This is just a simple library, I post it here to gather feedback. Lots of nice features are present in crystal-db/pool.cr at master · crystal-lang/crystal-db · GitHub, which may be worth to add (on a new child class).
A pool library was already existing: GitHub - ysbaddaden/pool: Generic (connection) pools for Crystal. However, I was not plainly satisfied on several aspects. I consider all the Fiber logic unnecessary, as the developer can just get one instance from the pool then use it/pass it around, instead of doing so with the Pool itself (did I miss an other advantage?).
Other added features are thread-safety and pool resizing. The later allows to add custom strategies on top to shrink back the pool after a traffic burst, or at the contrary grow it to prepare for an increased traffic for example.
Link to the repository: GitHub - j8r/pool.cr: A simple thread-safe generic pool.