Re: [PATCH] Fix race in DAP startup

[Tom de Vries <tdevries@suse.de>]

On 3/22/23 15:34, Tom Tromey via Gdb-patches wrote:
> Internal AdaCore DAP testing on Windows has had occasional failures
> that show:
> 
>      assert threading.current_thread() is _dap_thread
> 
> I think this is a race in DAP startup: the _dap_thread global is only
> set on return from start_thread, but it seems possible that the thread
> itself could already run and encounter a @in_dap_thread decorator.
> 
> This patch fixes the problem by setting the global before running any
> of the code in the new thread.  This also lets us remove a FIXME.

Hi,

I tried this patch out, hoping it would fix the stability issues I have 
with the gdb.dap tests, but I'm still seeing those, so I filed a PR: 
https://sourceware.org/bugzilla/show_bug.cgi?id=30268 .

Thanks,
- Tom

> ---
>   gdb/python/lib/gdb/dap/server.py  |  4 +---
>   gdb/python/lib/gdb/dap/startup.py | 12 +++++++++---
>   2 files changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/gdb/python/lib/gdb/dap/server.py b/gdb/python/lib/gdb/dap/server.py
> index 92b4eee1c5e..ff88282049f 100644
> --- a/gdb/python/lib/gdb/dap/server.py
> +++ b/gdb/python/lib/gdb/dap/server.py
> @@ -100,9 +100,7 @@ class Server:
>           log("WROTE: <<<" + json.dumps(obj) + ">>>")
>           self.write_queue.put(obj)
>   
> -    # This must be run in the DAP thread, but we can't use
> -    # @in_dap_thread here because the global isn't set until after
> -    # this starts running.  FIXME.
> +    @in_dap_thread
>       def main_loop(self):
>           """The main loop of the DAP server."""
>           # Before looping, start the thread that writes JSON to the
> diff --git a/gdb/python/lib/gdb/dap/startup.py b/gdb/python/lib/gdb/dap/startup.py
> index 22246a937ce..aa834cdb14c 100644
> --- a/gdb/python/lib/gdb/dap/startup.py
> +++ b/gdb/python/lib/gdb/dap/startup.py
> @@ -59,14 +59,20 @@ def start_thread(name, target, args=()):
>       with blocked_signals():
>           result = threading.Thread(target=target, args=args, daemon=True)
>           result.start()
> -        return result
>   
>   
>   def start_dap(target):
>       """Start the DAP thread and invoke TARGET there."""
> -    global _dap_thread
>       exec_and_log("set breakpoint pending on")
> -    _dap_thread = start_thread("DAP", target)
> +
> +    # Functions in this thread contain assertions that check for this
> +    # global, so we must set it before letting these functions run.
> +    def really_start_dap():
> +        global _dap_thread
> +        _dap_thread = threading.current_thread()
> +        target()
> +
> +    start_thread("DAP", really_start_dap)
>   
>   
>   def in_gdb_thread(func):