Re: Assert in pageinspect with NULL pages

I've suddenly found that the test in this patch is based on a fact that
heap pages don't have PageSpecial or it is of different size with btree
pages Special area:

CREATE TABLE test1 (a int8, b int4range);

SELECT bt_page_items(get_raw_page('test1', 0));

In the current state is is so, but it is not guaranteed. For example, in
the proposed 64xid patch [1] we introduce PageSpecial into heap pages and
its size on occasion equals to the size of btree page special so check from
above is false. Even if we pass heap page into pageinspect pretending it is
btree page. So the test will fail.

Generally it seems not only a wrong test but the consequence of a bigger
scale fact that we can not always distinguish different AM's pages. So I'd
propose at least that we don't come to a conclusion that a page is valid
based on PageSpecial size is right.

[1]
https://www.postgresql.org/message-id/flat/CACG%3DezZe1NQSCnfHOr78AtAZxJZeCvxrts0ygrxYwe%3DpyyjVWA%40mail.gmail.com
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Best regards,
Maxim Orlov.