func()

in internal/storage/fake/bucket.go [799:871]
51 lines of code
7 McCabe index (conditional complexity)

func (b *bucket) ComposeObjects(
	ctx context.Context,
	req *gcs.ComposeObjectsRequest) (o *gcs.Object, err error) {
	b.mu.Lock()
	defer b.mu.Unlock()

	// GCS doesn't like too few or too many sources.
	if len(req.Sources) < 1 {
		err = errors.New("you must provide at least one source component")
		return
	}

	if len(req.Sources) > gcs.MaxSourcesPerComposeRequest {
		err = errors.New("you have provided too many source components")
		return
	}

	// Find readers for all of the source objects, also computing the sum of
	// their component counts.
	var srcReaders []io.Reader
	var dstComponentCount int64

	for _, src := range req.Sources {
		var r io.Reader
		var srcIndex int

		r, srcIndex, err = b.newReaderLocked(&gcs.ReadObjectRequest{
			Name:       src.Name,
			Generation: src.Generation,
		})

		if err != nil {
			return
		}

		srcReaders = append(srcReaders, r)
		dstComponentCount += b.objects[srcIndex].metadata.ComponentCount
	}

	// GCS doesn't like the component count to go too high.
	if dstComponentCount > gcs.MaxComponentCount {
		err = errors.New("result would have too many components")
		return
	}

	// Create the new object.
	createReq := &gcs.CreateObjectRequest{
		Name:                       req.DstName,
		GenerationPrecondition:     req.DstGenerationPrecondition,
		MetaGenerationPrecondition: req.DstMetaGenerationPrecondition,
		Contents:                   io.MultiReader(srcReaders...),
		ContentType:                req.ContentType,
		Metadata:                   req.Metadata,
	}

	_, err = b.createObjectLocked(createReq)
	if err != nil {
		return
	}

	dstIndex := b.objects.find(req.DstName)
	metadata := &b.objects[dstIndex].metadata

	// Touchup: fix the component count.
	metadata.ComponentCount = dstComponentCount

	// Touchup: emulate the real GCS behavior of not exporting an MD5 hash for
	// composite objects.
	metadata.MD5 = nil

	o = copyObject(metadata)
	return
}