Turned out this scenario was happening, but due to other activity,
the writable block returned by the ReadResp would get downgraded
or invalidated before the "unnecessary" ReadExResp/UpgradeResp
returned, thus avoiding triggering the assertion that led us to
catch this.
--HG--
extra : convert_revision : 19dd0dbedb0227aed29da31a868ddf5ca4c50519