile event registry

Currently, Python code can use event registries to detect when gdb
loads a new objfile, and when gdb clears the objfile list.  However,
there's no way to detect the removal of an objfile, say when the
inferior calls dlclose.

This patch adds a gdb.free_objfile event registry and arranges for an
event to be emitted in this case.

gdb/doc/python.texi

@@ -3494,6 +3494,17 @@ A reference to the object file (@code{gdb.Objfile}) which has been loaded.
 @xref{Objfiles In Python}, for details of the @code{gdb.Objfile} object.
 @end defvar
 
+@item events.free_objfile
+Emits @code{gdb.FreeObjFileEvent} which indicates that an object file
+is about to be removed from @value{GDBN}.  One reason this can happen
+is when the inferior calls @code{dlclose}.
+@code{gdb.FreeObjFileEvent} has one attribute:
+
+@defvar NewObjFileEvent.objfile
+A reference to the object file (@code{gdb.Objfile}) which will be unloaded.
+@xref{Objfiles In Python}, for details of the @code{gdb.Objfile} object.
+@end defvar
+
 @item events.clear_objfiles
 Emits @code{gdb.ClearObjFilesEvent} which indicates that the list of object
 files for a program space has been reset.

gdb/python/py-all-events.def

@@ -27,6 +27,7 @@ GDB_PY_DEFINE_EVENT(stop)
 GDB_PY_DEFINE_EVENT(cont)
 GDB_PY_DEFINE_EVENT(exited)
 GDB_PY_DEFINE_EVENT(new_objfile)
+GDB_PY_DEFINE_EVENT(free_objfile)
 GDB_PY_DEFINE_EVENT(clear_objfiles)
 GDB_PY_DEFINE_EVENT(new_inferior)
 GDB_PY_DEFINE_EVENT(inferior_deleted)

gdb/python/py-event-types.def

@@ -86,6 +86,11 @@ GDB_PY_DEFINE_EVENT_TYPE (new_objfile,
 			  "GDB new object file event object",
 			  event_object_type);
 
+GDB_PY_DEFINE_EVENT_TYPE (free_objfile,
+			  "FreeObjFileEvent",
+			  "GDB free object file event object",
+			  event_object_type);
+
 GDB_PY_DEFINE_EVENT_TYPE (clear_objfiles,
 			  "ClearObjFilesEvent",
 			  "GDB clear object files event object",

gdb/python/py-event.h

@@ -74,6 +74,7 @@ extern gdbpy_ref<> create_thread_event_object (PyTypeObject *py_type,
 					       PyObject *thread);
 
 extern int emit_new_objfile_event (struct objfile *objfile);
+extern int emit_free_objfile_event (struct objfile *objfile);
 extern int emit_clear_objfiles_event (void);
 
 extern void evpy_dealloc (PyObject *self);

gdb/python/py-inferior.c

@@ -197,6 +197,20 @@ python_new_objfile (struct objfile *objfile)
     }
 }
 
+/* Emit a Python event when an objfile is about to be removed.  */
+
+static void
+python_free_objfile (struct objfile *objfile)
+{
+  if (!gdb_python_initialized)
+    return;
+
+  gdbpy_enter enter_py (objfile->arch ());
+
+  if (emit_free_objfile_event (objfile) < 0)
+    gdbpy_print_stack ();
+}
+
 /* Return a reference to the Python object of type Inferior
    representing INFERIOR.  If the object has already been created,
    return it and increment the reference count,  otherwise, create it.
@@ -853,6 +867,7 @@ gdbpy_initialize_inferior (void)
   gdb::observers::new_objfile.attach
     (python_new_objfile, "py-inferior",
      { &auto_load_new_objfile_observer_token });
+  gdb::observers::free_objfile.attach (python_free_objfile, "py-inferior");
   gdb::observers::inferior_added.attach (python_new_inferior, "py-inferior");
   gdb::observers::inferior_removed.attach (python_inferior_deleted,
 					   "py-inferior");

gdb/python/py-newobjfileevent.c

@@ -53,6 +53,42 @@ emit_new_objfile_event (struct objfile *objfile)
   return -1;
 }
 
