Is full_page_writes=off safe in conjunction with PITR?

While thinking about the patch I just made to allow full_page_writes to
be turned off again, it struck me that this patch only fixes the problem
for post-crash XLOG replay. There is still a hazard if the variable is
turned off in a PITR master system. The reason is that while a base
backup is being taken, the backup-taker might read an inconsistent state
of a page and include that in the backup. This is not a problem if
full_page_writes is ON --- it's logically equivalent to a torn page
write and will be fixed on the slave by XLOG replay. But it *is* a
problem if full_page_writes is OFF, for exactly the same reason that
torn page writes are a problem.

I think we had originally argued that there was no problem anyway
because the kernel should cause the page write to appear atomic to other
processes (since we issue it in a single write() command). But that's
only true if the backup-taker reads in units that are multiples of
BLCKSZ. If the backup-taker reads, say, 4K at a time then it's
certainly possible that it gets a later version of the second half of a
page than it got of the first half. I don't know about you, but I sure
don't feel comfortable making assumptions at that level about the
behavior of tar or cpio.

I fear we still have to disable full_page_writes (force it ON) if
XLogArchivingActive is on. Comments?

regards, tom lane