Re: [PATCHv5 05/11] gdb: don't always print breakpoint location after failed condition check

[Pedro Alves <pedro@palves.net>]

Hi!

Overall this looks good.  Some nits below.

On 2023-07-14 13:17, Andrew Burgess wrote:

> commit bd3309a50423114c2cb31241500820efa21d0452
> Author: Andrew Burgess <aburgess@redhat.com>
> Date:   Wed Jul 12 21:56:50 2023 +0100
> 
>     gdb: avoid double stop after failed breakpoint condition check
>     
>     This commit replaces this earlier commit:
>     
>       commit 2e411b8c68eb2b035b31d5b00d940d4be1a0928b
>       Date:   Fri Oct 14 14:53:15 2022 +0100
>     
>           gdb: don't always print breakpoint location after failed condition check
>     
>     and is a result of feedback received here[1].
>     
>     The original commit addressed a problem where, if a breakpoint
>     condition included an inferior function call, and if the inferior
>     function call fail, then GDB would announce the stop twice.  Here's an

"fail" -> "failed" or "fails", I guess.

>     example of GDB's output before the above commit that shows the problem
>     being addressed:
>     
>       (gdb) break foo if (some_func ())
>       Breakpoint 1 at 0x40111e: file bpcond.c, line 11.
>       (gdb) r
>       Starting program: /tmp/bpcond
>     
>       Program received signal SIGSEGV, Segmentation fault.
>       0x0000000000401116 in some_func () at bpcond.c:5
>       5       return *p;
>       Error in testing condition for breakpoint 1:
>       The program being debugged stopped while in a function called from GDB.
>       Evaluation of the expression containing the function
>       (some_func) will be abandoned.
>       When the function is done executing, GDB will silently stop.
>     
>       Breakpoint 1, 0x0000000000401116 in some_func () at bpcond.c:5
>       5       return *p;
>       (gdb)
>     
>     The original commit addressed this issue in breakpoint.c, by spotting
>     that the $pc had changed while evaluating the breakpoint condition,
>     and inferring from this that GDB must have stopped elsewhere.
>     
>     However, the way in which the original commit suppressed the second
>     stop announcement was to set bpstat::print to true -- this tells GDB
>     not to print the frame during the stop announcement, and for the CLI
>     this is fine, however, it was pointed out that for the MI this still
>     isn't really enough.  Below is an example from an MI session after the
>     above commit was applied, this shows the problem with the above
>     commit:
>     
>       -break-insert -c "cond_fail()" foo
>       ^done,bkpt={number="1",type="breakpoint",disp="keep",enabled="y",addr="0x000000000040111e",func="foo",file="/tmp/mi-condbreak-fail.c",line="30",thread-groups=["i1"],cond="cond_fail()",times="0",original-location="foo"}
>       (gdb)
>       -exec-run
>       =thread-group-started,id="i1",pid="2636270"
>       =thread-created,id="1",group-id="i1"
>       =library-loaded,id="/lib64/ld-linux-x86-64.so.2",target-name="/lib64/ld-linux-x86-64.so.2",host-name="/lib64/ld-linux-x86-64.so.2",symbols-loaded="0",thread-group="i1",ranges=[{from="0x00007ffff7fd3110",to="0x00007ffff7ff2bb4"}]
>       ^running
>       *running,thread-id="all"
>       (gdb)
>       =library-loaded,id="/lib64/libm.so.6",target-name="/lib64/libm.so.6",host-name="/lib64/libm.so.6",symbols-loaded="0",thread-group="i1",ranges=[{from="0x00007ffff7e59390",to="0x00007ffff7ef4f98"}]
>       =library-loaded,id="/lib64/libc.so.6",target-name="/lib64/libc.so.6",host-name="/lib64/libc.so.6",symbols-loaded="0",thread-group="i1",ranges=[{from="0x00007ffff7ca66b0",to="0x00007ffff7df3c5f"}]
>       ~"\nProgram"
>       ~" received signal SIGSEGV, Segmentation fault.\n"
>       ~"0x0000000000401116 in cond_fail () at /tmp/mi-condbreak-fail.c:24\n"
>       ~"24\t  return *p;\t\t\t/* Crash here.  */\n"
>       *stopped,reason="signal-received",signal-name="SIGSEGV",signal-meaning="Segmentation fault",frame={addr="0x0000000000401116",func="cond_fail",args=[],file="/tmp/mi-condbreak-fail.c",fullname="/tmp/mi-condbreak-fail.c",line="24",arch="i386:x86-64"},thread-id="1",stopped-threads="all",core="9"
>       &"Error in testing condition for breakpoint 1:\n"
>       &"The program being debugged was signaled while in a function called from GDB.\n"
>       &"GDB remains in the frame where the signal was received.\n"
>       &"To change this behavior use \"set unwindonsignal on\".\n"
>       &"Evaluation of the expression containing the function\n"
>       &"(cond_fail) will be abandoned.\n"
>       &"When the function is done executing, GDB will silently stop.\n"
>       =breakpoint-modified,bkpt={number="1",type="breakpoint",disp="keep",enabled="y",addr="0x000000000040111e",func="foo",file="/tmp/mi-condbreak-fail.c",fullname="/tmp/mi-condbreak-fail.c",line="30",thread-groups=["i1"],cond="cond_fail()",times="1",original-location="foo"}
