compact: group concurrency improvements (#1029)

* group concurrency improvements

* remove unnecessary error check

* add to wg in main goroutine

pkg/compact/compact.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"path/filepath"
 	"sort"
+	"strings"
 	"sync"
 	"time"
 
@@ -22,7 +23,6 @@ import (
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/tsdb"
 	"github.com/prometheus/tsdb/labels"
-	"golang.org/x/sync/errgroup"
 )
 
 type ResolutionLevel int64
@@ -921,17 +921,22 @@ func (c *BucketCompactor) Compact(ctx context.Context) error {
 	// Loop over bucket and compact until there's no work left.
 	for {
 		var (
-			errGroup, errGroupCtx = errgroup.WithContext(ctx)
-			groupChan             = make(chan *Group)
-			finishedAllGroups     = true
-			mtx                   sync.Mutex
+			wg                     sync.WaitGroup
+			workCtx, workCtxCancel = context.WithCancel(ctx)
+			groupChan              = make(chan *Group)
+			errChan                = make(chan error, c.concurrency)
+			finishedAllGroups      = true
+			mtx                    sync.Mutex
 		)
 
 		// Set up workers who will compact the groups when the groups are ready.
+		// They will compact available groups until they encounter an error, after which they will stop.
 		for i := 0; i < c.concurrency; i++ {
-			errGroup.Go(func() error {
+			wg.Add(1)
+			go func() {
+				defer wg.Done()
 				for g := range groupChan {
-					shouldRerunGroup, _, err := g.Compact(errGroupCtx, c.compactDir, c.comp)
+					shouldRerunGroup, _, err := g.Compact(workCtx, c.compactDir, c.comp)
 					if err == nil {
 						if shouldRerunGroup {
 							mtx.Lock()
@@ -942,17 +947,17 @@ func (c *BucketCompactor) Compact(ctx context.Context) error {
 					}
 
 					if IsIssue347Error(err) {
-						if err := RepairIssue347(errGroupCtx, c.logger, c.bkt, err); err == nil {
+						if err := RepairIssue347(workCtx, c.logger, c.bkt, err); err == nil {
 							mtx.Lock()
 							finishedAllGroups = false
 							mtx.Unlock()
 							continue
 						}
 					}
-					return errors.Wrap(err, "compaction")
+					errChan <- errors.Wrap(err, fmt.Sprintf("compaction failed for group %s", g.Key()))
+					return
 				}
-				return nil
-			})
+			}()
 		}
 
 		// Clean up the compaction temporary directory at the beginning of every compaction loop.
@@ -980,16 +985,26 @@ func (c *BucketCompactor) Compact(ctx context.Context) error {
 		}
 
 		// Send all groups found during this pass to the compaction workers.
+	groupLoop:
 		for _, g := range groups {
 			select {
-			case <-errGroupCtx.Done():
-				break
+			case err = <-errChan:
+				break groupLoop
 			case groupChan <- g:
 			}
 		}
 		close(groupChan)
-		if err := errGroup.Wait(); err != nil {
-			return err
+		wg.Wait()
+
+		close(errChan)
+		workCtxCancel()
+		if err != nil {
+			errMsgs := []string{err.Error()}
+			// Collect any other errors reported by the workers.
+			for e := range errChan {
+				errMsgs = append(errMsgs, e.Error())
+			}
+			return errors.New(strings.Join(errMsgs, "; "))
 		}
 
 		if finishedAllGroups {