cache: convert prune loop from recursive invocation to iterative

The prune logic would prune multiple times because one prune could cause
more things to be capable of pruning and change the logic. This was done
through a recursive invocation.

Since go doesn't have support for function tail calls, this would result
in a new stack entry for each loop. This unrolls the logic so the prune
function is invoked iteratively rather than recursively.

`prune` and `pruneOnce` have also had their names swapped. In general,
`pruneOnce` implies that it runs a single prune while `prune` sounds
like the top level function. The current code had this reversed and
`pruneOnce` would call `prune` and `prune` would call itself
recursively.

I've also updated a section in the controller that invoked prune on each
worker. In older versions of Go, the current version was correct because
those versions of Go would reuse the location for each loop which would
cause goroutines to all reference the same worker instead of different
workers.

Recent versions of Go have changed the behavior so this is no longer
needed.

Signed-off-by: Jonathan A. Sternberg <[email protected]>

cache/manager.go

@@ -1012,7 +1012,7 @@ func (cm *cacheManager) Prune(ctx context.Context, ch chan client.UsageInfo, opt
 	cm.muPrune.Lock()
 
 	for _, opt := range opts {
-		if err := cm.pruneOnce(ctx, ch, opt); err != nil {
+		if err := cm.prune(ctx, ch, opt); err != nil {
 			cm.muPrune.Unlock()
 			return err
 		}
@@ -1029,7 +1029,7 @@ func (cm *cacheManager) Prune(ctx context.Context, ch chan client.UsageInfo, opt
 	return nil
 }
 
-func (cm *cacheManager) pruneOnce(ctx context.Context, ch chan client.UsageInfo, opt client.PruneInfo) error {
+func (cm *cacheManager) prune(ctx context.Context, ch chan client.UsageInfo, opt client.PruneInfo) error {
 	filter, err := filters.ParseAll(opt.Filter...)
 	if err != nil {
 		return errors.Wrapf(err, "failed to parse prune filters %v", opt.Filter)
@@ -1066,14 +1066,21 @@ func (cm *cacheManager) pruneOnce(ctx context.Context, ch chan client.UsageInfo,
 		}
 	}
 
-	return cm.prune(ctx, ch, pruneOpt{
+	popt := pruneOpt{
 		filter:       filter,
 		all:          opt.All,
 		checkShared:  check,
 		keepDuration: opt.KeepDuration,
 		keepBytes:    calculateKeepBytes(totalSize, dstat, opt),
 		totalSize:    totalSize,
-	})
+	}
+	for {
+		releasedSize, releasedCount, err := cm.pruneOnce(ctx, ch, popt)
+		if err != nil || releasedCount == 0 {
+			return err
+		}
+		popt.totalSize -= releasedSize
+	}
 }
 
 func calculateKeepBytes(totalSize int64, dstat disk.DiskStat, opt client.PruneInfo) int64 {
@@ -1100,9 +1107,9 @@ func calculateKeepBytes(totalSize int64, dstat disk.DiskStat, opt client.PruneIn
 	return keepBytes
 }
 
-func (cm *cacheManager) prune(ctx context.Context, ch chan client.UsageInfo, opt pruneOpt) (err error) {
+func (cm *cacheManager) pruneOnce(ctx context.Context, ch chan client.UsageInfo, opt pruneOpt) (releasedSize, releasedCount int64, err error) {
 	if opt.keepBytes != 0 && opt.totalSize < opt.keepBytes {
-		return nil
+		return 0, 0, nil
 	}
 
 	var toDelete []*deleteRecord
@@ -1206,11 +1213,11 @@ func (cm *cacheManager) prune(ctx context.Context, ch chan client.UsageInfo, opt
 			// mark metadata as deleted in case we crash before cleanup finished
 			if err := cr.queueDeleted(); err != nil {
 				releaseLocks()
-				return err
+				return 0, 0, err
 			}
 			if err := cr.commitMetadata(); err != nil {
 				releaseLocks()
-				return err
+				return 0, 0, err
 			}
 		}
 		cr.mu.Unlock()
@@ -1221,7 +1228,7 @@ func (cm *cacheManager) prune(ctx context.Context, ch chan client.UsageInfo, opt
 	cm.mu.Unlock()
 
 	if len(toDelete) == 0 {
-		return nil
+		return 0, 0, nil
 	}
 
 	// calculate sizes here so that lock does not need to be held for slow process
@@ -1234,7 +1241,7 @@ func (cm *cacheManager) prune(ctx context.Context, ch chan client.UsageInfo, opt
 		if size == sizeUnknown {
 			// calling size will warm cache for next call
 			if _, err := cr.size(ctx); err != nil {
-				return err
+				return 0, 0, err
 			}
 		}
 	}
@@ -1277,15 +1284,18 @@ func (cm *cacheManager) prune(ctx context.Context, ch chan client.UsageInfo, opt
 			c.Size = cr.equalImmutable.getSize() // benefit from DiskUsage calc
 		}
 
-		opt.totalSize -= c.Size
+		releasedSize += c.Size
 
 		if cr.equalImmutable != nil {
 			if err1 := cr.equalImmutable.remove(ctx, false); err == nil {
 				err = err1
 			}
 		}
-		if err1 := cr.remove(ctx, true); err == nil {
+
+		if err1 := cr.remove(ctx, true); err1 != nil && err == nil {
 			err = err1
+		} else if err1 == nil {
+			releasedCount++
 		}
 
 		if err == nil && ch != nil {
@@ -1294,16 +1304,17 @@ func (cm *cacheManager) prune(ctx context.Context, ch chan client.UsageInfo, opt
 		cr.mu.Unlock()
 	}
 	cm.mu.Unlock()
+
 	if err != nil {
-		return err
+		return releasedSize, releasedCount, err
 	}
 
 	select {
 	case <-ctx.Done():
-		return context.Cause(ctx)
+		err = context.Cause(ctx)
 	default:
-		return cm.prune(ctx, ch, opt)
 	}
+	return releasedSize, releasedCount, err
 }
 
 func (cm *cacheManager) markShared(m map[string]*cacheUsageInfo) error {

control/control.go

@@ -623,14 +623,12 @@ func (c *Controller) gc() {
 	}()
 
 	for _, w := range workers {
-		func(w worker.Worker) {
-			eg.Go(func() error {
-				if policy := w.GCPolicy(); len(policy) > 0 {
-					return w.Prune(ctx, ch, policy...)
-				}
-				return nil
-			})
-		}(w)
+		eg.Go(func() error {
+			if policy := w.GCPolicy(); len(policy) > 0 {
+				return w.Prune(ctx, ch, policy...)
+			}
+			return nil
+		})
 	}
 
 	err = eg.Wait()

hack/composefiles/compose.yaml

@@ -4,16 +4,13 @@ services:
     container_name: buildkit-dev
     build:
       context: ../..
-      args:
-        BUILDKIT_DEBUG: 1
     image: moby/buildkit:local
     ports:
       - 127.0.0.1:5000:5000
       - 127.0.0.1:6060:6060
     restart: always
     privileged: true
     environment:
-      DELVE_PORT: 5000
       OTEL_SERVICE_NAME: buildkitd
       OTEL_EXPORTER_OTLP_ENDPOINT: http://otel-collector:4317
     configs: