assertion failure 'assert len(self._active_readers) >= self._required_shares' in mutable retrieve

[davidsarah]

Reported by Stuart Card:

c:\allmydata-tahoe-1.9.0-rebuild\src\allmydata\mutable\retrieve.py, line 502 in _activate_enough_peers
500    # XXX: don't just drop the Deferred. We need error-reporting
501    # but not flow-control here.
502    assert len(self._active_readers) >= self._required_shares

_active_readers	   List instance @ 0x3510850
_required_shares   1

This was seen with a 1.9.0 client connected to an LAE storage server using an S3 backend (on the ticket999-S3-backend branch), with 1/1/1 encoding parameters. It seems as though the client shouldn't have reported an error in this way even if it was caused by the server.

The assertion failure, at src/allmydata/mutable/retrieve.py#L502​, is in code that changed in 1.9.0, so it might be a regression.