Correctly handle select on multiple channels in Queues

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
 go-gitea#22145

Fix go-gitea#22145

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

modules/queue/queue_channel.go

@@ -109,32 +109,6 @@ func (q *ChannelQueue) Flush(timeout time.Duration) error {
 	return q.FlushWithContext(ctx)
 }
 
-// FlushWithContext is very similar to CleanUp but it will return as soon as the dataChan is empty
-func (q *ChannelQueue) FlushWithContext(ctx context.Context) error {
-	log.Trace("ChannelQueue: %d Flush", q.qid)
-	paused, _ := q.IsPausedIsResumed()
-	for {
-		select {
-		case <-paused:
-			return nil
-		case data, ok := <-q.dataChan:
-			if !ok {
-				return nil
-			}
-			if unhandled := q.handle(data); unhandled != nil {
-				log.Error("Unhandled Data whilst flushing queue %d", q.qid)
-			}
-			atomic.AddInt64(&q.numInQueue, -1)
-		case <-q.baseCtx.Done():
-			return q.baseCtx.Err()
-		case <-ctx.Done():
-			return ctx.Err()
-		default:
-			return nil
-		}
-	}
-}
-
 // Shutdown processing from this queue
 func (q *ChannelQueue) Shutdown() {
 	q.lock.Lock()

modules/queue/unique_queue_channel.go

@@ -8,7 +8,6 @@ import (
 	"fmt"
 	"runtime/pprof"
 	"sync"
-	"sync/atomic"
 	"time"
 
 	"code.gitea.io/gitea/modules/container"
@@ -167,35 +166,6 @@ func (q *ChannelUniqueQueue) Flush(timeout time.Duration) error {
 	return q.FlushWithContext(ctx)
 }
 
-// FlushWithContext is very similar to CleanUp but it will return as soon as the dataChan is empty
-func (q *ChannelUniqueQueue) FlushWithContext(ctx context.Context) error {
-	log.Trace("ChannelUniqueQueue: %d Flush", q.qid)
-	paused, _ := q.IsPausedIsResumed()
-	for {
-		select {
-		case <-paused:
-			return nil
-		default:
-		}
-		select {
-		case data, ok := <-q.dataChan:
-			if !ok {
-				return nil
-			}
-			if unhandled := q.handle(data); unhandled != nil {
-				log.Error("Unhandled Data whilst flushing queue %d", q.qid)
-			}
-			atomic.AddInt64(&q.numInQueue, -1)
-		case <-q.baseCtx.Done():
-			return q.baseCtx.Err()
-		case <-ctx.Done():
-			return ctx.Err()
-		default:
-			return nil
-		}
-	}
-}
-
 // Shutdown processing from this queue
 func (q *ChannelUniqueQueue) Shutdown() {
 	log.Trace("ChannelUniqueQueue: %s Shutting down", q.name)

modules/queue/workerpool.go

@@ -463,13 +463,43 @@ 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 data := <-p.dataChan:
+		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)
 			}
@@ -495,6 +525,7 @@ func (p *WorkerPool) doWork(ctx context.Context) {
 	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)
@@ -515,8 +546,19 @@ func (p *WorkerPool) doWork(ctx context.Context) {
 				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