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forgejo/modules/queue/workerpool.go
zeripath a609cae9fb
Correctly handle select on multiple channels in Queues (#22146)
There are a few places in FlushQueueWithContext which make an incorrect
assumption about how `select` on multiple channels works.

The problem is best expressed by looking at the following example:

```go
package main

import "fmt"

func main() {
    closedChan := make(chan struct{})
    close(closedChan)
    toClose := make(chan struct{})
    count := 0

    for {
        select {
        case <-closedChan:
            count++
            fmt.Println(count)
            if count == 2 {
                close(toClose)
            }
        case <-toClose:
            return
        }
    }
}
```

This PR double-checks that the contexts are closed outside of checking
if there is data in the dataChan. It also rationalises the WorkerPool
FlushWithContext because the previous implementation failed to handle
pausing correctly. This will probably fix the underlying problem in
 #22145

Fix #22145

Signed-off-by: Andrew Thornton <art27@cantab.net>

Signed-off-by: Andrew Thornton <art27@cantab.net>
2022-12-30 02:06:47 +02:00

613 lines
17 KiB
Go

// Copyright 2019 The Gitea Authors. All rights reserved.
// SPDX-License-Identifier: MIT
package queue
import (
"context"
"fmt"
"runtime/pprof"
"sync"
"sync/atomic"
"time"
"code.gitea.io/gitea/modules/log"
"code.gitea.io/gitea/modules/process"
"code.gitea.io/gitea/modules/util"
)
// WorkerPool represent a dynamically growable worker pool for a
// provided handler function. They have an internal channel which
// they use to detect if there is a block and will grow and shrink in
// response to demand as per configuration.
type WorkerPool struct {
// This field requires to be the first one in the struct.
// This is to allow 64 bit atomic operations on 32-bit machines.
// See: https://pkg.go.dev/sync/atomic#pkg-note-BUG & Gitea issue 19518
numInQueue int64
lock sync.Mutex
baseCtx context.Context
baseCtxCancel context.CancelFunc
baseCtxFinished process.FinishedFunc
paused chan struct{}
resumed chan struct{}
cond *sync.Cond
qid int64
maxNumberOfWorkers int
numberOfWorkers int
batchLength int
handle HandlerFunc
dataChan chan Data
blockTimeout time.Duration
boostTimeout time.Duration
boostWorkers int
}
var (
_ Flushable = &WorkerPool{}
_ ManagedPool = &WorkerPool{}
)
// WorkerPoolConfiguration is the basic configuration for a WorkerPool
type WorkerPoolConfiguration struct {
Name string
QueueLength int
BatchLength int
BlockTimeout time.Duration
BoostTimeout time.Duration
BoostWorkers int
MaxWorkers int
}
// NewWorkerPool creates a new worker pool
func NewWorkerPool(handle HandlerFunc, config WorkerPoolConfiguration) *WorkerPool {
ctx, cancel, finished := process.GetManager().AddTypedContext(context.Background(), fmt.Sprintf("Queue: %s", config.Name), process.SystemProcessType, false)
dataChan := make(chan Data, config.QueueLength)
pool := &WorkerPool{
baseCtx: ctx,
baseCtxCancel: cancel,
baseCtxFinished: finished,
batchLength: config.BatchLength,
dataChan: dataChan,
resumed: closedChan,
paused: make(chan struct{}),
handle: handle,
blockTimeout: config.BlockTimeout,
boostTimeout: config.BoostTimeout,
boostWorkers: config.BoostWorkers,
maxNumberOfWorkers: config.MaxWorkers,
}
return pool
}
