soumya92/barista

// Copyright 2018 Google 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 localtz watches for changes to the local machine's time zone.
package localtz // import "barista.run/base/watchers/localtz"

import (
    "errors"
    "fmt"
    "os"
    "strings"
    "sync/atomic"
    "time"

    "barista.run/base/value"
    "barista.run/base/watchers/file"
    l "barista.run/logging"
)

// Overridden in tests.
var tzFile = "/etc/localtime"

var current value.Value // of *time.Location
var testMode uint32     // atomic bool

// Get returns the machine's current time zone.
func Get() *time.Location {
    return current.Get().(*time.Location)
}

// Next returns a channel that signals when the machine's time zone changes.
func Next() <-chan struct{} {
    return current.Next()
}

// SetForTest allows simulating a timezone change in tests.
func SetForTest(newZone *time.Location) {
    current.Set(newZone)
    atomic.StoreUint32(&testMode, 1)
}

func init() {
    current.Set(time.Local) // At startup, time.Local is correct.
    if tzFile != "" {
        go watchTz(tzFile)
    }
}

var errCount = int32(0)

func watchTz(tzFile string) {
    w := file.Watch(tzFile)
    defer w.Unsubscribe()

    for {
        err := watchTzOnce(w, tzFile)
        if err == errTestMode {
            return
        }
        l.Log("Timezone watcher exited: %v, falling back to time.Local", err)
        // fallback to time.Local on any errors.
        current.Set(time.Local)
        // throttle retries, in case the problem is transient.
        time.Sleep(time.Second)
        // limit retries. If three consecutive attempts fail, bail out.
        if atomic.AddInt32(&errCount, 1) > 3 {
            l.Log("Too many failures in timezone watcher, giving up")
            return
        }
    }
}

func watchTzOnce(w *file.Watcher, tzFile string) error {
    for {
        err := updateTz(tzFile)
        if err != nil && !os.IsNotExist(err) {
            return err
        }
        select {
        case <-w.Updates:
        case e := <-w.Errors:
            return e
        }
    }
}

var errTestMode = errors.New("TestMode")

func updateTz(tzFile string) error {
    if atomic.LoadUint32(&testMode) > 0 {
        return errTestMode
    }
    link, err := os.Readlink(tzFile)
    if err != nil {
        return err
    }
    l.Fine("Updating timezone from %s->%s", tzFile, link)
    ps := strings.Split(link, "/")
    len := len(ps)
    if len < 2 {
        return fmt.Errorf("Failed parsing zoneinfo: %s->%s", tzFile, link)
    }
    loc, err := time.LoadLocation(ps[len-2] + "/" + ps[len-1])
    if err != nil {
        return err
    }
    atomic.StoreInt32(&errCount, 0)
    current.Set(loc)
    l.Fine("Machine timezone changed to %v", loc)
    return nil
}