Re: [PATCH] [PR python/29603] Fix deletion of Watchpoints

[Lancelot SIX <lsix@lancelotsix.com>]

Hi,

Thanks for the patch!

First, I have not really explored why, but running your testcase with
current master GDB gives me:

    $ make check-gdb TESTS=gdb.python/py-watchpoint.exp
    Running .../gdb/testsuite/gdb.python/py-watchpoint.exp ...

                === gdb Summary ===

    # of expected passes            6

which is strange.  I was expecting FAILs there.  Is it only on my system
(ubuntu 22.04 on x86_64)?

I can however reproduce the problem by running the ticket’s testcase
manually and observe the issue you describe.

On Sun, Sep 25, 2022 at 01:33:50PM +0800, Johnson Sun via Gdb-patches wrote:
> Currently, during normal stop (i.e., stop and waiting for the next command),
> GDB automatically deletes local Watchpoints that are out of scope. However,
> local Watchpoints are not deleted if a Python script decides not to normal
> stop upon hit, causing them to be hit many more times than they should, as
> reported in PR python/29603. This was happening because the watchpoint is not
> disabled when going out of scope.
> 
> This commit fixes this issue by disabling the watchpoint when going out of
> scope. It also adds a test to ensure this feature isn't regressed in the
> future.
> 
> Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=29603
> ---
>  gdb/breakpoint.c                           |  1 +
>  gdb/testsuite/gdb.python/py-watchpoint.c   | 27 +++++++++++++
>  gdb/testsuite/gdb.python/py-watchpoint.exp | 44 ++++++++++++++++++++++
>  gdb/testsuite/gdb.python/py-watchpoint.py  | 31 +++++++++++++++
>  4 files changed, 103 insertions(+)
>  create mode 100644 gdb/testsuite/gdb.python/py-watchpoint.c
>  create mode 100644 gdb/testsuite/gdb.python/py-watchpoint.exp
>  create mode 100644 gdb/testsuite/gdb.python/py-watchpoint.py
> 
> diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
> index bff3bac7d1a..b78ae9c4993 100644
> --- a/gdb/breakpoint.c
> +++ b/gdb/breakpoint.c
> @@ -1832,6 +1832,7 @@ watchpoint_del_at_next_stop (struct watchpoint *w)
>        w->related_breakpoint = w;
>      }
>    w->disposition = disp_del_at_next_stop;
> +  disable_breakpoint(w);

From what I understand, the problem you are trying to solve is linked to
extension language (python in this case), so it seems odd to me that the
fix is in a non-extension related code path.

If my understanding is correct, the python extension you provide should
work similarly to typing say "watch i if 0".  So my approach would be to
have something similar between checking the watchpoint condition and
evaluating the `stop` method from you python class.

In the condition case, `disable_breakpoint` is not used before the
breakpoint is deleted.  Instead, the code checks "b->disposition !=
disp_del_at_next_stop" (in bpstat_check_breakpoint_conditions) before
trying to evaluate the condition.  I guess that this is what you also
want to do in your case.

In the same function, you can have the following change:

    diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
    index 002f4a935b1..e8d3fd0d50a 100644
    --- a/gdb/breakpoint.c
    +++ b/gdb/breakpoint.c
    @@ -5339,7 +5339,8 @@ bpstat_check_breakpoint_conditions (bpstat *bs, thread_info *thread)
     
       /* Evaluate extension language breakpoints that have a "stop" method
          implemented.  */
    -  bs->stop = breakpoint_ext_lang_cond_says_stop (b);
    +  if (b->disposition != disp_del_at_next_stop)
    +    bs->stop = breakpoint_ext_lang_cond_says_stop (b);
     
       if (is_watchpoint (b))
         {

I believe this is what you are looking for.  That being said, this is my
approach to solve your problem, a maintainer might have a better way to
address this.

>  }
>  
>  /* Extract a bitfield value from value VAL using the bit parameters contained in
> diff --git a/gdb/testsuite/gdb.python/py-watchpoint.c b/gdb/testsuite/gdb.python/py-watchpoint.c
> new file mode 100644
> index 00000000000..6d02e87e571
> --- /dev/null
> +++ b/gdb/testsuite/gdb.python/py-watchpoint.c
> @@ -0,0 +1,27 @@
> +/* 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 <stdio.h>
> +
> +int
> +main ()
> +{
> +  int i;
> +  for (i = 0; i < 3; i++)
> +    printf ("%d", i);
> +  return 0;
> +}
> diff --git a/gdb/testsuite/gdb.python/py-watchpoint.exp b/gdb/testsuite/gdb.python/py-watchpoint.exp
> new file mode 100644
> index 00000000000..863f3eff66a
> --- /dev/null
> +++ b/gdb/testsuite/gdb.python/py-watchpoint.exp
> @@ -0,0 +1,44 @@
> +# 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/>.
> +
> +# This file is part of the GDB testsuite.  It checks Watchpoints
> +# are deleted after used.
> +
> +load_lib gdb-python.exp
> +
> +standard_testfile
> +
> +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
> +}
> +
> +#
> +# Check Watchpoints are deleted after used
> +#
> +
> +gdb_test "set can-use-hw-watchpoints 0" ".*" "Don't use hardware watchpoints"

In this case, you could probably use gdb_test_no_output

> +gdb_test "python print (len(gdb.breakpoints()))" "1" "check default BP count"

It looks odd to have a space after print and not before the other
parenthesis.  To follow python coding convention, I would remove the
space between "print" and the "(".  Same remark holds for the two other
occurrences of this pattern below.

Best,
Lancelot.

> +gdb_test "source ${srcdir}/${subdir}/${testfile}.py" ".*Python script imported.*" \
> +	 "import python scripts"
> +gdb_test "python print (len(gdb.breakpoints()))" "2" "check modified BP count"
> +gdb_test "continue" ".*Watchpoint Hit: 4.*" "run until program stops"
> +gdb_test "python print (len(gdb.breakpoints()))" "1" "check BP count"
> diff --git a/gdb/testsuite/gdb.python/py-watchpoint.py b/gdb/testsuite/gdb.python/py-watchpoint.py
> new file mode 100644
> index 00000000000..855c820b245
> --- /dev/null
> +++ b/gdb/testsuite/gdb.python/py-watchpoint.py
> @@ -0,0 +1,31 @@
> +# 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/>.
> +
> +# This file is part of the GDB testsuite.  It checks Watchpoints
> +# are deleted after used.
> +
> +class MyBreakpoint(gdb.Breakpoint):
> +    def __init__(self, *args, **kwargs):
> +        gdb.Breakpoint.__init__(self, *args, **kwargs)
> +        self.i = 0
> +    def stop(self):
> +        self.i += 1
> +        print("Watchpoint Hit:", self.i)
> +        gdb.execute('print i')
> +        return False
> +
> +MyBreakpoint('i', gdb.BP_WATCHPOINT)
> +
> +print("Python script imported")
> -- 
> 2.34.1
> 
> base-commit: c99b2113a478a075e8d8f7c3848a92f1ce73f847