// Done returns when this worker pool's base context has been cancelled
func (p *WorkerPool) Done() <-chan struct{} {
return p.baseCtx.Done()
}
// Push pushes the data to the internal channel
func (p *WorkerPool) Push(data Data) {
atomic.AddInt64(&p.numInQueue, 1)
p.lock.Lock()
select {
case <-p.paused:
p.lock.Unlock()
p.dataChan <- data
return
default:
}
if p.blockTimeout > 0 && p.boostTimeout > 0 && (p.numberOfWorkers <= p.maxNumberOfWorkers || p.maxNumberOfWorkers < 0) {
if p.numberOfWorkers == 0 {
p.zeroBoost()
} else {
p.lock.Unlock()
}
p.pushBoost(data)
} else {
p.lock.Unlock()
p.dataChan <- data
}
}
// HasNoWorkerScaling will return true if the queue has no workers, and has no worker boosting
func (p *WorkerPool) HasNoWorkerScaling() bool {
p.lock.Lock()
defer p.lock.Unlock()
return p.hasNoWorkerScaling()
}
func (p *WorkerPool) hasNoWorkerScaling() bool {
return p.numberOfWorkers == 0 && (p.boostTimeout == 0 || p.boostWorkers == 0 || p.maxNumberOfWorkers == 0)
}
// zeroBoost will add a temporary boost worker for a no worker queue
// p.lock must be locked at the start of this function BUT it will be unlocked by the end of this function
// (This is because addWorkers has to be called whilst unlocked)
func (p *WorkerPool) zeroBoost() {
ctx, cancel := context.WithTimeout(p.baseCtx, p.boostTimeout)
mq := GetManager().GetManagedQueue(p.qid)
boost := p.boostWorkers
if (boost+p.numberOfWorkers) > p.maxNumberOfWorkers && p.maxNumberOfWorkers >= 0 {
boost = p.maxNumberOfWorkers - p.numberOfWorkers
}
if mq != nil {
log.Debug("WorkerPool: %d (for %s) has zero workers - adding %d temporary workers for %s", p.qid, mq.Name, boost, p.boostTimeout)
start := time.Now()
pid := mq.RegisterWorkers(boost, start, true, start.Add(p.boostTimeout), cancel, false)
cancel = func() {
mq.RemoveWorkers(pid)
}
} else {
log.Debug("WorkerPool: %d has zero workers - adding %d temporary workers for %s", p.qid, p.boostWorkers, p.boostTimeout)
}
p.lock.Unlock()
p.addWorkers(ctx, cancel, boost)
}
func (p *WorkerPool) pushBoost(data Data) {
select {
case p.dataChan <- data:
default:
p.lock.Lock()
if p.blockTimeout <= 0 {
p.lock.Unlock()
p.dataChan <- data
return
}
ourTimeout := p.blockTimeout
timer := time.NewTimer(p.blockTimeout)
p.lock.Unlock()
select {
case p.dataChan <- data:
util.StopTimer(timer)
case <-timer.C:
p.lock.Lock()
if p.blockTimeout > ourTimeout || (p.numberOfWorkers > p.maxNumberOfWorkers && p.maxNumberOfWorkers >= 0) {
p.lock.Unlock()
p.dataChan <- data
return
}
p.blockTimeout *= 2
boostCtx, boostCtxCancel := context.WithCancel(p.baseCtx)
mq := GetManager().GetManagedQueue(p.qid)
boost := p.boostWorkers
if (boost+p.numberOfWorkers) > p.maxNumberOfWorkers && p.maxNumberOfWorkers >= 0 {
boost = p.maxNumberOfWorkers - p.numberOfWorkers
}
if mq != nil {
log.Debug("WorkerPool: %d (for %s) Channel blocked for %v - adding %d temporary workers for %s, block timeout now %v", p.qid, mq.Name, ourTimeout, boost, p.boostTimeout, p.blockTimeout)
start := time.Now()
pid := mq.RegisterWorkers(boost, start, true, start.Add(p.boostTimeout), boostCtxCancel, false)
go func() {
<-boostCtx.Done()
mq.RemoveWorkers(pid)
boostCtxCancel()
}()
} else {
log.Debug("WorkerPool: %d Channel blocked for %v - adding %d temporary workers for %s, block timeout now %v", p.qid, ourTimeout, p.boostWorkers, p.boostTimeout, p.blockTimeout)
}
go func() {
<-time.After(p.boostTimeout)
