From: Eric Wong <e@80x24.org>
Date: Mon, 27 Jun 2016 23:19:49 GMT
Subject: Re: [PATCH 1/2] xread: retry after poll on EAGAIN/EWOULDBLOCK
Message-ID: <20160627231949.GA23661@dcvr.yhbt.net>
In-Reply-To: <20160627222238.GA23645@sigill.intra.peff.net>

Jeff King <peff@peff.net> wrote:
> On Mon, Jun 27, 2016 at 05:49:48PM -0400, Jeff King wrote:
> 
> > So in general I would say that handing non-blocking descriptors to git
> > is not safe.

Indeed.  This also makes me wonder if our output to stdout/stderr
suffer from the same theoretical problems if given non-blocking
outputs; I suspect they do.

>>  I think it's possible to loop on getdelim() when we see
>> EAGAIN, but I'm not sure if it's worth it.

> The patch for that is below, for the curious. It works with even this:
> 
>   {
>     for i in H E A D; do
>       sleep 1
>       printf $i
>     done
>     printf "\n"
>   } | nonblock git cat-file --batch-check
> 
> Note that I folded the "did we see EAGAIN" check into my sub-function
> (which is the equivalent of your io_wait). You might want to do that in
> the xread() code path, too, as it makes the resulting code there a very
> nice:
> 
>   if (errno == EINTR)
> 	continue;
>   if (handle_nonblock(fd, POLLIN))
> 	continue;

Yes :)

> +int handle_nonblock(FILE *fp, short poll_events)
> +{
> +	if (ferror(fp) && (errno == EAGAIN || errno == EWOULDBLOCK)) {

<snip>

> +		clearerr(fp);
> +		return 1;
> +	}
> +	return 0;
> +}
> +
> +static int getline_stdio_loop(struct strbuf *sb, FILE *fp, int term)
> +{
> +	int ch;
> +	do {
> +		errno = 0;
> +		flockfile(fp);
> +		while ((ch = getc_unlocked(fp)) != EOF) {

<snip>

> +		}
> +		funlockfile(fp);
> +	} while (handle_nonblock(fp, POLLIN));

I haven't used stdio in a while and I'm glad :)
Error handling with ferror + clearerr + errno checking is
difficult and error-prone.

Linus said this back in 2006, too:
http://mid.gmane.org/Pine.LNX.4.64.0609141023130.4388@g5.osdl.org

So I wonder if we're better off relying entirely on xread/xwrite
+ strbuf for all our buffering.