This is a cherry which you know well by now. Sloe, Black thorn, Prunus spinosa.
At this time of the year the subject of fungus in deciduous trees is usually discussed on many forums. The majority of folks are scared to death if they see it. Professionals know that the overwhelming majority of fungi that sit in dead wood are harmless. I would go so far to say that more or less every deciduous tree has a fungus in it#s deadwood. One only sees it when the fungus produces a mushroom. This Prunus has had the fungus since ever. I have never even tried to do something about it. If you can see this even cutting out all dead wood does to help to get rid of it. so I use the mushroom for decoration