Re: [RFC/PATCH] pipe_command(): mark stdin descriptor as non-blocking

[Junio C Hamano <gitster@pobox.com>]

Jeff King <peff@peff.net> writes:

> Our pipe_command() helper lets you both write to and read from a child
> process on its stdin/stdout. It's supposed to work without deadlocks
> because we use poll() to check when descriptors are ready for reading or
> writing. But there's a bug: if both the data to be written and the data
> to be read back exceed the pipe buffer, we'll deadlock.
> ...
> If you set add.interactive.useBuiltin to false, the problem goes away,
> because now we're not using pipe_command() anymore (instead, that part
> happens in perl). But this isn't a bug in the interactive code at all.
> It's the underlying pipe_command() code which is broken, and has been
> all along.
> ...
> The obvious fix is to put the descriptor into non-blocking mode, and
> indeed, that makes the problem go away. Callers shouldn't need to
> care, because they never see the descriptor (they hand us a buffer to
> feed into it).

Thanks for a very well reasoned and explained patch.

>   - more importantly, I'm not sure of the portability implications of
>     the fix. This is our first use of O_NONBLOCK outside of the
>     compat/simple-ipc unix-socket code. Do we need to abstract this
>     behind a compat/ layer for Windows?

Yup.  A very good question to ask for the platform maintainer.

>  run-command.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff --git a/run-command.c b/run-command.c
> index 14f17830f5..45bffb4b11 100644
> --- a/run-command.c
> +++ b/run-command.c
> @@ -1418,6 +1418,14 @@ static int pump_io(struct io_pump *slots, int nr)
>  	return 0;
>  }
>  
> +static int make_nonblock(int fd)
> +{
> +	int flags = fcntl(fd, F_GETFL);
> +	if (flags < 0)
> +		return -1;
> +	flags |= O_NONBLOCK;
> +	return fcntl(fd, F_SETFL, flags);
> +}
>  
>  int pipe_command(struct child_process *cmd,
>  		 const char *in, size_t in_len,
> @@ -1438,6 +1446,15 @@ int pipe_command(struct child_process *cmd,
>  		return -1;
>  
>  	if (in) {
> +		if (make_nonblock(cmd->in) < 0) {
> +			error_errno("unable to make pipe non-blocking");
> +			close(cmd->in);
> +			if (out)
> +				close(cmd->out);
> +			if (err)
> +				close(cmd->err);
> +			return -1;
> +		}
>  		io[nr].fd = cmd->in;
>  		io[nr].type = POLLOUT;
>  		io[nr].u.out.buf = in;