Re: [PATCH v2 7/7] t/helper/simple-ipc: convert test-simple-ipc to use start_bg_command

[Ramsay Jones <ramsay@ramsayjones.plus.com>]

On 08/11/2021 23:59, Johannes Schindelin wrote:
[snip]
> I had a look at this and could reproduce... partially. I only managed to
> make it fail every once in a while.
> 
> Digging deeper, it turns out that the `lstat()` call in
> `ipc_get_active_state()` does not receive an `st_mode` indicating a
> socket, but rather a file (in my tests, it was usually 0100644, but
> sometimes even 0100755).
> 
> The reason is, of course, that Cygwin _emulates_ Unix sockets. What is in
> the file system is just a special file, it is marked with the `system` bit
> (which only exists on Windows), and its contents start with the tell-tale
> `!<socket>`.
> 
> And as you might have guessed, there is a race going on between Cygwin
> writing that file _and_ flipping that `system` bit, and Git trying to
> access the Unix socket and encountering an unexpected file.
> 
> Now, why this only happens in your 32-bit setup, I have no idea.
> 
> In my tests, the following patch works around the issue. Could I ask you
> to test it in your environment?

Just FYI, I just tried the patch below (on 64-bit cygwin) and this test
now works fine for me. (well, run 5 times by hand - not with --stress).

This is on windows 10 21H1 and cygwin:

  $ uname -a
  CYGWIN_NT-10.0 satellite 3.3.2(0.341/5/3) 2021-11-08 16:55 x86_64 Cygwin
  $ 

[Yes, I updated last night!]

ATB,
Ramsay Jones

> -- snip --
> diff --git a/compat/simple-ipc/ipc-unix-socket.c
> b/compat/simple-ipc/ipc-unix-socket.c
> index 4e28857a0a..1c591b2adf 100644
> --- a/compat/simple-ipc/ipc-unix-socket.c
> +++ b/compat/simple-ipc/ipc-unix-socket.c
> @@ -36,6 +36,23 @@ enum ipc_active_state ipc_get_active_state(const char
> *path)
>  	}
> 
>  	/* also complain if a plain file is in the way */
> +#ifdef __CYGWIN__
> +	{
> +		static const int delay[] = { 1, 10, 20, 40, -1 };
> +		int i;
> +
> +		for (i = 0; S_ISREG(st.st_mode) && delay[i] > 0; i++) {
> +			/*
> +			 * Cygwin might still be in the process of marking the
> +			 * underlying file as a system file.
> +			 */
> +			sleep_millisec(delay[i]);
> +			if (lstat(path, &st) == -1)
> +				return IPC_STATE__INVALID_PATH;
> +		}
> +	}
> +#endif
> +
>  	if ((st.st_mode & S_IFMT) != S_IFSOCK)
>  		return IPC_STATE__INVALID_PATH;
> 
> -- snap --
> 
> FWIW it looks as if the loop might be a bit of an overkill, as I could not
> get the code to need more than a single one-millisecond sleep, but it's
> probably safer to just keep the delay loop in place as-is.
> 
> Ciao,
> Dscho
>