Re: new heapcheck contrib module

> On Oct 22, 2020, at 6:41 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
>
> I wrote:
>> So now I think this is a REDIRECT on either architecture, but the
>> offset and length fields have different values, causing the redirect
>> pointer to point to different places.  Maybe it happens to point
>> at a DEAD tuple in the big-endian case.
>
> Just to make sure, I tried this test program:
>
> #include <stdio.h>
> #include <string.h>
>
> typedef struct ItemIdData
> {
>    unsigned    lp_off:15,      /* offset to tuple (from start of page) */
>                lp_flags:2,     /* state of line pointer, see below */
>                lp_len:15;      /* byte length of tuple */
> } ItemIdData;
>
> int main()
> {
>    ItemIdData lp;
>
>    memset(&lp, 0x77, sizeof(lp));
>    printf("off = %x, flags = %x, len = %x\n",
>           lp.lp_off, lp.lp_flags, lp.lp_len);
>    return 0;
> }
>
> I get
>
> off = 7777, flags = 2, len = 3bbb
>
> on a little-endian machine, and
>
> off = 3bbb, flags = 2, len = 7777
>
> on big-endian.  It'd be less symmetric if the bytes weren't
> all the same ...

I think we're going in the wrong direction here.  The idea behind this test was to have as little knowledge about the
layoutof pages as possible and still verify that damaging the pages would result in corruption reports.  Of course, not
alldamage will result in corruption reports, because some damage looks legit.  I think it was just luck (good or bad
dependingon your perspective) that the damage in the test as committed works on little-endian but not big-endian. 

I can embed this knowledge that you have researched into the test if you want me to, but my instinct is to go the other
directionand have even less knowledge about pages in the test.  That would work if instead of expecting corruption for
everytime the test writes the file, instead to have it just make sure that it gets corruption reports at least some of
thetimes that it does so.  That seems more maintainable long term. 

—
Mark Dilger
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