task.go

/*
   Copyright 2014 CoreOS, Inc.

   Licensed under the Apache License, Version 2.0 (the "License");
   you may not use this file except in compliance with the License.
   You may obtain a copy of the License at

       http://www.apache.org/licenses/LICENSE-2.0

   Unless required by applicable law or agreed to in writing, software
   distributed under the License is distributed on an "AS IS" BASIS,
   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   See the License for the specific language governing permissions and
   limitations under the License.
*/

package agent

import (
	"errors"
	"fmt"
	"strings"

	"github.com/coreos/fleet/job"
	"github.com/coreos/fleet/pkg"
)

const (
	taskTypeLoadUnit   = "LoadUnit"
	taskTypeUnloadUnit = "UnloadUnit"
	taskTypeStartUnit  = "StartUnit"
	taskTypeStopUnit   = "StopUnit"

	taskReasonScheduledButNotRunnable    = "unit scheduled locally but unable to run"
	taskReasonScheduledButUnloaded       = "unit scheduled here but not loaded"
	taskReasonLoadedButNotScheduled      = "unit loaded but not scheduled here"
	taskReasonLoadedButHashDiffers       = "unit loaded but hash differs to expected"
	taskReasonLoadedDesiredStateLaunched = "unit currently loaded but desired state is launched"
	taskReasonLaunchedDesiredStateLoaded = "unit currently launched but desired state is loaded"
	taskReasonPurgingAgent               = "purging agent"
)

type taskChain struct {
	unit  *job.Unit
	tasks []task
}

func newTaskChain(u *job.Unit, t ...task) taskChain {
	return taskChain{
		unit:  u,
		tasks: t,
	}
}

func (tc *taskChain) Add(t task) {
	tc.tasks = append(tc.tasks, t)
}

func (tc taskChain) String() (out string) {
	tasks := make([]string, len(tc.tasks))
	for i, t := range tc.tasks {
		tasks[i] = fmt.Sprintf("(%s, %q)", t.typ, t.reason)
	}
	return fmt.Sprintf("{%s %s}", tc.unit.Name, strings.Join(tasks, ", "))
}

type task struct {
	typ    string
	reason string
}

type taskResult struct {
	task task
	err  error
}

type taskManager struct {
	processing pkg.Set
	mapper     taskMapperFunc
}

func newTaskManager() *taskManager {
	return &taskManager{
		processing: pkg.NewUnsafeSet(),
		mapper:     mapTaskToFunc,
	}
}

// Do attempts to complete a task against an Agent. If the task is unable
// to be attempted, an error is returned. A task is unable to be attempted
// if there exists in-flight any task with the same unit name. The returned
// error channel will be non-nil only if the task could be attempted. The
// channel will be closed when the task completes. If the task failed, an
// error will be sent to the channel. Do is not threadsafe.
func (tm *taskManager) Do(tc taskChain, a *Agent) (chan taskResult, error) {
	if tc.unit == nil {
		return nil, errors.New("unable to handle task with nil Job")
	}

	if tm.processing.Contains(tc.unit.Name) {
		return nil, errors.New("task already in flight")
	}

	// Do is not threadsafe due to the race between Contains and Add
	tm.processing.Add(tc.unit.Name)

	reschan := make(chan taskResult, len(tc.tasks))
	go func() {
		defer tm.processing.Remove(tc.unit.Name)
		for _, t := range tc.tasks {
			t := t
			res := taskResult{
				task: t,
			}

			taskFunc, err := tm.mapper(t, tc.unit, a)
			if err != nil {
				res.err = err
			} else {
				res.err = taskFunc()
			}

			reschan <- res
		}

		close(reschan)
	}()

	return reschan, nil
}

type taskMapperFunc func(t task, u *job.Unit, a *Agent) (func() error, error)

func mapTaskToFunc(t task, u *job.Unit, a *Agent) (fn func() error, err error) {
	switch t.typ {
	case taskTypeLoadUnit:
		fn = func() error { return a.loadUnit(u) }
	case taskTypeUnloadUnit:
		fn = func() error { a.unloadUnit(u.Name); return nil }
	case taskTypeStartUnit:
		fn = func() error { a.startUnit(u.Name); return nil }
	case taskTypeStopUnit:
		fn = func() error { a.stopUnit(u.Name); return nil }
	default:
		err = fmt.Errorf("unrecognized task type %q", t.typ)
	}

	return
}