Hi Kefu, Patrick, Rishabh, Python 3 enthusiasts, and future Python 3
users (i.e. everybody on this list, presumably):
so we already enable WITH_PYTHON3 for deb/rpm packaging, and for "make
check" runs.
Taken literally, this is true. But running cmake with WITH_PYTHON3=ON is
not sufficient to cause Ceph to actually use Python 3. (CAVEAT: I am
only familiar with the RPM spec file builds, so this commentary is
limited to those, and the usual "correct me if I'm wrong about any of
this" caveat applies as well!)
With that out of the way, I'll start my exposition which - I'm sorry to
say - has no "tl;dr" because I honestly do not know how to explain this
via a one-liner.
Unless special measures [1] are taken at build time, the spec file runs
cmake with -DWITH_PYTHON2=ON -DWITH_PYTHON3=ON. At first glance, this
seems reasonable and correct: we do want our builds to support both
Python 2 and Python 3, right? Turns out there are design considerations
that currently make this "both-ness" impossible to achieve.
At the spec file level, the problem is there are packages, like
ceph-common, which "Require" the Ceph python bindings. And it is not
possible to do "Require: python2-rados" and "Require: python3-rados" at
the same time: you have to choose one or the other.
(It's probably useful to note here that, as far as RPM is concerned in
currently supported distros, "python-rados" is just a synonym for
"python2-rados".)
Even at the cmake level, it's not possible to do both because the MGR
has an embedded Python subinterpreter which cannot be both Python 2 and
Python 3 at the same time.
So, what to do? How could we obtain a "real" Python 3 build that could
be tested using teuthology? For that, the RPMs would need to be built
with [1]. But the world at large is not ready for Ceph to be built this
way (not even for Nautilus AFAICT)!
My proposed solution for this is to implement a "python3" build flavor
in the CI infrastructure - similar to the existing "notcmalloc" build
flavor. Then, it would be possible to include "flavor: python3" in test
yaml for consumption by teuthology. I have not yet written any code in
this direction, though, so for the time being this "solution" has a
hollow ring to it.
HTH,
Nathan
[1] -DWITH_PYTHON2=OFF -DMGR_PYTHON_VERSION=3
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