Re: [PATCH 1/2] xread: retry after poll on EAGAIN/EWOULDBLOCK
- From
- Eric Wong <e@80x24.org>
- Date
- Jun 27, 2016, 23:19 UTC
- 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.
Show 19 quoted lines
> 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>
Show 13 quoted lines
> + 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.