boostCtxCancel()
p.lock.Lock()
p.blockTimeout /= 2
p.lock.Unlock()
}()
p.lock.Unlock()
p.addWorkers(boostCtx, boostCtxCancel, boost)
p.dataChan <- data
}
}
}
// NumberOfWorkers returns the number of current workers in the pool
func (p *WorkerPool) NumberOfWorkers() int {
p.lock.Lock()
defer p.lock.Unlock()
return p.numberOfWorkers
}
// NumberInQueue returns the number of items in the queue
func (p *WorkerPool) NumberInQueue() int64 {
return atomic.LoadInt64(&p.numInQueue)
}
// MaxNumberOfWorkers returns the maximum number of workers automatically added to the pool
func (p *WorkerPool) MaxNumberOfWorkers() int {
p.lock.Lock()
defer p.lock.Unlock()
return p.maxNumberOfWorkers
}
// BoostWorkers returns the number of workers for a boost
func (p *WorkerPool) BoostWorkers() int {
p.lock.Lock()
defer p.lock.Unlock()
return p.boostWorkers
}
// BoostTimeout returns the timeout of the next boost
func (p *WorkerPool) BoostTimeout() time.Duration {
p.lock.Lock()
defer p.lock.Unlock()
return p.boostTimeout
}
// BlockTimeout returns the timeout til the next boost
func (p *WorkerPool) BlockTimeout() time.Duration {
p.lock.Lock()
defer p.lock.Unlock()
return p.blockTimeout
}
// SetPoolSettings sets the setable boost values
func (p *WorkerPool) SetPoolSettings(maxNumberOfWorkers, boostWorkers int, timeout time.Duration) {
p.lock.Lock()
defer p.lock.Unlock()
p.maxNumberOfWorkers = maxNumberOfWorkers
p.boostWorkers = boostWorkers
p.boostTimeout = timeout
}
// SetMaxNumberOfWorkers sets the maximum number of workers automatically added to the pool
// Changing this number will not change the number of current workers but will change the limit
// for future additions
func (p *WorkerPool) SetMaxNumberOfWorkers(newMax int) {
p.lock.Lock()
defer p.lock.Unlock()
p.maxNumberOfWorkers = newMax
}
func (p *WorkerPool) commonRegisterWorkers(number int, timeout time.Duration, isFlusher bool) (context.Context, context.CancelFunc) {
var ctx context.Context
var cancel context.CancelFunc
start := time.Now()
end := start
hasTimeout := false
if timeout > 0 {
ctx, cancel = context.WithTimeout(p.baseCtx, timeout)
end = start.Add(timeout)
hasTimeout = true
} else {
ctx, cancel = context.WithCancel(p.baseCtx)
}
mq := GetManager().GetManagedQueue(p.qid)
if mq != nil {
pid := mq.RegisterWorkers(number, start, hasTimeout, end, cancel, isFlusher)
log.Trace("WorkerPool: %d (for %s) adding %d workers with group id: %d", p.qid, mq.Name, number, pid)
return ctx, func() {
mq.RemoveWorkers(pid)
}
}
log.Trace("WorkerPool: %d adding %d workers (no group id)", p.qid, number)
return ctx, cancel
}
// AddWorkers adds workers to the pool - this allows the number of workers to go above the limit
func (p *WorkerPool) AddWorkers(number int, timeout time.Duration) context.CancelFunc {
ctx, cancel := p.commonRegisterWorkers(number, timeout, false)
p.addWorkers(ctx, cancel, number)
return cancel
}
// addWorkers adds workers to the pool
func (p *WorkerPool) addWorkers(ctx context.Context, cancel context.CancelFunc, number int) {
for i := 0; i < number; i++ {
p.lock.Lock()
if p.cond == nil {
p.cond = sync.NewCond(&p.lock)
}
p.numberOfWorkers++
p.lock.Unlock()
go func() {
pprof.SetGoroutineLabels(ctx)
p.doWork(ctx)
p.lock.Lock()
p.numberOfWorkers--
if p.numberOfWorkers == 0 {
p.cond.Broadcast()
cancel()
} else if p.numberOfWorkers < 0 {
// numberOfWorkers can't go negative but...
