Re: new heapcheck contrib module

On Wed, Apr 22, 2020 at 10:43 PM Mark Dilger
<mark.dilger@enterprisedb.com> wrote:
> It's simple enough to extend the tap test a little to check for those things.  In v3, the tap test skips tests if the
pagesize is not 8k, and also if the tuples do not fall on the page where expected (which would happen due to alignment
issues,gremlins, or whatever.). 

Skipping the test if the tuple isn't in the expected location sounds
really bad. That will just lead to the tests passing without actually
doing anything. If the tuple isn't in the expected location, the tests
should fail.

> There are other approaches, though.  The HeapFile/HeapPage/HeapTuple perl modules recently submitted on another
thread*could* be used here, but only if those modules are likely to be committed. 

Yeah, I don't know if we want that stuff or not.

> This test *could* be extended to autodetect the page size and alignment issues and calculate at runtime where tuples
willbe on the page, but only if folks don't mind the test having that extra complexity in it.  (There is a school of
thoughtthat regression tests should avoid excess complexity.). Do you have a recommendation about which way to go with
this?

How much extra complexity are we talking about? It feels to me like
for a heap page, the only things that are going to affect the position
of the tuples on the page -- supposing we know the tuple size -- are
the page size and, I think, MAXALIGN, and that doesn't sound too bad.
Another possibility is to use pageinspect's heap_page_items() to
determine the position within the page (lp_off), which seems like it
might simplify things considerably. Then, we're entirely relying on
the backend to tell us where the tuples are, and we only need to worry
about the offsets relative to the start of the tuple.

I kind of like that approach, because it doesn't involve having Perl
code that knows how heap pages are laid out; we rely entirely on the C
code for that. I'm not sure if it'd be a problem to have a TAP test
for one contrib module that uses another contrib module, but maybe
there's some way to figure that problem out.

--
Robert Haas
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