Jeff King <peff@xxxxxxxx> 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;