log.Warn("Number of Workers < 0 for QID %d - this shouldn't happen", p.qid)
p.numberOfWorkers = 0
p.cond.Broadcast()
cancel()
}
select {
case <-p.baseCtx.Done():
// Don't warn or check for ongoing work if the baseCtx is shutdown
case <-p.paused:
// Don't warn or check for ongoing work if the pool is paused
default:
if p.hasNoWorkerScaling() {
log.Warn(
"Queue: %d is configured to be non-scaling and has no workers - this configuration is likely incorrect.\n"+
"The queue will be paused to prevent data-loss with the assumption that you will add workers and unpause as required.", p.qid)
p.pause()
} else if p.numberOfWorkers == 0 && atomic.LoadInt64(&p.numInQueue) > 0 {
// OK there are no workers but... there's still work to be done -> Reboost
p.zeroBoost()
// p.lock will be unlocked by zeroBoost
return
}
}
p.lock.Unlock()
}()
}
}
// Wait for WorkerPool to finish
func (p *WorkerPool) Wait() {
p.lock.Lock()
defer p.lock.Unlock()
if p.cond == nil {
p.cond = sync.NewCond(&p.lock)
}
if p.numberOfWorkers <= 0 {
return
}
p.cond.Wait()
}
// IsPaused returns if the pool is paused
func (p *WorkerPool) IsPaused() bool {
p.lock.Lock()
defer p.lock.Unlock()
select {
case <-p.paused:
return true
default:
return false
}
}
// IsPausedIsResumed returns if the pool is paused and a channel that is closed when it is resumed
func (p *WorkerPool) IsPausedIsResumed() (<-chan struct{}, <-chan struct{}) {
p.lock.Lock()
defer p.lock.Unlock()
return p.paused, p.resumed
}
// Pause pauses the WorkerPool
func (p *WorkerPool) Pause() {
p.lock.Lock()
defer p.lock.Unlock()
p.pause()
}
func (p *WorkerPool) pause() {
select {
case <-p.paused:
default:
p.resumed = make(chan struct{})
close(p.paused)
}
}
// Resume resumes the WorkerPool
func (p *WorkerPool) Resume() {
p.lock.Lock() // can't defer unlock because of the zeroBoost at the end
select {
case <-p.resumed:
// already resumed - there's nothing to do
p.lock.Unlock()
return
default:
}
p.paused = make(chan struct{})
close(p.resumed)
// OK now we need to check if we need to add some workers...
if p.numberOfWorkers > 0 || p.hasNoWorkerScaling() || atomic.LoadInt64(&p.numInQueue) == 0 {
// We either have workers, can't scale or there's no work to be done -> so just resume
p.lock.Unlock()
return
}
// OK we got some work but no workers we need to think about boosting
select {
case <-p.baseCtx.Done():
// don't bother boosting if the baseCtx is done
p.lock.Unlock()
return
default:
}
// OK we'd better add some boost workers!
p.zeroBoost()
// p.zeroBoost will unlock the lock
}
// CleanUp will drain the remaining contents of the channel
// This should be called after AddWorkers context is closed
func (p *WorkerPool) CleanUp(ctx context.Context) {
log.Trace("WorkerPool: %d CleanUp", p.qid)
close(p.dataChan)
for data := range p.dataChan {
if unhandled := p.handle(data); unhandled != nil {
if unhandled != nil {
log.Error("Unhandled Data in clean-up of queue %d", p.qid)
}
}
atomic.AddInt64(&p.numInQueue, -1)
select {
case <-ctx.Done():
log.Warn("WorkerPool: %d Cleanup context closed before finishing clean-up", p.qid)
return
default:
}
}
log.Trace("WorkerPool: %d CleanUp Done", p.qid)
}
// Flush flushes the channel with a timeout - the Flush worker will be registered as a flush worker with the manager
func (p *WorkerPool) Flush(timeout time.Duration) error {
ctx, cancel := p.commonRegisterWorkers(1, timeout, true)
defer cancel()
return p.FlushWithContext(ctx)
}
// IsEmpty returns if true if the worker queue is empty
func (p *WorkerPool) IsEmpty() bool {
return atomic.LoadInt64(&p.numInQueue) == 0
}
// contextError returns either ctx.Done(), the base context's error or nil
func (p *WorkerPool) contextError(ctx context.Context) error {
select {
case <-p.baseCtx.Done():
return p.baseCtx.Err()
case <-ctx.Done():
return ctx.Err()
default:
return nil
}
}
// FlushWithContext is very similar to CleanUp but it will return as soon as the dataChan is empty
// NB: The worker will not be registered with the manager.
