Add helpers to define Settings with multiple impls (#16365)

* Add helpers to define Settings with multiple impls

* Add docstrings

* Fix missing :require

* Add namespace docstring per @dpsutton suggestion

src/metabase/models/setting/multi_setting.clj

+(ns metabase.models.setting.multi-setting
+  "Helper macros for defining Settings that can have multiple getter/setter implementations. The implementation that
+  gets used is determined at runtime when the getter or setter is invoked by a dispatch function.
+
+  This functionality was originally intended to facilitate separate EE and OSS versions of Settings, but rather than
+  restrict the impls to just `:oss` and `:ee`, these macros allow an arbitrary dispatch function and any number of
+  implementations.
+
+  See PR #16365 for more context."
+  (:require [metabase.models.setting :as setting]
+            [metabase.util.i18n :refer [tru]]))
+
+(defmulti dispatch-multi-setting
+  "Determine the dispatch value for a multi-Setting defined by `define-multi-setting`."
+  {:arglists '([setting-key])}
+  keyword)
+
+(defmulti get-multi-setting
+  "Get the value of a multi-Setting defined by `define-multi-setting` for the `impl` obtained by
+  calling `(dispatch-multi-setting setting-key)`."
+  {:arglists '([setting-key impl])}
+  (fn [setting-key impl]
+    [(keyword setting-key) (keyword impl)]))
+
+(defmulti set-multi-setting
+  "Update the value of a multi-Setting defined by `define-multi-setting` for the `impl` obtained by
+  calling `(dispatch-multi-setting setting-key)`."
+  {:arglists '([setting-key impl new-value])}
+  (fn [setting-key impl _]
+    [(keyword setting-key) (keyword impl)]))
+
+(defmacro define-multi-setting
+  "Define a Setting that can have multiple getter/setter implementations. The implementation used is determined by
+  calling `dispatch-thunk` when the Setting getter or setter is invoked. And `:getter` or `:setter` defined here will
+  be used for all impls; you can use this to make a multi-Setting read-only, for example by specifying `:setter` none
+  here.
+
+    defsetting : define-multi-setting :: defn : defmulti"
+  {:style/indent :defn}
+  [setting-symbol doc dispatch-thunk & {:as options}]
+  (let [setting-key (keyword setting-symbol)
+        options     (merge {:getter `(fn []
+                                       (get-multi-setting ~setting-key (dispatch-multi-setting ~setting-key)))
+                            :setter `(fn [new-value#]
+                                       (set-multi-setting ~setting-key (dispatch-multi-setting ~setting-key) new-value#))}
+                       options)]
+    `(do
+       (let [dispatch-thunk# ~dispatch-thunk]
+         (defmethod dispatch-multi-setting ~setting-key
+           [~'_]
+           (dispatch-thunk#)))
+       (setting/defsetting ~setting-symbol
+         ~doc
+         ~@(mapcat identity options)))))
+
+(defmacro define-multi-setting-impl
+  "Define a implementation for a Setting defined by `define-multi-setting`. Accepts options `:getter` (a function that
+  takes no args) and/or `:setter` (a function that takes a single arg, or the keyword `:none`), the same as
+  `defsetting`. Note that any of these options defined by `define-multi-setting` will be used for all impls and
+  ignored here.
+
+    define-multi-setting : define-multi-setting-impl :: defmulti : defmethod
+
+  See `define-multi-setting` for more details."
+  [setting-symbol dispatch-value & {:keys [getter setter]}]
+  (let [setting-key    (keyword setting-symbol)
+        dispatch-value (keyword dispatch-value)]
+    `(do
+       ~(when getter
+          `(let [getter# ~getter]
+             (defmethod get-multi-setting [~setting-key ~dispatch-value]
+               [~'_ ~'_]
+               (getter#))))
+       ~(when setter
+          (if (= setter :none)
+            `(defmethod set-multi-setting [~setting-key ~dispatch-value]
+               [~'_ ~'_ ~'_]
+               (throw (UnsupportedOperationException. (tru "You cannot set {0}; it is a read-only setting." ~setting-key))))
+            `(let [setter# ~setter]
+               (defmethod set-multi-setting [~setting-key ~dispatch-value]
+                 [~'_ ~'_ new-value#]
+                 (setter# new-value#))))))))

test/metabase/models/setting/multi_setting_test.clj

+(ns metabase.models.setting.multi-setting-test
+  (:require [clojure.test :refer :all]
+            [metabase.models.setting :as setting]
+            [metabase.models.setting.multi-setting :as multi-setting]
+            [metabase.test.fixtures :as fixtures]))
+
+(use-fixtures :once (fixtures/initialize :db))
+
+(def ^:dynamic ^:private *parakeet* :green-friend)
+
+(multi-setting/define-multi-setting ^:private multi-setting-test-bird-name
+  "A test Setting."
+  (fn [] *parakeet*)
+  :visibility :internal)
+
+(multi-setting/define-multi-setting-impl multi-setting-test-bird-name :green-friend
+  :getter (constantly "Green Friend")
+  :setter :none)
+
+(multi-setting/define-multi-setting-impl multi-setting-test-bird-name :yellow-friend
+  :getter (partial setting/get-string :multi-setting-test-bird-name)
+  :setter (partial setting/set-string! :multi-setting-test-bird-name))
+
+(deftest multi-setting-test
+  (testing :green-friend
+    (is (= "Green Friend"
+           (multi-setting-test-bird-name)))
+    (is (thrown-with-msg?
+         UnsupportedOperationException
+         #"You cannot set :multi-setting-test-bird-name; it is a read-only setting"
+         (multi-setting-test-bird-name "Parroty"))))
+  (testing :yellow-friend
+    (binding [*parakeet* :yellow-friend]
+      (is (= "Yellow Friend"
+             (multi-setting-test-bird-name "Yellow Friend")))
+      (is (= "Yellow Friend"
+             (multi-setting-test-bird-name))))))
+
+(multi-setting/define-multi-setting ^:private multi-setting-read-only
+  "A test setting that is always read-only."
+  (fn [] *parakeet*)
+  :visibility :internal
+  :getter     (constantly "Parroty")
+  :setter     :none)
+
+(multi-setting/define-multi-setting-impl multi-setting-read-only :green-friend
+  :getter (constantly "Green Friend")
+  :setter (partial setting/set-string! :multi-setting-read-only))
+
+(multi-setting/define-multi-setting-impl multi-setting-read-only :yellow-friend
+  :getter (constantly "Yellow Friend")
+  :setter (partial setting/set-string! :multi-setting-read-only))
+
+(deftest keys-in-definition-should-overshadow-keys-in-impls
+  (testing "Specifying :getter or :setter in `define-multi-setting` should mean ones in any `impl` are ignored"
+    (doseq [parakeet [:green-friend :yellow-friend :parroty]]
+      (testing parakeet
+        (binding [*parakeet* parakeet]
+          (is (= "Parroty"
+                 (multi-setting-read-only)))
+          (is (thrown-with-msg?
+               UnsupportedOperationException
+               #"You cannot set multi-setting-read-only; it is a read-only setting"
+               (multi-setting-read-only "Parroty"))))))))