Re: [HACKERS] A design for amcheck heapam verification

On Fri, Sep 29, 2017 at 10:54 AM, Peter Geoghegan <pg@bowt.ie> wrote:
>> Something that allocates new memory as the patch's bloom_init()
>> function does I'd tend to call 'make' or 'create' or 'new' or
>> something, rather than 'init'.
>
> I tend to agree. I'll adopt that style in the next version. I just
> didn't want the caller to have to manage the memory themselves.

v3 of the patch series, attached, does it that way -- it adds a
bloom_create(). The new bloom_create() function still allocates its
own memory, but does so while using a FLEXIBLE_ARRAY_MEMBER. A
separate bloom_init() function (that works with dynamic shared memory)
could easily be added later, for the benefit of parallel hash join.

Other notable changes:

* We now support bloom filters that have bitsets of up to 512MB in
size. The previous limit was 128MB.

* We now use TransactionXmin for our AccessShareLock xmin cutoff,
rather than calling GetOldestXmin(). This is the same cut-off used by
xacts that must avoid broken hot chains for their earliest snapshot.
This avoids a scan of the proc array, and allows more thorough
verification, since GetOldestXmin() was overly restrictive here.

* Expanded code comments describing the kinds of problems the new
verification capability is expected to be good at catching.

For example, there is now a passing reference to the CREATE INDEX
CONCURRENTLY bug that was fixed back in February (it's given as an
example of a more general problem -- faulty HOT safety assessment).
With the new heapallindexed enhancement added by this patch, amcheck
can be expected to catch that issue much of the time. We also go into
heap-only tuple handling within IndexBuildHeapScan(). The way that
CREATE INDEX tries to index the most recent tuple in a HOT chain
(while locating the root tuple in the chain, to get the right heap TID
for the index) has proven to be very useful as a smoke test while
investigating HOT/VACUUM FREEZE bugs in the past couple of weeks [1].
I believe it would have caught several historic MultiXact/recovery
bugs, too. This all seems worth noting explicitly.

[1] https://postgr.es/m/CAH2-Wznm4rCrhFAiwKPWTpEw2bXDtgROZK7jWWGucXeH3D1fmA@mail.gmail.com
-- 
Peter Geoghegan

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