[PR,python/29603] Fix deletion of Watchpoints

  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
  

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
  

Hi Lancelot,

Thanks for reviewing this!  I revised the patch and 

will send version 2 shortly.

Cheers,
Johnson