Failure in atlas_fctest_trans_unstructured

[DJDavies2]

What happened?

Running atlas_fctest_trans_unstructured with certain configs gives this failure:

183/216 Test #183: atlas_fctest_trans_unstructured ...........................Subprocess aborted***Exception: 0.23 sec
Runtime Error: *** Arithmetic exception: Floating overflow - aborting
/home/users/david.davies/cylc-run/mi-bg671/work/1/get_source_atlas/atlas/src/atlas_f/trans/atlas_Trans_module.F90, line 352: Error occurred in ATLAS_TRANS_MODULE:INVTRANS_VORDIV2WIND_FIELD
/data/users/david.davies/cylc-run/mi-bg671/work/1/get_source_atlas/atlas/src/tests/trans/fctest_trans_unstructured.F90, line 84: Called by FCTEST_ATLAS_TRANS_UNSTR:TEST_TRANS
/home/users/david.davies/cylc-run/mi-bg671/work/1/build_atlas_nag/build/src/tests/trans/fctest_trans_unstructured_main.F90, line 21: Called by RUN_FCTEST_ATLAS_TRANS_UNSTR

What are the steps to reproduce the bug?

Building and runinng with NAG/GCC, building with ectrans support. However this failure only occurs with some configs. I don't think the NAG is particularly relevant here (see below)(

Version

Head

Platform (OS and architecture)

Linus

Relevant log output

No response

Accompanying data

No response

Organisation

Met Office

[DJDavies2]

I dug around a bit and printed some stuff out. I think the failure occurs in this line in src/atlas/trans/local/VorDivToUVLocal.cc:

                    rv[ir + ji]  = -chiIm * rvor[ii + ji] - psiM1 * rdiv[ir + ji + 1] + psiP1 * rdiv[ir + ji - 1];

However I think the root of the problem lies earlier than that. In src/tests/trans/fctest_trans_unstructured.F90 the call to spectral%create_field for sp_div_field and friends produce arrays that have 6 elements. I have traced the path of sp_div_field down into the code and I believe it ends up in extend_trunction in the file src/atlas/trans/local/TransLocal.cc as the old_spectra parameter. Printing some values and array indices out show that this line:

                      new_spectra[k++] = old_spectra[k_old++];

is going out of bounds in terms of the access to old_spectra. This results in undefined values being copied into new_spectra, which in some circumstances result in arithmetic exceptions in subsequent calculations such as the line noted above.

I have checked the hypothesis by adding some special code to extend_truncation so that only the first 6 elements of old_spectra are used (0 otherwise); this seems to fix atlas_fctest_trans_unstructured but is of course unacceptable as it breaks other tests.

I don't know what to do next about this. There is no way in extend_trunction of knowing how many elements old_spectra has so there would be no way of generalizing my hack even if it was considered okay in principle.

[wdeconinck]

Thank you @DJDavies2 for digging! I will dig myself a bit further in the New Year.
I can refactor this and add some assertions to make sure we don't go out of bound silently, and then see how to prevent it.
These functions are implementation details.

[wdeconinck]

@DJDavies2 I have nailed down the bug and fixed it in #254.
Thanks once more for the discovery and report!