>       *stopped
>       (gdb)
>     
>     Notice that we still see two '*stopped' lines, the first includes the
>     full frame information, while the second has no frame information,
>     this is a result of bpstat::print having been set.  Ideally, the
>     second '*stopped' line should not be present.
>     
>     By setting bpstat::print I was addressing the problem too late, this
>     flag really only changes how interp::on_normal_stop prints the stop
>     event, and interp::on_normal_stop is called (indirectly) from the
>     normal_stop function in infrun.c.  A better solution is to avoid
>     calling normal_stop at all for the stops which should not be reported
>     to the user, and this is what I do in this commit.
>     
>     This commit has 3 parts:
>     
>       1. In breakpoint.c, revert the above commit,
>     
>       2. In fetch_inferior_event (infrun.c), capture the stop-id before
>       calling handle_inferior_event.  If, after calling
>       handle_inferior_event, the stop-id has changed, then this indicates
>       that somewhere within handle_inferior_event, a stop was announced to
>       the user.  If this is the case then GDB should not call normal_stop,
>       and we should rely on whoever announced the stop to ensure that we
>       are in a PROMPT_NEEDED state, which means the prompt will be
>       displayed once fetch_inferior_event returns.  And,
>     
>       3. In infcall.c, do two things:
>     
>          (a) In run_inferior_call, after making the inferior call, ensure
>          that either async_disable_stdin or async_enable_stdin is called
>          to put the prompt state, and stdin handling into the correct
>          state based on whether the inferior call completed successfully
>          or not, and
>     
>          (b) In call_thread_fsm::should_stop, call async_enable_stdin
>          rather than changing the prompt state directly.  This isn't
>          strictly necessary, but helped me understand this code more.
>          This async_enable_stdin call is only reached if normal_stop is
>          not going to be called, and replaces the async_enable_stdin call
>          that exists in normal_stop.  Though we could just adjust the
>          prompt state if felt (to me) much easier to understand when I
>          could see this call and the corresponding call in normal_stop.
>     
>     With these changes in place now, when the inferior call (from the
>     breakpoint condition) fails, infcall.c leaves the prompt state as
>     PROMPT_NEEDED, and leaves stdin registered with the event loop.
>     
>     Back in fetch_inferior_event GDB notices that the stop-id has changed
>     and so avoids calling normal_stop.
>     
>     And on return from fetch_inferior_event GDB will display the prompt
>     and handle input from stdin.
>     
>     As normal_stop is not called the MI problem is solved, and the test
>     added in the earlier mentioned commit still passes just fine, so the
>     CLI has not regressed.
>     
>     [1] https://inbox.sourceware.org/gdb-patches/6fd4aa13-6003-2563-5841-e80d5a55d59e@palves.net/
> 
> diff --git a/gdb/breakpoint.c b/gdb/breakpoint.c
> index d898167b7e1..93634bd7e51 100644
> --- a/gdb/breakpoint.c
> +++ b/gdb/breakpoint.c
> @@ -5535,7 +5535,6 @@ bpstat_check_breakpoint_conditions (bpstat *bs, thread_info *thread)
>  	  else
>  	    within_current_scope = false;
>  	}
> -      CORE_ADDR pc_before_check = get_frame_pc (get_selected_frame (nullptr));
>        if (within_current_scope)
>  	{
>  	  try
> @@ -5555,17 +5554,6 @@ bpstat_check_breakpoint_conditions (bpstat *bs, thread_info *thread)
>  		  (gdb_stderr, ex,
>  		   "Error in testing condition for breakpoint %d:\n",
>  		   b->number);
> -
> -	      /* If the pc value changed as a result of evaluating the
> -		 condition then we probably stopped within an inferior
> -		 function call due to some unexpected stop, e.g. the thread
> -		 hit another breakpoint, or the thread received an
> -		 unexpected signal.  In this case we don't want to also
> -		 print the information about this breakpoint.  */
> -	      CORE_ADDR pc_after_check
> -		= get_frame_pc (get_selected_frame (nullptr));
> -	      if (pc_before_check != pc_after_check)
> -		bs->print = 0;
>  	    }
>  	}
>        else
> diff --git a/gdb/infcall.c b/gdb/infcall.c
> index ddf325a62a5..0944097e18d 100644
> --- a/gdb/infcall.c
> +++ b/gdb/infcall.c
> @@ -564,10 +564,13 @@ call_thread_fsm::should_stop (struct thread_info *thread)
>  	 call..  */
>        return_value = get_call_return_value (&return_meta_info);
>  
> -      /* Break out of wait_sync_command_done.  */
> +      /* Break out of wait_sync_command_done.  This is similar to the
> +	 async_enable_stdin call in normal_stop (which we don't call),
> +	 however, in this case we only change the WAITING_UI, this is

