cubicweb #831816 connections-pool-size=1 seems broken [rejected]

I try to upgrade a test instance with this configuration but without success (works with size>1).

Traceback (most recent call last):
  File "/home/juj/src/fcubicweb/cubicweb/cwctl.py", line 132, in run_arg
        cmdmeth(appid)
  File "/home/juj/src/fcubicweb/cubicweb/cwctl.py", line 759, in upgrade_instance
        vcconf = repo.get_versions()
  File "/home/juj/src/fcubicweb/logilab/common/decorators.py", line 65, in cache_wrapper3
        _cache[args] = callableobj(self, *args)
  File "/home/juj/src/fcubicweb/cubicweb/server/repository.py", line 454, in get_versions
        session = self.internal_session()
  File "/home/juj/src/fcubicweb/cubicweb/server/repository.py", line 773, in internal_session
        session.set_pool()
  File "/home/juj/src/fcubicweb/cubicweb/server/session.py", line 537, in set_pool
        self._threaddata.pool = pool = self.repo._get_pool()
  File "/home/juj/src/fcubicweb/cubicweb/server/repository.py", line 293, in _get_pool
        raise Exception('no pool available after 5 secs, probably either a '
Exception: no pool available after 5 secs, probably either a bug in code (too many uncommited/rollbacked connections) or too much load on the server (in which case you can try to set a bigger connections pools size)
instance jujspie not upgraded: no pool available after 5 secs, probably either a bug in code (too many uncommited/rollbacked connections) or too much load on the server (in which case you can try to set a bigger connections pools size)

Note: 4 additionnal connections are created silently. But It can be normal.

prioritynormal
typebug
done in<not specified>
load0.000
load left0.000
closed by<not specified>