Added simple if-modified-since / 304 Not Modified support

@@ -1,7 +1,9 @@
 (ns ring.middleware.file-info
   "Augment Ring File responses."
   (:use [clojure.contrib.def :only (defvar-)])
-  (:import java.io.File))
+  (:import java.io.File)
+  (:import java.text.SimpleDateFormat)
+  (:import java.util.SimpleTimeZone))
 
 (defvar- base-mime-types
   {"ai"    "application/postscript"
@@ -70,18 +72,47 @@
   [#^File file mime-types]
   (get mime-types (get-extension file) "application/octet-stream"))
 
+(defvar- http-date-formatter
+  ;"A SimpleDateFormat instance, set to format using RFC 822/1123"
+  (let [formatter (SimpleDateFormat. "EEE, dd MMM yyyy HH:mm:ss ZZZ")]
+    (do 
+      ; We use GMT because it makes testing much easier
+      (.setTimeZone formatter (SimpleTimeZone. 0 "GMT"))
+      formatter)))
+
+(defn- http-date
+  "Takes a Date or Long, returns a String in HTTP Date (RFC 822/1123) format"
+  [date]
+  (.format http-date-formatter date))
+
 (defn wrap-file-info
   "Wrap an app such that responses with a file a body will have 
-  corresponding Content-Type and Content-Length headers added if they can be
-  determined from the file.
+  corresponding Content-Type, Content-Length, and Last Modified headers added 
+  if they can be determined from the file.
   If two arguments are given, the second is taken to be a map of file extensions
-  to content types that will supplement the default, built-in map."
+  to content types that will supplement the default, built-in map.
+  If the request specifies If-Modified-Since in its header, and it is a literal 
+  match of the string returned as Last-Modified, a 304 with no body will be 
+  sent instead."
   [app & [custom-mime-types]]
   (let [mime-types (merge base-mime-types custom-mime-types)]
     (fn [req]
       (let [{:keys [headers body] :as response} (app req)]
         (if (instance? File body)
-          (assoc response :headers
-            (assoc headers "Content-Length" (str (.length #^File body))
-                           "Content-Type"   (guess-mime-type body mime-types)))
+          (let [
+                 file-size        (str (.length #^File body))
+                 content-type     (guess-mime-type body mime-types)
+                 server-lmodified (http-date (.lastModified body))
+                 client-lmodified (get (:headers req) "if-modified-since")
+                 ;it'd be nice to have a real date comparison at some point
+                 not-modified     (= client-lmodified server-lmodified)]
+            (if not-modified
+              (assoc response :status 304 :body "" :headers 
+                (assoc headers "Content-Length" "0"
+                               "Content-Type"   content-type
+                               "Last-Modified"  server-lmodified))
+              (assoc response :headers
+                (assoc headers "Content-Length" file-size
+                               "Content-Type"   content-type
+                               "Last-Modified"  server-lmodified))))
           response)))))

@@ -11,6 +11,17 @@
 (def unknown-file (File. "test/ring/assets/random.xyz"))
 (def unknown-file-app (wrap-file-info (constantly {:headers {} :body unknown-file})))
 
+(defmacro with-custom-last-modified [file new-time form]
+  "Lets us use a known file modification time for tests, without permanently changing
+   the file's modification time"
+  `(let [old-time# (.lastModified ~file)]
+        (do
+          (.setLastModified ~file (* 1000 ~new-time));use seconds, not millis
+          (let [result# ~form]
+            (do
+              (.setLastModified ~file old-time#)
+              result#)))))
+
 (def custom-type-app
   (wrap-file-info
     (constantly {:headers {} :body known-file})
@@ -20,16 +31,35 @@
   (is (= {:headers {} :body "body"} (non-file-app {}))))
 
 (deftest wrap-file-info-known-file-response
-  (is (= {:headers {"Content-Type" "text/plain" "Content-Length" "6"}
-          :body    known-file}
-         (known-file-app {}))))
+  (with-custom-last-modified known-file 1263506400
+    (is (= {:headers {"Content-Type" "text/plain" "Content-Length" "6"
+                      "Last-Modified" "Thu, 14 Jan 2010 22:00:00 +0000"}
+            :body    known-file}
+           (known-file-app {})))))
 
 (deftest wrap-file-info-unknown-file-response
   (is (= {:headers {"Content-Type" "application/octet-stream" "Content-Length" "7"}
           :body    unknown-file}
          (unknown-file-app {}))))
 
 (deftest wrap-file-info-custom-mime-types
-  (is (= {:headers {"Content-Type" "custom/type" "Content-Length" "6"}
-          :body known-file}
-         (custom-type-app {}))))
+  (with-custom-last-modified known-file 0
+    (is (= {:headers {"Content-Type" "custom/type" "Content-Length" "6"
+                      "Last-Modified" "Thu, 01 Jan 1970 00:00:00 +0000"}
+            :body known-file}
+           (custom-type-app {})))))
+
+(deftest wrap-file-info-if-modified-since-hit
+  (with-custom-last-modified known-file 1263506400
+    (is (= {:status  304
+            :headers {"Content-Type" "text/plain" "Content-Length" "0"
+                      "Last-Modified" "Thu, 14 Jan 2010 22:00:00 +0000"}
+            :body    ""}
+           (known-file-app {:headers {"if-modified-since" "Thu, 14 Jan 2010 22:00:00 +0000" }})))))
+
+(deftest wrap-file-info-if-modified-miss
+  (with-custom-last-modified known-file 1263506400
+    (is (= {:headers {"Content-Type" "text/plain" "Content-Length" "6"
+                      "Last-Modified" "Thu, 14 Jan 2010 22:00:00 +0000"}
+            :body    known-file}
+           (known-file-app {:headers {"if-modified-since" "Wed, 13 Jan 2010 22:00:00 +0000"}})))))