Re: O_DIRECT on macOS

On Mon, May 31, 2021 at 8:12 AM Andres Freund <andres(at)anarazel(dot)de> wrote:
> On 2021-05-30 16:39:48 +1200, Thomas Munro wrote:
> > I thought about a few different ways to encapsulate this API
> > difference in PostgreSQL, and toyed with two:
> >
> > 1. We could define our own fake O_DIRECT flag, and translate that to
> > the right thing inside BasicOpenFilePerm(). That seems a bit icky.
> > We'd have to be careful not to collide with system defined flags and
> > worry about changes. We do that sort of thing for Windows, though
> > that's a bit different, there we translate *all* the flags from
> > POSIXesque to Windowsian.
> >
> > 2. We could make an extended BasicOpenFilePerm() variant that takes a
> > separate boolean parameter for direct, so that we don't have to hijack
> > any flag space, but now we need new interfaces just to tolerate a
> > rather niche system.
>
> I don't think 2) really covers the problem on its own. It's fine for
> things that directly use BasicOpenFilePerm(), but what about "virtual
> file descriptors" (PathNameOpenFile())? I.e. what md.c et al use? There
> we need to store the fact that we want non-buffered IO as part of the
> vfd, otherwise we'll loose that information when re-opening the file
> later.

Right, a bit more API perturbation is required to traffic the separate
flags around for VFDs, which is all a bit unpleasant for a feature
that most people don't care about.

For comparison, here is my sketch of idea #1. I pick an arbitrary
value to use as PG_O_DIRECT (I don't want to define O_DIRECT for fear
of breaking other code that might see it and try to pass it into
open()... for all I know, it might happen to match OS-internal value
O_NASAL_DEMONS), and statically assert that it doesn't collide with
standard flags we're using, and I strip it out of the flags I pass in
to open(). As I said, a bit icky, but it's a tiny and localised
patch, which is nice.

I also realised that it probably wasn't right to raise an ERROR, so in
this version I return -1 when fcntl() fails.