datatype unpack: potential incorrect assert

[hoopoepg]

Thank you for taking the time to submit an issue!

Background information

During debugging of issue #8442 we found strange assert, details below

What version of Open MPI are you using? (e.g., v3.0.5, v4.0.2, git branch name and hash, etc.)

Found at least in 4.1.x and master branches

Details of the problem

there is assert at

ompi/opal/datatype/opal_datatype_unpack.h

Line 99 in 56ed3a6

assert( 0 == (cando_count % _elem->blocklen) ); /* no partials here */

but call from

ompi/opal/datatype/opal_datatype_unpack.c

Line 204 in 56ed3a6

UNPACK_PREDEFINED_DATATYPE( pConvertor, pElem, count_desc,

is unpack_partial_blocklen(... count_desc=1, ...)
because

ompi/opal/datatype/opal_datatype_unpack.c

Line 172 in 56ed3a6

size_t count_desc = 1;

which produces assert shot in any case on this call path.

is this issue or I missed something in this world? :)

@jsquyres could it be artefact of some refactoring?

[jsquyres]

@bosilca will have a look; datastuff is in his domain...

[jsquyres]

@bosilca mentioned on the Tuesday call on 9 Mar 2021 that this assert is correct. This code path shouldn't be used for partial datatypes, so the assert is actually doing its job correctly here. The fact that this assert is tripping means that the real problem is somewhere else (perhaps related to #8442).

[hoopoepg]

hmmm, then call

ompi/opal/datatype/opal_datatype_unpack.c

Line 204 in 56ed3a6

UNPACK_PREDEFINED_DATATYPE( pConvertor, pElem, count_desc,

is incorrect

[jsquyres]

Re-opening because the fix has been merged on master, but still needs to be merged to v4.0.x, v4.1.x, and v5.0.x.