[PATCH V2] posix: insert seek between reads and writes on Solaris (issue4943)

[Matt Mackall]

On Sat, 2015-12-05 at 21:26 -0800, Gregory Szorc wrote:
> On Fri, Dec 4, 2015 at 6:51 PM, Pierre-Yves David <
> pierre-yves.david at ens-lyon.org> wrote:
> 
> > So what is the state of this patch.
> > 
> > Is ther any chance that we eventually take it as is or are we going
> > to use
> > a modified approach anyway?
> 
> 
> Some Solaris distributions are definitely busted. I believe mpm was
> investigating the scope of the bustage (OS-level POSIX semantics/bug
> versus
> Python behavior/bug).

As best I can tell from poring over the reports, it's a bug in the
stdio buffering layer in the C library and not in Python or the kernel.
To confirm that, we'd really need to write a pure C test case to mirror
Greg's minimal Python test.

The fact that we have the same bug in both Windows and (some) Solaris
makes me pretty anxious that it's more widespread and we're opening
ourselves up to corruption more widely.

Because the buffer is a complicated read/write cache that emulates the
file offset and the rules around writes in "a+" mode are a bit subtle,
it seems likely to me that bugs in this code could be somewhat
widespread. If we look at Py3's rewrite of this code in the io module,
it has indeed had a few bugs in this area of its own.

So I'm frankly leaning towards just inserting a seek in the revlog code
to be safe.

-- 
Mathematics is the supreme nostalgia of our time.