When we pread, we don't force the read to read all of the next block header.
However, when we get into a race condition where two opener threads try to
cache the same block and one thread read all of the next block header and
the other one didn't, it will fail the open process. This is especially important
in a splitting case where it will potentially fail the split process.
Instead, in the caches, we should only fail if the required blocks are different.
Signed-off-by: Andrew Purtell <apurtell@apache.org>
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