Re: Ensuring hash tuples are properly maxaligned

Andres Freund <andres(at)anarazel(dot)de> writes:
> On 2018-01-02 19:08:49 -0500, Tom Lane wrote:
>> Now, the existing definition of the struct seems safe on all
>> architectures we support, but it would not take much to break it.
>> I think we ought to do what we did recently in the memory-context
>> code: insert an explicit padding calculation and a static assertion
>> that it's right. Hence, the attached proposed patch (in which
>> I also failed to resist the temptation to make the two code paths
>> in dense_alloc() look more alike). Any objections?

> But note that dsa_pointer can be wider than a regular pointer on
> platforms without atomics support.

Hm. I did not get that impression from the comments in dsa.h,
but if it's true then this approach won't work --- and indeed the
hash code would be actively broken in such a case, so it's a problem
we must fix.

The other idea I was considering was to get rid of the data[] member
entirely, define HASH_CHUNK_HEADER_SIZE as
MAXALIGN(sizeof(HashMemoryChunkData)), and replace the references to
data[] with pointer arithmetic along the lines of
((char *) chunk) + HASH_CHUNK_HEADER_SIZE
This would be a bit more invasive, but probably not too ugly if we
encapsulated that pointer arithmetic in a macro. And it'd certainly
be a bunch more robust against people throwing random fields into
the struct.

regards, tom lane