Re: Implementing an expanded object in C

Replying to my own problem here, I guess this was a situation where
explaining it in detail revealed the problem to me. By specifying my type
is 'passedbyvalue' and 'alignment = double' it now works!

CREATE TYPE matrix (
internallength = 8,
input = matrix_in,
output = matrix_out,
passedbyvalue,
alignment = double
);

Thanks for being a sounding board.

-Michel

On Sun, Jan 27, 2019 at 8:59 AM Michel Pelletier <pelletier(dot)michel(at)gmail(dot)com>
wrote:

> Hello,
>
> Apologies in advance for the long question. I've made a lot of progress
> on my GraphBLAS extension and getting close to having most of the API
> usefully exposed to postgres, but I'm been struggling with an issue related
> to when i switched to using an expanded representation of matrix types.
>
> I've tried to follow closely how arrays work, but the answer still eludes
> me. The situation is slightly different in that where an array's flat
> representation is useful, a sparse matrix's flat form is an edge-list, so
> it's not useful unexpanded so I have a macro, PGGRB_GETARG_MATRIX(n) always
> returns the expanded form by checking VARATT_IS_EXTERNAL_EXPANDED_RW and
> then DatumGetEOHP if true, otherwise expanding from the flat representation:
>
>
> https://github.com/michelp/pggraphblas/blob/bug/eohsegfault/src/matrix.c#L203
>
> I also have a PGGRB_RETURN_MATRIX(m) macro that calls `return
> EOHPGetRWDatum(&(A)->hdr)`
>
>
> https://github.com/michelp/pggraphblas/blob/bug/eohsegfault/src/pggraphblas.h#L140
>
> This chain of calls works for me in some cases, for example an operator
> function, 'matrix_mxm' which overloads the '*' operator, can be used to
> multiply two matrices:
>
> postgres=# select '{{0,1,2},{1,2,0},{4,5,6}}'::matrix *
> '{{0,1,2},{1,2,0},{4,5,6}}'::matrix;
> ?column?
> ------------------------------
> {{0,1,2},{2,0,1},{20,30,24}}
> (1 row)
>
> Works great! Internally this was `matrix_out(matrix_mxm(matrix_in(),
> matrix_in()))` where the data flow fine both in and out of the functions.
> But I have another function, 'matrix_agg', that aggregates edges from a
> query into a matrix. It builds and returns the result matrix similarly to
> matrix_mxm does and returns it using the same macro, but matrix_out's call
> to get the agregates final value segfaults.
>
> select matrix_agg(i, j, v) from edges; -- segfaults in matrix_out at
> PG_GETARG_MATRIX(0)
>
> at
>
>
> https://github.com/michelp/pggraphblas/blob/bug/eohsegfault/src/matrix.c#L207
>
> Afaict matrix_agg and matrix_mxm are both creating and returning matrices
> the same way, using the same function to build them and the same macro that
> `return EOHPGetRWDatum(&(A)->hdr)`, but when matrix_out fetches the
> argument to print the result it bombs on the aggregate's final value. The
> only salient different I can see if the agg's final function calls:
>
> if (!AggCheckCallContext(fcinfo, &resultcxt)) {
> resultcxt = CurrentMemoryContext;
> }
>
> oldcxt = MemoryContextSwitchTo(resultcxt);
> // do matrix creation stuff
> MemoryContextSwitchTo(oldcxt);
>
>
> But even if I remove that and do not switch contexts, it still crashes the
> same way.
>
> It must be possible to return expanded objects from aggregates so I'm
> clearly doing something wrong. The final function actually worked before I
> was using expanded representation and just using PG_RETURN_POINTER, but
> despite having all these clues I've been staring at this segfault in gdb
> for a couple of days now.
>
> Any pointers on this subject would be greatly appreciated! I know someone
> else out there recently was working on an expanded object posted on the
> list, if you don't see this, I may reach out to you. :)
>
> -Michel
>
>
>