Full stop after WAITING_UI.

> +	 enough for wait_sync_command_done.  */
>        scoped_restore save_ui = make_scoped_restore (&current_ui, waiting_ui);
> -      target_terminal::ours ();
> -      waiting_ui->prompt_state = PROMPT_NEEDED;
> +      gdb_assert (current_ui->prompt_state == PROMPT_BLOCKED);
> +      async_enable_stdin ();
>      }
>  
>    return true;
> @@ -661,14 +664,30 @@ run_inferior_call (std::unique_ptr<call_thread_fsm> sm,
>    infcall_debug_printf ("thread is now: %s",
>  			inferior_ptid.to_string ().c_str ());
>  
> -  /* If GDB has the prompt blocked before, then ensure that it remains
> -     so.  normal_stop calls async_enable_stdin, so reset the prompt
> -     state again here.  In other cases, stdin will be re-enabled by
> -     inferior_event_handler, when an exception is thrown.  */
> +  /* After the inferior call async_enable_stdin has been called, either
> +     from normal_stop or from call_thread_fsm::should_stop, and the prompt
> +     state has been restored by the scoped_restore in the try block above.

It took me several reads to understand this comment.  I kept misreading this
as "After ... async_enable_stdin has been called, then ..."

I think putting a comma here

 /* After the inferior call, async_enable_stdin has been called, either
                          ^^^

would have helped me.  Maybe even say:

   "After the inferior call finishes, ".

?

> +
> +     If the inferior call finished successfully then we should disable
> +     stdin as we don't know yet whether the inferior will be stopping,

The last comma should be a period, IMO.

> +     calling async_disable_stdin restores things as they were when this
> +     function was called.

"restores things as they were" -> "restores things to how they were" ?

> +
> +     If the inferior call didn't complete successfully then normal_stop has

Comma before "then".

