Additional write barrier in AdvanceXLInsertBuffer().

First, mark the xlblocks member with InvalidXLogRecPtr, then issue a
write barrier, then initialize it. That ensures that the xlblocks
member doesn't appear valid while the contents are being initialized.

In preparation for reading WAL buffer contents without a lock.

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

src/backend/access/transam/xlog.c

@@ -1932,6 +1932,14 @@ AdvanceXLInsertBuffer(XLogRecPtr upto, TimeLineID tli, bool opportunistic)
 
 		NewPage = (XLogPageHeader) (XLogCtl->pages + nextidx * (Size) XLOG_BLCKSZ);
 
+		/*
+		 * Mark the xlblock with InvalidXLogRecPtr and issue a write barrier
+		 * before initializing. Otherwise, the old page may be partially
+		 * zeroed but look valid.
+		 */
+		pg_atomic_write_u64(&XLogCtl->xlblocks[nextidx], InvalidXLogRecPtr);
+		pg_write_barrier();
+
 		/*
 		 * Be sure to re-zero the buffer so that bytes beyond what we've
 		 * written will look like zeroes and not valid XLOG records...