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/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)