Re: [PATCH 01/10] strbuf: Add strbuf_read_noblock

[Jeff King <peff@peff.net>]

On Thu, Sep 17, 2015 at 09:44:19AM -0700, Junio C Hamano wrote:

> > Arguably we should actually return the number of bytes we _did_ read,
> > but then caller cannot easily tell the difference between EOF and
> > EAGAIN.
> 
> Why can't it check errno==EAGAIN/EWOULDBLOCK?

Is it trustworthy to check errno if read() has not actually ever
returned -1? E.g., consider this sequence:

  1. somebody (maybe even us calling strbuf_read) calls read() and it
     returns -1, setting errno to EAGAIN

  2. we call strbuf_read()

    2a. it calls read(), which returns N bytes

    2b. it calls read() again, which returns 0 for EOF

    2c. it returns N, because that's how many bytes it read

  3. we wonder if we hit EOF, or if we simply need to read again. We
     check errno == EAGAIN

I don't think the read calls in step 2 are guaranteed to clear errno,
and we might read the cruft from step 1.

-Peff