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

[Andrew Burgess <aburgess@redhat.com>]

Pedro Alves <pedro@palves.net> writes:

> On 2023-07-07 22:18, Andrew Burgess wrote:
>> Pedro Alves <pedro@palves.net> writes:
>> 
>>> On 2023-07-07 16:20, Pedro Alves wrote:
>>>> I'd think the second *stopped shouldn't even be there at all, following the
>>>> logic used to remove the stop from the CLI.
>>>>
>>>> Maybe we should try moving or copying this "don't report this stop" logic to
>>>> fetch_inferior_event, to avoid calling the second normal_stop at all.
>>>> I gave it a quick try, but what I tried without thinking much about it
>>>> just hung GDB.  I didn't dig too much as I want to move on to review the rest
>>>> of your series.
>>>
>>> Here's the totally broken naive patch I tried.
>>>
>>> From e1fde974fa58351de9f6dc0661162d77fc8be078 Mon Sep 17 00:00:00 2001
>>> From: Pedro Alves <pedro@palves.net>
>>> Date: Fri, 7 Jul 2023 16:10:29 +0100
>>> Subject: [PATCH] mi
>>>
>>> Change-Id: Ie49a36dc67b9584c40daabfffae1f87f6dd98c5c
>>> ---
>>>  gdb/infrun.c | 11 +++++++----
>>>  1 file changed, 7 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/gdb/infrun.c b/gdb/infrun.c
>>> index 3dd24fccf6e..ab1fd13abe0 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);
>>>  
>>> +    stop_context saved_context;
>>> +
>>>      /* Now figure out what to do with the result of the result.  */
>>>      handle_inferior_event (&ecs);
>>>  
>>> @@ -4415,16 +4417,17 @@ fetch_inferior_event ()
>>>  	  }
>>>  	else
>>>  	  {
>>> -	    bool should_notify_stop = true;
>>>  	    bool proceeded = false;
>>>  
>>>  	    stop_all_threads_if_all_stop_mode ();
>>>  
>>>  	    clean_up_just_stopped_threads_fsms (&ecs);
>>>  
>>> -	    if (thr != nullptr && thr->thread_fsm () != nullptr)
>>> -	      should_notify_stop
>>> -	       = thr->thread_fsm ()->should_notify_stop ();
>>> +	    bool should_notify_stop
>>> +		= (!saved_context.changed ()
>>> +		   && thr != nullptr
>>> +		   && thr->thread_fsm () != nullptr
>>> +		   && thr->thread_fsm ()->should_notify_stop ());
>> 
>> So, I got this working, as in, not locking up GDB.  The problem was that
>> thr->thread_fsm() is often nullptr, so we end up never calling
>> normal_stop.
>> 
>> However, I don't think this is going to work out.  There's a couple of
>> problems.
>> 
>> First, at the point fetch_inferior_event is called, there is no thread
>> selected, so the ptid and thread captured in saved_context are always
>> null_ptid and nullptr respectively.  By the time we call
>> saved_context.changed() we do have a thread selected, so the context
>> appears to have always changed.
>
> Yeah.
>
>> However, I figured I could work around this be replacing the full
>> stop_context with a simple 'int stop_id = get_stop_id ();', I figured
>> this might be enough, this will tell me if GDB has notified the user of
>> a stop somewhere within the handle_inferior_event code.
>> 
>> This works fine in the sense that I do now correctly spot that the user
>> has been notified of a stop and don't call normal_stop again...
>> 
>> ... but as a consequence, GDB now doesn't realise that is has stopped,
>> and doesn't display a prompt back to the user.
>> 
>> I'm going to have to leave this for the weekend now, but will continue
>> looking at this next week.  I've included my WIP hack below.
>> 
>> Would be interested on whether you see any problems with just using
>> get_stop_id() to determine if the user has already seen a stop or not? 
>
> I don't see a problem offhand.

Pedro,

It took a little longer than I'd have liked, but I think I have
something which addresses the problems you pointed out.

Let me know what you think.

Thanks,
Andrew

---

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
    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
+	 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.
+
+     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 as 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.
+
+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\",.*}"] \
+    "wait for stop"