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value.go
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value.go
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// Copyright (c) 2018 Uber Technologies, Inc.
//
// Permission is hereby granted, free of charge, to any person obtaining a copy
// of this software and associated documentation files (the "Software"), to deal
// in the Software without restriction, including without limitation the rights
// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
// copies of the Software, and to permit persons to whom the Software is
// furnished to do so, subject to the following conditions:
//
// The above copyright notice and this permission notice shall be included in
// all copies or substantial portions of the Software.
//
// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
// THE SOFTWARE.
package watch
import (
"errors"
"fmt"
"sync"
"time"
"go.uber.org/zap"
)
var (
errInitWatchTimeout = errors.New("init watch timeout")
errNilValue = errors.New("nil kv value")
)
// Value is a resource that can be updated during runtime.
type Value interface {
// Watch starts watching for value updates.
Watch() error
// Unwatch stops watching for value updates.
Unwatch()
}
// NewUpdatableFn creates an updatable.
type NewUpdatableFn func() (Updatable, error)
// GetUpdateFn returns the latest value.
type GetUpdateFn func(updatable Updatable) (interface{}, error)
// ProcessFn processes an update.
type ProcessFn func(update interface{}) error
// processWithLockFn updates a value while holding a lock.
type processWithLockFn func(update interface{}) error
type valueStatus int
const (
valueNotWatching valueStatus = iota
valueWatching
)
type value struct {
sync.RWMutex
opts Options
log *zap.Logger
newUpdatableFn NewUpdatableFn
getUpdateFn GetUpdateFn
processFn ProcessFn
processWithLockFn processWithLockFn
updatable Updatable
status valueStatus
}
// NewValue creates a new value.
func NewValue(
opts Options,
) Value {
v := &value{
opts: opts,
log: opts.InstrumentOptions().Logger(),
newUpdatableFn: opts.NewUpdatableFn(),
getUpdateFn: opts.GetUpdateFn(),
processFn: opts.ProcessFn(),
}
v.processWithLockFn = v.processWithLock
return v
}
func (v *value) Watch() error {
v.Lock()
defer v.Unlock()
if v.status == valueWatching {
return nil
}
updatable, err := v.newUpdatableFn()
if err != nil {
return CreateWatchError{innerError: err}
}
v.status = valueWatching
v.updatable = updatable
// NB(xichen): we want to start watching updates even though
// we may fail to initialize the value temporarily (e.g., during
// a network partition) so the value will be updated when the
// error condition is resolved.
defer func() { go v.watchUpdates(v.updatable) }()
select {
case <-v.updatable.C():
case <-time.After(v.opts.InitWatchTimeout()):
return InitValueError{innerError: errInitWatchTimeout}
}
update, err := v.getUpdateFn(v.updatable)
if err != nil {
return InitValueError{innerError: err}
}
if err = v.processWithLockFn(update); err != nil {
return InitValueError{innerError: err}
}
return nil
}
func (v *value) Unwatch() {
v.Lock()
defer v.Unlock()
// If status is nil, it means we are not watching.
if v.status == valueNotWatching {
return
}
v.updatable.Close()
v.status = valueNotWatching
v.updatable = nil
}
func (v *value) watchUpdates(updatable Updatable) {
for range updatable.C() {
v.Lock()
// If we are not watching, or we are watching with a different
// watch because we stopped the current watch and started a new
// one, return immediately.
if v.status != valueWatching || v.updatable != updatable {
v.Unlock()
return
}
update, err := v.getUpdateFn(updatable)
if err != nil {
v.log.Error("error getting update", zap.Error(err))
v.Unlock()
continue
}
if err = v.processWithLockFn(update); err != nil {
v.log.Error("error updating update", zap.Error(err))
}
v.Unlock()
}
}
func (v *value) processWithLock(update interface{}) error {
if update == nil {
return errNilValue
}
return v.processFn(update)
}
// CreateWatchError is returned when encountering an error creating a watch.
type CreateWatchError struct {
innerError error
}
func (e CreateWatchError) Error() string {
return fmt.Sprintf("create watch error:%v", e.innerError)
}
// InitValueError is returned when encountering an error when initializing a value.
type InitValueError struct {
innerError error
}
func (e InitValueError) Error() string {
return fmt.Sprintf("initializing value error:%v", e.innerError)
}