Add rate limiting to the image processor.

lib/util/requtil.js

@@ -26,3 +26,25 @@ module.exports.validate = function(req, rules) {
     }
   })
 }
+
+module.exports.limit = function(limit, handler) {
+  var queue = []
+  var running = 0
+
+  function done() {
+    running -= 1
+    maybeNext()
+  }
+
+  function maybeNext() {
+    while (running < limit && queue.length) {
+      running += 1
+      handler.apply(null, queue.shift()).finally(done)
+    }
+  }
+
+  return function() {
+    queue.push(arguments)
+    maybeNext()
+  }
+}