We pasken like Rebbi that kohanim can drink wine. However, there is a machlokes rashi and rabbeinu chananel to explain rebbi. Rashi explains that even a kohein who nows exactly his mishmar and bais av is allowed to drink wine, but Rabbeinu Chananel says that he is only arguing on the last situation when he doesn't know his mishmar and bais av. This machlokes is also an argument between the Ramabam (bias mikdash 1:7) who says like Rabbeinu Chananel that a kohein who knows his beis av cannot drink wine that day, and if he only knows his mishmar he can't drink wine the whole week, and the Ra'avad says like rashi that any kohein may drink wine.
Rashi/ra'avad seem to be gores "kilkalaso takanaso", meaning that the amount of time that elapsed that would be assur to drink (kilkalaso) is indicative of not having to be concerned for the imminent building of the bais hamikdash (takanaso). However, the girsa in our gemara and the Rambam is "takanaso kilkalaso" which is explained by the maharsha (chidushei agados) to mean that his takana to be able to drink wine which is not knowing his beis av and mishmar, is kilkalaso that perhaps he is not a kohein at all and can drink wine always - to the exclusion of a kohein who knows his mishmar and/or beis av who cannot drink wine on that day/week.
The difficulty with rashi is that we come out paskening like rebbi that we are not concerned for the rebuilding of the Beis Hamikdash. This is addressed by the Ra'avad that even if we assume it will be build imminently, the restoration of the mishmaros and beis av will take some time.
