a very minor bug and a couple of comment changes for basebackup.c

Here are a few small patches for basebackup.c:

0001 fixes what I believe to be a slight logical error in sendFile(),
introduced by me during the v15 development cycle when I introduced
the bbsink abstraction. I believe that it is theoretically possible
for this to cause an assertion failure, although the chances of that
actually happening seem extremely remote in practice. I don't believe
there are any consequences worse than that; for instance, I don't
think this can result in your backup getting corrupted. See the
proposed commit message for full details. Because I believe that this
is formally a bug, I am inclined to think that this should be
back-patched, but I also think it's fairly likely that no one would
ever notice if we didn't. However, patch authors have been known to be
wrong about the consequences of their own bugs from time to time, so
please do let me know if this seems more serious to you than what I'm
indicating, or conversely if you think it's not a problem at all for
some reason.

0002 removes an old comment from the file that I find useless and
slightly misleading.

0003 rewrites a comment about the way that we verify checksums during
backups. If we get a checksum mismatch, we reread the block and see if
the perceived problem goes away. If it doesn't, then we report it.
This is intended as protection against the backup reading a block
while some other process is in the midst of writing it, but there's no
guarantee that any concurrent write will finish quickly enough for our
second read attempt to see the updated contents. The comment claims
otherwise, and that's false, and I'm getting tired of reading this
false claim every time I read this code, so I rewrote the comment to
say what I believe to be true, namely, that our algorithm is flaky and
we have no good way to fix that right now. I'm pretty sure that Andres
pointed this problem out when this feature was under discussion, but
somehow it's still like this. There's another nearby comment which is
also false or at least misleading for basically the same reasons which
probably should be rewritten too, but I was a bit less certain how to
rewrite it and it wasn't making me as annoyed as this one, so for now
I only rewrote the one.

Comments?

--
Robert Haas
EDB: http://www.enterprisedb.com