api: support the expires_in param on all GET requests.

`expires_in` is an alternative to `expiry` that supports durations other
than days:

* https://danbooru.donmai.us/posts.json?expires_in=30s
* https://danbooru.donmai.us/posts.json?expires_in=5min
* https://danbooru.donmai.us/posts.json?expires_in=1hour
* https://danbooru.donmai.us/posts.json?expires_in=1week
* https://danbooru.donmai.us/posts.json?expires_in=1month

app/logical/application_responder.rb

@@ -3,9 +3,14 @@
 class ApplicationResponder < ActionController::Responder
   # this is called by respond_with for non-html, non-js responses.
   def to_format
+    params = request.params
+
     if get?
-      expiry = request.params["expiry"]
-      controller.expires_in expiry.to_i.days if expiry.present?
+      if params["expires_in"]
+        controller.expires_in(DurationParser.parse(params["expires_in"]))
+      elsif request.params["expiry"]
+        controller.expires_in(params["expiry"].to_i.days)
+      end
     end
 
     if format == :xml

app/logical/duration_parser.rb

@@ -0,0 +1,27 @@
+module DurationParser
+  def self.parse(string)
+    string =~ /(\d+)(s(econds?)?|mi(nutes?)?|h(ours?)?|d(ays?)?|w(eeks?)?|mo(nths?)?|y(ears?)?)?/i
+
+    size = $1.to_i
+    unit = $2
+
+    case unit
+    when /^s/i
+      size.seconds
+    when /^mi/i
+      size.minutes
+    when /^h/i
+      size.hours
+    when /^d/i
+      size.days
+    when /^w/i
+      size.weeks
+    when /^mo/i
+      size.months
+    when /^y/i
+      size.years
+    else
+      size.seconds
+    end
+  end
+end

app/models/tag.rb

@@ -313,29 +313,7 @@ class Tag < ApplicationRecord
         end
 
       when :age
-        object =~ /(\d+)(s(econds?)?|mi(nutes?)?|h(ours?)?|d(ays?)?|w(eeks?)?|mo(nths?)?|y(ears?)?)?/i
-
-        size = $1.to_i
-        unit = $2
-
-        case unit
-        when /^s/i
-          size.seconds.ago
-        when /^mi/i
-          size.minutes.ago
-        when /^h/i
-          size.hours.ago
-        when /^d/i
-          size.days.ago
-        when /^w/i
-          size.weeks.ago
-        when /^mo/i
-          size.months.ago
-        when /^y/i
-          size.years.ago
-        else
-          size.seconds.ago
-        end
+        DurationParser.parse(object).ago
 
       when :ratio
         object =~ /\A(\d+(?:\.\d+)?):(\d+(?:\.\d+)?)\Z/i

test/functional/application_controller_test.rb

@@ -199,6 +199,13 @@ class ApplicationControllerTest < ActionDispatch::IntegrationTest
       assert_equal("max-age=#{1.day}, private", response.headers["Cache-Control"])
     end
 
+    should "support the expires_in parameter" do
+      get posts_path, as: :json, params: { expires_in: "5min" }
+
+      assert_response :success
+      assert_equal("max-age=#{5.minutes}, private", response.headers["Cache-Control"])
+    end
+
     should "return the correct root element name for empty xml responses" do
       get tags_path, as: :xml