> +     already been called, and we know for sure that we are going to present
> +     this stop to the user, in this case we call async_enable_stdin, this

Period after "the user" instead of comma.  Comma after "this case".
Also period after async_enable_stdin; alternatively, say "which" instead of
"this".

> +     changes the prompt state to PROMPT_NEEDED.
> +
> +     If the previous prompt state was PROMPT_NEEDED then, as

I think you mean to put the comma before "then" instead of after?
Or is this "then" refering to some previously-referred-to situation?

> +     async_enable_stdin has already been called, nothing additional needs
> +     to be done here.  */

I found it much easier to understand the whole comment block without applying
my suggestions above locally.  Here is the result for your reference, and to
double check I'm parsing this all correctly:

  /* After the inferior call finished, async_enable_stdin has been
     called, either from normal_stop or from
     call_thread_fsm::should_stop, and the prompt state has been
     restored by the scoped_restore in the try block above.

     If the inferior call finished successfully, then we should
     disable stdin as we don't know yet whether the inferior will be
     stopping.  Calling async_disable_stdin restores things to how
     they were when this function was called.

     If the inferior call didn't complete successfully, then
     normal_stop has already been called, and we know for sure that we
     are going to present this stop to the user.  In this case, we
     call async_enable_stdin.  This changes the prompt state to
     PROMPT_NEEDED.

     If the previous prompt state was PROMPT_NEEDED, then as
     async_enable_stdin has already been called, nothing additional
     needs to be done here.  */


>    if (current_ui->prompt_state == PROMPT_BLOCKED)
> -    current_ui->unregister_file_handler ();
> -  else
> -    current_ui->register_file_handler ();
> +    {
> +      if (call_thread->thread_fsm ()->finished_p ())
> +	async_disable_stdin ();
> +      else
> +	async_enable_stdin ();
> +    }
>  
>    /* If the infcall does NOT succeed, normal_stop will have already
>       finished the thread states.  However, on success, normal_stop
> diff --git a/gdb/infrun.c b/gdb/infrun.c
> index 58da1cef29e..95ab7c6f7dc 100644
> --- a/gdb/infrun.c
> +++ b/gdb/infrun.c
> @@ -4395,6 +4395,8 @@ fetch_inferior_event ()
>      auto defer_delete_threads
>        = make_scope_exit (delete_just_stopped_threads_infrun_breakpoints);
>  
> +    int stop_id = get_stop_id ();
> +
>      /* Now figure out what to do with the result of the result.  */
>      handle_inferior_event (&ecs);
>  
> @@ -4422,7 +4424,19 @@ fetch_inferior_event ()
>  
>  	    clean_up_just_stopped_threads_fsms (&ecs);
>  
> -	    if (thr != nullptr && thr->thread_fsm () != nullptr)
> +	    if (stop_id != get_stop_id ())
> +	      {
> +		/* If the stop-id has changed then a stop has already been
> +		   presented to the user in handle_inferior_event, this is
> +		   likely a failed inferior call.  As the stop has already
> +		   been announced then we should not notify again.
> +
> +		   Also, if the prompt state is not PROMPT_NEEDED then GDB
> +		   will not be ready for user input after this function.  */
> +		should_notify_stop = false;
> +		gdb_assert (current_ui->prompt_state == PROMPT_NEEDED);
> +	      }
> +	    else if (thr != nullptr && thr->thread_fsm () != nullptr)
>  	      should_notify_stop
>  	       = thr->thread_fsm ()->should_notify_stop ();
>  
> diff --git a/gdb/testsuite/gdb.mi/mi-condbreak-fail.c b/gdb/testsuite/gdb.mi/mi-condbreak-fail.c
> new file mode 100644
> index 00000000000..94bd2484934
> --- /dev/null
> +++ b/gdb/testsuite/gdb.mi/mi-condbreak-fail.c
> @@ -0,0 +1,39 @@
> +/* Copyright 2023 Free Software Foundation, Inc.
> +
> +   This file is part of GDB.
> +
> +   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/>.  */
> +
> +volatile int global_counter = 0;
> +
> +int
> +cond_fail ()
> +{
> +  volatile int *p = 0;
> +  return *p;			/* Crash here.  */
> +}
> +
> +int
> +foo ()
> +{
> +  global_counter += 1;		/* Set breakpoint here.  */
> +  return 0;
> +}
> +
> +int
> +main ()
> +{
> +  int res = foo ();
> +  return res;
> +}
> diff --git a/gdb/testsuite/gdb.mi/mi-condbreak-fail.exp b/gdb/testsuite/gdb.mi/mi-condbreak-fail.exp
> new file mode 100644
> index 00000000000..86b1db5a8dd
> --- /dev/null
> +++ b/gdb/testsuite/gdb.mi/mi-condbreak-fail.exp
> @@ -0,0 +1,67 @@
> +# Copyright (C) 2023 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 when GDB fails to evaluate the condition of a conditional
> +# breakpoint we only get one *stopped message.