func (p *WorkerPool) FlushWithContext(ctx context.Context) error {
log.Trace("WorkerPool: %d Flush", p.qid)
paused, _ := p.IsPausedIsResumed()
for {
// Because select will return any case that is satisified at random we precheck here before looking at dataChan.
select {
case <-paused:
// Ensure that even if paused that the cancelled error is still sent
return p.contextError(ctx)
case <-p.baseCtx.Done():
return p.baseCtx.Err()
case <-ctx.Done():
return ctx.Err()
default:
}
select {
case <-paused:
return p.contextError(ctx)
case data, ok := <-p.dataChan:
if !ok {
return nil
}
if unhandled := p.handle(data); unhandled != nil {
log.Error("Unhandled Data whilst flushing queue %d", p.qid)
}
atomic.AddInt64(&p.numInQueue, -1)
case <-p.baseCtx.Done():
return p.baseCtx.Err()
case <-ctx.Done():
return ctx.Err()
default:
return nil
}
}
}
func (p *WorkerPool) doWork(ctx context.Context) {
pprof.SetGoroutineLabels(ctx)
delay := time.Millisecond * 300
// Create a common timer - we will use this elsewhere
timer := time.NewTimer(0)
util.StopTimer(timer)
paused, _ := p.IsPausedIsResumed()
data := make([]Data, 0, p.batchLength)
for {
// Because select will return any case that is satisified at random we precheck here before looking at dataChan.
select {
case <-paused:
log.Trace("Worker for Queue %d Pausing", p.qid)
if len(data) > 0 {
log.Trace("Handling: %d data, %v", len(data), data)
if unhandled := p.handle(data...); unhandled != nil {
log.Error("Unhandled Data in queue %d", p.qid)
}
atomic.AddInt64(&p.numInQueue, -1*int64(len(data)))
}
_, resumed := p.IsPausedIsResumed()
select {
case <-resumed:
paused, _ = p.IsPausedIsResumed()
log.Trace("Worker for Queue %d Resuming", p.qid)
util.StopTimer(timer)
case <-ctx.Done():
log.Trace("Worker shutting down")
return
}
case <-ctx.Done():
if len(data) > 0 {
log.Trace("Handling: %d data, %v", len(data), data)
if unhandled := p.handle(data...); unhandled != nil {
log.Error("Unhandled Data in queue %d", p.qid)
}
atomic.AddInt64(&p.numInQueue, -1*int64(len(data)))
}
log.Trace("Worker shutting down")
return
default:
}
select {
case <-paused:
// go back around
case <-ctx.Done():
if len(data) > 0 {
log.Trace("Handling: %d data, %v", len(data), data)
if unhandled := p.handle(data...); unhandled != nil {
log.Error("Unhandled Data in queue %d", p.qid)
}
atomic.AddInt64(&p.numInQueue, -1*int64(len(data)))
}
log.Trace("Worker shutting down")
return
case datum, ok := <-p.dataChan:
if !ok {
// the dataChan has been closed - we should finish up:
if len(data) > 0 {
log.Trace("Handling: %d data, %v", len(data), data)
if unhandled := p.handle(data...); unhandled != nil {
log.Error("Unhandled Data in queue %d", p.qid)
}
atomic.AddInt64(&p.numInQueue, -1*int64(len(data)))
}
log.Trace("Worker shutting down")
return
}
data = append(data, datum)
util.StopTimer(timer)
if len(data) >= p.batchLength {
log.Trace("Handling: %d data, %v", len(data), data)
if unhandled := p.handle(data...); unhandled != nil {
log.Error("Unhandled Data in queue %d", p.qid)
}
atomic.AddInt64(&p.numInQueue, -1*int64(len(data)))
data = make([]Data, 0, p.batchLength)
} else {
timer.Reset(delay)
}
case <-timer.C:
delay = time.Millisecond * 100
if len(data) > 0 {
log.Trace("Handling: %d data, %v", len(data), data)
if unhandled := p.handle(data...); unhandled != nil {
log.Error("Unhandled Data in queue %d", p.qid)
}
atomic.AddInt64(&p.numInQueue, -1*int64(len(data)))
data = make([]Data, 0, p.batchLength)
}
}
}
}