pgsql: Use 64-bit atomics for xlblocks array elements.

Use 64-bit atomics for xlblocks array elements.

In preparation for reading the contents of WAL buffers without a
lock. Also, avoids the previously-needed comment in GetXLogBuffer()
explaining why it's safe from torn reads.

Author: Bharath Rupireddy
Discussion: https://postgr.es/m/CALj2ACVfFMfqD5oLzZSQQZWfXiJqd-NdX0_317veP6FuB31QWA@mail.gmail.com
Reviewed-by: Andres Freund

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/c3a8e2a7cb16d55e3b757934b538cb8b8a0eab02

Modified Files
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src/backend/access/transam/xlog.c | 40 +++++++++++++++++++--------------------
1 file changed, 19 insertions(+), 21 deletions(-)