Don't call PageGetSpecialPointer() on page until it's been initialized.
After calling XLogInitBufferForRedo(), the page might be all-zeros if it was not in page cache already. btree_xlog_unlink_page initialized the page correctly, but it called PageGetSpecialPointer before initializing it, which would lead to a corrupt page at WAL replay, if the unlinked page is not in page cache. Backpatch to 9.4, the bug came with the rewrite of B-tree page deletion.
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@ -864,9 +864,10 @@ btree_xlog_unlink_page(uint8 info, XLogReaderState *record)
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buffer = XLogInitBufferForRedo(record, 3);
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page = (Page) BufferGetPage(buffer);
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pageop = (BTPageOpaque) PageGetSpecialPointer(page);
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_bt_pageinit(page, BufferGetPageSize(buffer));
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pageop = (BTPageOpaque) PageGetSpecialPointer(page);
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pageop->btpo_flags = BTP_HALF_DEAD | BTP_LEAF;
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pageop->btpo_prev = xlrec->leafleftsib;
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pageop->btpo_next = xlrec->leafrightsib;
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