Re: [PATCH] gdb: fix target_ops reference count for some cases

[Andrew Burgess <aburgess@redhat.com>]

Simon Marchi <simark@simark.ca> writes:

>> diff --git a/gdb/inferior.c b/gdb/inferior.c
>> index 7eb2bd97907..0c0b4b78ff5 100644
>> --- a/gdb/inferior.c
>> +++ b/gdb/inferior.c
>> @@ -70,6 +70,16 @@ inferior::~inferior ()
>>  {
>>    inferior *inf = this;
>>  
>> +  /* Before the inferior is deleted, all target_ops should be popped from
>> +     the target stack, this leaves just the dummy_target behind.  If this
>> +     is not done, then any target left in the target stack will be left
>> +     with an artificially high reference count.  As the dummy_target is
>> +     still on the target stack then we are about to loose a reference to
> loose -> lose?
>
>> @@ -191,6 +201,26 @@ delete_inferior (struct inferior *inf)
>>  
>>    gdb::observers::inferior_removed.notify (inf);
>>  
>> +  {
>> +    /* Limit the change of inferior to an inner scope so that the original
>> +       inferior and program space will have been restored by the time that
>> +       we delete the inferior INF and (possibly) its related  program
>
> two spaces between related and program.
>
>> +       space.  */
>> +    scoped_restore_current_pspace_and_thread restore_pspace_and_thread;
>
> Just wondering, why do we need to restore explicitly the current
> pspace, instead of using just scoped_restore_current_thread?
>
> scoped_restore_current_pspace_and_thread's doc says:
>
> /* Save/restore the current program space, thread, inferior and frame.
>    Use this when you need to call
>    switch_to_program_space_and_thread.  */
>
> ... but you are not using switch_to_program_space_and_thread here.
> Maybe it's ok and I just don't understand.  Same in
> ~scoped_mock_context.

I suspect the comment you quote is just out of date.

switch_to_program_space_and_thread can end up calling
switch_to_inferior_no_thread if there are no running threads in the
program space being switched too.  But, even if switch_to_thread does
end up being called we:

  - set the program space,
  - set the inferior,
  - set the current thread,
  - reinit the frame cache,

By comparison, switch_to_inferior_no_thread does:

  - sets the program space,
  - sets the inferior,
  - sets the current thread (to nullptr this time though),
  - reinits the frame cache,

As you can see they do the same set of things, all of which I think
should be reverted once we leave the scope, hence
scoped_restore_current_pspace_and_thread seems like the way to go.

>
>> diff --git a/gdb/testsuite/gdb.python/py-connection-removed.exp b/gdb/testsuite/gdb.python/py-connection-removed.exp
>> new file mode 100644
>> index 00000000000..1b139cedc0d
>> --- /dev/null
>> +++ b/gdb/testsuite/gdb.python/py-connection-removed.exp
>> @@ -0,0 +1,92 @@
>> +# 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/>.
>> +
>> +# Check that the gdb.connect_removed event triggers when we expect it
>> +# too.
>
> too -> to
>
>> +#
>> +# Checking this event has wider implications that simply some corner
>
> simply -> imply?  Or I don't get what you mean.
>
>> +# of the Python API working or not.  The connection_removed event
>> +# triggers when the reference count of a process_stratum_target
>> +# reaches zero.  If these events stop triggering when expected then
>> +# GDB might be getting the reference counting on target_ops objects
>> +# wrong.
>> +
>> +load_lib gdb-python.exp
>> +
>> +standard_testfile py-connection.c
>> +
>> +if { [prepare_for_testing "failed to prepare" ${testfile} ${srcfile}] } {
>> +    return -1
>> +}
>> +
>> +# Skip all tests if Python scripting is not enabled.
>> +if { [skip_python_tests] } { continue }
>> +
/>> +if ![runto_main] then {
>> +    return 0
>> +}
>> +
>> +# Register a callback that will trigger when a connection is removed
>> +# (deleted) within GDB.
>> +gdb_test_multiline "Add connection_removed event" \
>> +    "python" "" \
>> +    "def connection_removed_handler(event):" "" \
>> +    "   num = event.connection.num" "" \
>> +    "   type = event.connection.type" "" \
>> +    "   print(f'Connection {num} ({type}) removed')" "" \
>
> I think unfortunately need to support Python versions that don't know
> about f-strings.

OK.

Thanks,
Andrew


>
> Simon