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* Note: buf_hdr_lock must be held to examine or change the tag, flags,
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* usage_count, refcount, or wait_backend_pid fields. buf_id field never
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* protected by the BufFreelistLock not buf_hdr_lock. The LWLocks can take
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* the data in the buffer!
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* An exception is that if we have the buffer pinned, its tag can't change