Final(?) proposal for wal_sync_method changes

After reviewing the two ongoing threads about fixing the wal_sync_method
fiasco, I think there is general agreement on these two points:

1. open_datasync shouldn't be the default choice
2. O_DIRECT shouldn't be forcibly bundled in with O_DSYNC/O_SYNC

What I suggest we do about the latter is to invent two new
wal_sync_method values,
open_datasync_direct
open_sync_direct
which are defined only on platforms that define O_DIRECT (and O_DSYNC
or O_SYNC respectively). That puts it in the hands of the DBA whether
we try to use O_DIRECT or not. We'll still keep the hard-wired
optimization of disabling O_DIRECT when archiving or walreceiver are
active.

Dropping open_datasync as the first-choice default is something we have
to back-patch, but I'm less sure about it being a good idea to
back-patch the rearrangement of O_DIRECT management. Somebody who'd
explicitly specified open_sync or open_datasync as wal_sync_method
would find its behavior changing under him, which might be bad.

Comments?

regards, tom lane