[petsc-dev] building PETSc plugins separately

[Jed Brown]

Barry Smith <bsmith at mcs.anl.gov> writes:

>    This discussion is two vague about what cases you are considering
>
>    1)  Dynamic or  2) shared libraries

I don't understand this distinction.  Shared libraries can be opened
with dlopen().  Static libraries are the issue.

>    a) -prefix install was already made and the plugin install
>    installer does not have write access there

As in, someone else installed PETSc.  Such as a system administrator on
a Cray.  Being able to add external packages to such a build would be a
neat feature, though we probably need to talk to the Cray folks to
ensure that it works properly (changing petscconf.h behind our backs can
break things like this).

>    b) user has old $PETSC_DIR and write access to install location
>    (either prefix or not)
>
>     Do we plan to support 1 a and b, 2 a and b?  What are the
>     similarities in the four cases and differences that need to be
>     handled.

If we only do plugins when shared libraries are available, then we just
need plugin paths.  That could be an environment variable
PETSC_PLUGIN_PATH containing a list of directories that PetscInitialize
walks through, loading any shared libraries.  There could even be a
default global plugin path and a default local (per user) path, so that
typical users don't have to mess with the environment variable, though
if we do that, PETSC_ARCH should appear somewhere so that they don't
collide.

It's messier for static libraries because the user needs to link them
explicitly.  If we create a petsc-config program (similar to pkg-config,
but I think we need to execute our code), it could walk through the
designated plugin paths to construct a suitable link line that would
include all the plugins.
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