Syncing sql extension versions with shared library versions

Hi All,

I am developing the TimescaleDB extension for postgres (
https://github.com/timescale/timescaledb) and have some questions about
versioning. First of all, I have to say that the versioning system on the
sql side is wonderful. It's really simple to write migrations etc.

However when thinking through the implications of having a database cluster
with databases having different extension versions installed, it was not
apparently clear to me how to synchronize the installed extension version
with a shared library version. For example, if I have timescaledb version
0.1.0 in one db and version 0.2.0 in another db, I'd like for
timescaledb-0.1.0.so and timescaledb-0.2.0.so to be used, respectively. (I
want to avoid having to keep backwards-compatibility for all functions in
future shared-libraries). In our case, this is further complicated by the
fact that we need to preload the shared library since we are accessing the
planner hooks etc. Below, I'll describe some solutions I have been thinking
about, but wanted to hear if anyone else on this list has already solved
this problem and has some insight. It is also quite possible I am missing
something.

Issue 1: Preloading the right shared library.
When preloading libraries (either via local_preload_libraries, or
session_preload_libraries, shared_preload_libraries), it would be nice to
preload the shared_library according to it's version. But, I looked through
the code and found no logic for adding version numbers to shared library
names.
Solution 1: Set session_preload_libraries on the database via ALTER
DATABASE SET. This can be done in the sql and the sql version-migration
scripts can change this value as you change extensions versions. I think
this would work, but it seems very hack-ish and less-than-ideal.
Solution 2: Create some kind of stub shared-library that will, in turn,
load another shared library of the correct version. This seems like the
cleaner approach. Has anybody seen/implemented something like this already?

Issue 2: module_pathname
I believe that for user defined function the MODULE_PATHNAME substitution
will not work since that setting is set once per-extension. Thus, for
example, the migration scripts that include function definitions for older
versions would use the latest .so file if MODULE_PATHNAME was used in the
definition. This would be a problem if upgrading to an intermediate (not
latest) version.
Solution: MODULE_PATHNAME cannot be used, and we should build our own
templating/makefile infrastructure to link files to the right-versioned
shared library in the CREATE FUNCTION definition.

Thanks a lot in advance,
Mat Arye
http://www.timescale.com/