When Master Thief enters the battlefield, gain control of target artifact for as long as you control Master Thief.
If Master Thief ceases to be under your control before its ability resolves, you won’t gain control of the targeted artifact at all.
If Master Thief leaves the battlefield, you no longer control it, and its control-change effect ends.
If another player gains control of Master Thief, its control-change effect ends. Regaining control of Master Thief won’t cause you to regain control of the artifact.