checkpoint reliability

I was wondering, when we start to reuse a WAL file, do we know that all
dirty buffers modified in that WAL file have been flushed to disk?  Do
we fsync() dirty buffers at time of checkpoint, and do we also make sure
that buffers we wrote to disk and later reused before the checkpoint
also made it to disk?

My point is that writing it to the kernel doesn't guarantee it made it
to disk.

I see the WAL records being fsync'ed in xlog.c but I don't see the
buffer pages being fsynced.

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