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