"message" -> "async record" or "notification".

> +
> +load_lib mi-support.exp
> +set MIFLAGS "-i=mi"
> +
> +standard_testfile
> +
> +if [build_executable ${testfile}.exp ${binfile} ${srcfile}] {
> +    return -1
> +}
> +
> +if {[mi_clean_restart $binfile]} {
> +    return
> +}
> +
> +if {[mi_runto_main] == -1} {
> +    return
> +}
> +
> +# Create the conditional breakpoint.
> +set bp_location [gdb_get_line_number "Set breakpoint here"]
> +mi_create_breakpoint "-c \"cond_fail ()\" $srcfile:$bp_location" \
> +    "insert conditional breakpoint" \
> +    -func foo -file ".*$srcfile" -line "$bp_location" \
> +    -cond "cond_fail \\(\\)"
> +
> +# Number of the previous breakpoint.
> +set bpnum [mi_get_valueof "/d" "\$bpnum" "INVALID" \
> +	       "get number for breakpoint"]
> +
> +# The line where we expect the inferior to crash.
> +set crash_linenum [gdb_get_line_number "Crash here"]
> +
> +# Run the inferior and wait for it to stop.
> +mi_send_resuming_command "exec-continue" "continue the inferior"
> +mi_gdb_test "" \
> +    [multi_line \
> +	 "~\"\\\\nProgram\"" \
> +	 "~\" received signal SIGSEGV, Segmentation fault\\.\\\\n\"" \
> +	 "~\"$hex in cond_fail \\(\\) at \[^\r\n\]+\"" \
> +	 "~\"${crash_linenum}\\\\t\\s+return \\*p;\[^\r\n\]+\\\\n\"" \
> +	 "\\*stopped,reason=\"signal-received\",signal-name=\"SIGSEGV\"\[^\r\n\]+" \
> +	 "&\"Error in testing condition for breakpoint $bpnum:\\\\n\"" \
> +	 "&\"The program being debugged was signaled while in a function called from GDB\\.\\\\n\"" \
> +	 "&\"GDB remains in the frame where the signal was received\\.\\\\n\"" \
> +	 "&\"To change this behavior use \\\\\"set unwindonsignal on\\\\\"\\.\\\\n\"" \
> +	 "&\"Evaluation of the expression containing the function\\\\n\"" \
> +	 "&\"\\(cond_fail\\) will be abandoned\\.\\\\n\"" \
> +	 "&\"When the function is done executing, GDB will silently stop\\.\\\\n\"" \
> +	 "=breakpoint-modified,bkpt={number=\"$bpnum\",type=\"breakpoint\",.*}"] \

I wonder about matching times="1" too, to confirm we're incrementing the hit
count even if the condition fails.

Also, that ".*" in the last line gave me pause.  It makes it so that a future
change that results in an output like:

  =breakpoint-modified,bkpt={number=\"$bpnum\",type=\"breakpoint\", [snip]
  *stopped
  =something-else-that-ends-in-curly-braces,{...}

would still pass.  Maybe use "\[^\r\n\]*" instead.

With the nits fixed, this LGTM.

Thanks!

Pedro Alves