Replace the stopgap fix I made in 0e758ae89 with a cleaner one.
The real problem with 4ab5dae94 is that it contorted this function's
logic substantially, by introducing a third code path that required
different behavior in the function's main loop. That seems quite
unnecessary on closer inspection: the new IsBinaryUpgrade case can
just share the behavior of the other immediate-unlink cases. Hence,
revert 4ab5dae94 and most of 0e758ae89 (keeping the latter's
save/restore errno fix), and add IsBinaryUpgrade to the set of
conditions tested to choose immediate unlink.
Also fix some additional places with sloppy handling of errno,
to ensure we have an invariant that we always continue processing
after any non-ENOENT failure of do_truncate. I doubt that that's
fixing any bug of field importance, so I don't feel it necessary to
back-patch; but we might as well get it right while we're here.
Also improve the comments, which had drifted a bit from what the
code actually does, and neglected to mention some important
considerations.
Back-patch to v15, not because this is fixing any bug but because
it doesn't seem like a good idea for v15's mdunlinkfork logic to be
significantly different from both v14 and v16.
Discussion: https://postgr.es/m/3797575.1667924888@sss.pgh.pa.us
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