[petsc-users] Bug report: using a custom ksp convergence function

[Daniel Stone]

Hello PETSc Community,

I think I've found a bug -

Go to $PETSC_DIR/src/ksp/ksp/tutorials/

open ex5f.F90, and add the following line in MyKSPConverged(), say around
line 336:

 call KSPConvergedDefault(ksp,n,rnorm,flag,dummy,ierr)

It should make sense why someone would want to do this - within a
definition of custom convergence behaviour, get the default convergence
flags, and, based on certain conditions, overwrite it.

Now, building and running the exercise, making sure to include the flag to
use the custom convergence:

 boston at boston-SYS-540A-TR:~/DATA_DRIVE/PETSC_DIRS/petsc_3.22/src/ksp/ksp/tutorials$
./ex2f -my_ksp_convergence
[0]PETSC ERROR:
------------------------------------------------------------------------
[0]PETSC ERROR: Caught signal number 11 SEGV: Segmentation Violation,
probably memory access out of range[...]

>From my debugging, the issue is at line 1520 of iterativ.c:

  KSPConvergedDefaultCtx *cctx = (KSPConvergedDefaultCtx *)ctx;

Because a dummy is used for the context object, this cast fails (my
debugger says "cannot access memory..." for cctx). Then the crash happens
when trying to access members of cctx:

  if (cctx->convmaxits && n >= ksp->max_it) {


I'm not sure how to fix it. Crucially, we must note that that this did work
in older versions of PETSc - I can repeat this test in 3.19.1, for example,
and it works fine - a debugger shows the cast succeeding and producing some
defaulted version of the ctx object, the subsequent crash does not happen,
etc.

Does anybody have any advice for working around this for the time being? A
piece of software I work with uses a custom ksp convergence test in the
manner described above, and is only functional with older versions of petsc
because of this.
Like in ex2f, I have no need of the ctx object, and just as in ex2f, I use
0 instead.

Things I'll try next: defining a proper ctx object (I use 0, like in ex2f),
or looking for some PETSC_NULL_CTX definition somewhere.

Many Thanks,

Daniel
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