Re: [PATCH v3 1/4] t5000: test tar files that overflow ustar headers

[Jeff King <peff@peff.net>]

On Fri, Jun 24, 2016 at 03:07:44PM -0400, Jeff King wrote:

> > "dd bs=1 count=4096" is hopefully more portable.
> 
> Hmm. I always wonder whether dd is actually very portable, but we do use
> it already, at least.
> 
> Perhaps the perl monstrosity in t9300 could be replaced with that, too.

Hrm. So I wrote a patch for t9300 for this. But I wanted to flip the
order to:

  dd bs=4096 count=1

because otherwise, dd will call read() 4096 times, for 1 byte each.

But it's not safe to do that on a pipe. For example:

  {
	echo 1
	sleep 1
	echo 2
  } | dd bs=4 count=1

will copy only 2 bytes. So it's racily wrong, depending on how the
writer feeds the data to write().

The 1-byte reads do work (assuming blocking descriptors and that dd
restarts a read after a signal, which mine seems to). But yuck.

The difference in time between the two is measurable on my system, but
it's only a few milliseconds (for 4096 bytes). So maybe it's not worth
worrying about (though as a general technique, it does make me worry
that it's easy to get wrong in a way that will fail racily).

-Peff