[Bug gdb/29831] New: event loop deadlocks when using continue in breakpoint command and using gdb.execute in stop event hook

["dominik.b.czarnota+bugzilla at gmail dot com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=29831

            Bug ID: 29831
           Summary: event loop deadlocks when using continue in breakpoint
                    command and using gdb.execute in stop event hook
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: dominik.b.czarnota+bugzilla at gmail dot com
  Target Milestone: ---

Hi,

There is a case when GDB deadlocks or hang indefinetely when a breakpoint is
hit and:
1) the breakpoint has attached commands and when one of the commands is
`continue`
2) a stop event hook was registered before with a Python script and that hook
executes a `gdb.execute` function

This issue is painful for automating GDB and hits many GDB users that use
plugins written in Python (see [0]). I would appreciate if it is fixed sooner
than later, or, if you can provide any information that may be helpful for
fixing or diagnosing this further.

The bug can be reproduced by doing the following steps:

```
$ echo 'void foo(){}; int main() {foo();}' > example.c && gcc example.c

$ cat gdbs.py
import gdb
def stop_handler (event):
    print("Python stop handler started")
    print("Thread is stopped: " + str(gdb.selected_thread().is_stopped()))
    print("Thread is running: " + str(gdb.selected_thread().is_running()))
    (gdb.execute("",to_string=True))
    print("Thread is stopped: " + str(gdb.selected_thread().is_stopped()))
    print("Thread is running: " + str(gdb.selected_thread().is_running()))
    print("Python Stop handler finished\n")
gdb.events.stop.connect (stop_handler)

$ gdb --quiet --nx ./a.out
Reading symbols from ./a.out...
(No debugging symbols found in ./a.out)
(gdb) starti
Starting program: /home/dc/a.out

Program stopped.
0x00007ffff7dd4090 in _start () from /lib64/ld-linux-x86-64.so.2
(gdb) break foo
Breakpoint 1 at 0x5555555545fe
(gdb) commands
Type commands for breakpoint(s) 1, one per line.
End with a line saying just "end".
>continue
>end
(gdb) source gdbs.py
(gdb) c
Continuing.

Breakpoint 1, 0x00005555555545fe in foo ()
Python stop handler started
Thread is stopped: True
Thread is running: False
Thread is stopped: False
Thread is running: True
Python Stop handler finished

```

I was able to reproduce this bug with the following GDB versions:
- 10.2-0ubuntu1~18.04~2
- 12.1 compiled from sources (../configure && make -j)

[0] https://github.com/pwndbg/pwndbg/issues/425

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