Allow configuration to be overridden by environment variables

This is (I think) the final piece in enabling Smeagol to be run neatly from Docker and, indeed, from a pre-packaged jar file.

src/smeagol/configuration.clj

@@ -44,10 +44,58 @@
     (str (io/resource-path) "../config.edn")))
 
 
+(defn- from-env-vars
+  "Read a map from those of these environment variables which have values"
+  [& vars]
+  (reduce
+    #(let [v (env %2)]
+       (if v (assoc %1 %2 v) %1))
+    {}
+    vars))
+
+
+(defn transform-map
+  "transform this map `m` by applying these `transforms`. Each transforms
+  is expected to comprise a map with the keys :from and :to, whose values
+  are respectively a key to match and a key to replace that match with,
+  and optionally a key :transform, whose value is a function of one
+  argument to be used to transform the value of that key."
+  [m tuples]
+  (reduce
+    (fn [m tuple]
+      (if
+        (and (map? tuple) (map? m) (m (:from tuple)))
+        (let [old-val (m (:from tuple))
+              t (:transform tuple)
+              new-val (if t (apply t (list old-val)) old-val)]
+          (assoc (dissoc m (:from tuple)) (:to tuple) new-val))
+        m))
+    m
+    tuples))
+
+
+(def config-env-transforms
+  "Transforms to use with `transform-map` to convert environment
+  variable names (which need to be specific) into the shorter names
+  used internally"
+  '( {:from :smeagol-site-title :to :site-title}
+     {:from :smeagol-default-locale :to :default-locale}
+     {:from :smeagol-formatters :to :formatters :transform read-string}))
+
+
 (def config
-  "The actual configuration, as a map."
+  "The actual configuration, as a map. The idea here is that the config
+  file is read (if it is specified and present), but that individual
+  values "
   (try
-    (read-string (slurp config-file-path))
+    (let [file-contents (try
+                          (read-string (slurp config-file-path))
+                          (catch Exception _ {}))]
+      (merge
+        file-contents
+        (transform-map
+          (from-env-vars :smeagol-site-title :smeagol-default-locale)
+          config-env-transforms)))
     (catch Exception any
       (timbre/error any "Could not load configuration")
       {})))

src/smeagol/handler.clj

@@ -2,6 +2,7 @@
       :author "Simon Brooke"}
   smeagol.handler
   (:require [clojure.java.io :as cjio]
+            [clojure.string :refer [lower-case]]
             [compojure.core :refer [defroutes]]
             [compojure.route :as route]
             [cronj.core :as cronj]
@@ -43,6 +44,7 @@
 (defn user-access [request]
   (session/get :user))
 
+
 (defroutes base-routes
   (route/resources "/")
   (route/not-found "Not Found"))
@@ -69,7 +71,13 @@
        {:rotor (rotor/rotor-appender
                  {:path "smeagol.log"
                   :max-size (* 512 1024)
-                  :backlog 10})}})
+                  :backlog 10})}
+       :level (or
+                (read-string (env :timbre-level))
+                (let [level (read-string (env :log-level))]
+                  (if (string? level) (lower-case (keyword level))))
+                (if (env :dev) :debug)
+                :info)})
     (cronj/start! session-manager/cleanup-job)
     (if (env :dev) (parser/cache-off!))
     ;;start the expired session cleanup job