Implement If-Modified-Since header in httpd

A browser sends If-Modified-Since when requesting a resource, the server should send a '304 Not Modified' if the Last-Modified time of the resource matches the date in 'If-Modified-Since'. Googlebot also uses If-Modified-Since so this addition to httpd can save some bandwidth.

contrib/httpd/file-responder.factor

@@ -2,24 +2,45 @@
 ! See http://factorcode.org/license.txt for BSD license.
 IN: file-responder
 USING: calendar embedded errors html httpd io kernel math namespaces parser
-sequences strings ;
+sequences strings hashtables ;
 
 : serving-path ( filename -- filename )
     [ "" ] unless* "doc-root" get swap append ;
 
+: file-http-date ( filename -- string )
+  #! Return the date in HTTP Date format (see RFC 2616).
+  #! Returns false if no time information available for the file.
+  stat [ fourth unix>gmt timestamp>http-string ] [ f ] if* ;
+
 : file-response ( filename mime-type length -- )
     [
         number>string "Content-Length" set
         "Content-Type" set
-        stat [ fourth unix>gmt timestamp>http-string "Last-Modified" set ] when*
+        file-http-date [ "Last-Modified" set ] when*
         now timestamp>http-string "Date" set
     ] make-hash "200 OK" response terpri ;
 
+: last-modified-matches? ( filename -- bool )
+  file-http-date [
+    "If-Modified-Since" "header" get hash = 
+  ] [
+    f
+  ] if* ;
+
+: not-modified-response ( -- )
+    [
+        now timestamp>http-string "Date" set
+    ] make-hash "304 Not Modified" response terpri ;  
+
 : serve-static ( filename mime-type -- )
-    dupd pick file-length file-response "method" get "head" = [
+    over last-modified-matches? [
-        drop
+      drop not-modified-response
     ] [
-        <file-reader> stdio get stream-copy
+      dupd pick file-length file-response "method" get "head" = [
+          drop
+      ] [
+          <file-reader> stdio get stream-copy
+      ] if 
     ] if ;
 
 SYMBOL: page