SECURITY: Don't allow a particular site to monopolize the defer queue

lib/work_queue.rb

@@ -0,0 +1,111 @@
+# frozen_string_literal: true
+
+require "monitor"
+
+module WorkQueue
+  class WorkQueueFull < StandardError
+  end
+
+  class ThreadSafeWrapper
+    include MonitorMixin
+
+    def initialize(queue)
+      mon_initialize
+
+      @queue = queue
+      @has_items = new_cond
+    end
+
+    def push(task, force:)
+      synchronize do
+        previously_empty = @queue.empty?
+        @queue.push(task, force: force)
+
+        @has_items.signal if previously_empty
+      end
+    end
+
+    def shift(block:)
+      synchronize do
+        loop do
+          if task = @queue.shift
+            break task
+          elsif block
+            @has_items.wait
+          else
+            break nil
+          end
+        end
+      end
+    end
+
+    def empty?
+      synchronize { @queue.empty? }
+    end
+
+    def size
+      synchronize { @queue.size }
+    end
+  end
+
+  class FairQueue
+    attr_reader :size
+
+    def initialize(limit, &blk)
+      @limit = limit
+      @size = 0
+      @elements = Hash.new { |h, k| h[k] = blk.call }
+    end
+
+    def push(task, force:)
+      raise WorkQueueFull if !force && @size >= @limit
+      key, task = task.values_at(:key, :task)
+      @elements[key].push(task, force: force)
+      @size += 1
+      nil
+    end
+
+    def shift
+      unless @elements.empty?
+        key, queue = @elements.shift
+
+        task = queue.shift
+
+        @elements[key] = queue unless queue.empty?
+
+        @size -= 1
+
+        { key: key, task: task }
+      end
+    end
+
+    def empty?
+      @elements.empty?
+    end
+  end
+
+  class BoundedQueue
+    def initialize(limit)
+      @limit = limit
+      @elements = []
+    end
+
+    def push(task, force:)
+      raise WorkQueueFull if !force && @elements.size >= @limit
+      @elements << task
+      nil
+    end
+
+    def shift
+      @elements.shift
+    end
+
+    def empty?
+      @elements.empty?
+    end
+
+    def size
+      @elements.size
+    end
+  end
+end