fix(tasks): cancel immediately

Previously the loop would wait for the next yield but instead we can
just step forward immediately when cancelling.

See #15

lua/nio/tasks.lua

@@ -60,6 +60,7 @@ end
 function nio.tasks.run(func, cb)
   local co = coroutine.create(func)
   local cancelled = false
+  local step
   local task = { parent = nio.tasks.current_task() }
   if task.parent then
     child_tasks[task.parent] = child_tasks[task.parent] or {}
@@ -75,6 +76,7 @@ function nio.tasks.run(func, cb)
       child.cancel()
     end
     cancelled = true
+    step()
   end
 
   function task.trace()
@@ -104,7 +106,7 @@ function nio.tasks.run(func, cb)
 
   tasks[co] = task
 
-  local function step(...)
+  step = function(...)
     if cancelled then
       close_task(nil, format_error("Task was cancelled"))
       return

tests/tasks_spec.lua

@@ -42,6 +42,24 @@ describe("task", function()
     assert.Nil(should_be_nil)
   end)
 
+  a.it("cancels immediately", function()
+    local cancelled = nio.control.future()
+    local start = os.time()
+    local reached_end = false
+
+    local task = tasks.run(function()
+      nio.sleep(1000)
+      reached_end = true
+    end, function()
+      cancelled.set(os.time())
+    end)
+    task.cancel()
+    local cancel_time = cancelled.wait()
+
+    assert.False(reached_end)
+    assert.Equal(cancel_time - start, 0)
+  end)
+
   a.it("assigns parent task", function()
     local current = tasks.current_task()
     local task = tasks.run(function()