+/* Create an event object representing a to-be-freed objfile.  Return
+   nullptr, with the Python exception set, on error.  */
+
+static gdbpy_ref<>
+create_free_objfile_event_object (struct objfile *objfile)
+{
+  gdbpy_ref<> objfile_event
+    = create_event_object (&free_objfile_event_object_type);
+  if (objfile_event == nullptr)
+    return nullptr;
+
+  gdbpy_ref<> py_objfile = objfile_to_objfile_object (objfile);
+  if (py_objfile == nullptr
+      || evpy_add_attribute (objfile_event.get (), "objfile",
+			     py_objfile.get ()) < 0)
+    return nullptr;
+
+  return objfile_event;
+}
+
+/* Callback function which notifies observers when a free objfile
+   event occurs.  This function will create a new Python event object.
+   Return -1 if emit fails.  */
+
+int
+emit_free_objfile_event (struct objfile *objfile)
+{
+  if (evregpy_no_listeners_p (gdb_py_events.free_objfile))
+    return 0;
+
+  gdbpy_ref<> event = create_free_objfile_event_object (objfile);
+  if (event == nullptr)
+    return -1;
+  return evpy_emit_event (event.get (), gdb_py_events.free_objfile);
+}
+
 
 /* Subroutine of emit_clear_objfiles_event to simplify it.  */
 

gdb/testsuite/gdb.python/py-event-load.c

@@ -0,0 +1,42 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2022 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see  <http://www.gnu.org/licenses/>.  */
+
+#include <unistd.h>
+
+#ifdef __WIN32__
+#include <windows.h>
+#define dlopen(name, mode) LoadLibrary (TEXT (name))
+#define dlclose(handle) FreeLibrary (handle)
+#else
+#include <dlfcn.h>
+#endif
+
+/* This is updated by the .exp file.  */
+char *libname = "py-events-shlib.so";
+
+int
+main ()
+{
+  void *h;
+
+  h = dlopen (libname, RTLD_LAZY);
+
+  dlclose (h);
+
+  h = NULL;			/* final breakpoint here */
+  return 0;
+}

gdb/testsuite/gdb.python/py-event-load.exp

@@ -0,0 +1,67 @@
+# Copyright 2022 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+#
+
+# Test the Python free_objfile event.
+
+load_lib gdb-python.exp
+
+if {[skip_shlib_tests]} {
+    untested "skipping shared library tests"
+    return -1
+}
+
+if {[get_compiler_info]} {
+    warning "Could not get compiler info"
+    untested "no compiler info"
+    return -1
+}
+
+standard_testfile .c
+if {[gdb_compile "${srcdir}/${subdir}/${srcfile}" "${binfile}" \
+	 executable {debug shlib_load}] != ""} {
+    untested "failed to compile"
+    return -1
+}
+
+set testfile2 py-events-shlib
+set srcfile2 ${testfile2}.c
+set binfile2 [standard_output_file ${testfile2}.so]
+set binfile2_dlopen [shlib_target_file ${testfile2}.so]
+if {[gdb_compile_shlib "${srcdir}/${subdir}/${srcfile2}" \
+	 ${binfile2} {debug}] != ""} {
+    untested "failed to compile shared library"
+    return -1
+}
+
+clean_restart $testfile
+
+if {![runto_main]} {
+    return
+}
+
+if { [skip_python_tests] } { return }
+
+gdb_test_no_output "set var libname = \"$binfile2_dlopen\""
+
+set pyfile [gdb_remote_download host ${srcdir}/${subdir}/py-event-load.py]
+gdb_test_no_output "source ${pyfile}" "load python file"
+
+gdb_breakpoint [gdb_get_line_number "final breakpoint here"]
+
+gdb_continue_to_breakpoint "run to final breakpoint"
+
+gdb_test "python print(freed_objfile)" [string_to_regexp $binfile2_dlopen] \
+    "print name of unloaded objfile"

gdb/testsuite/gdb.python/py-event-load.py

@@ -0,0 +1,30 @@
+# Copyright (C) 2022 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+# Test Python free_objfile event.
+
+import gdb
+
+
+freed_objfile = None
+
+
+def free_objfile_handler(event):
+    assert isinstance(event, gdb.FreeObjFileEvent)
+    global freed_objfile
+    freed_objfile = event.objfile.username
+
+
+gdb.events.free_objfile.connect(free_objfile_handler)