Add remote.WithJobs and use it in remote.MultiWrite (#803)

- default parallel is 4
- an error is returned if WithParallel is called with a value <= 0

We can probably reuse this to fetch and push blobs elsewhere in remote.

MultiWrite'ing 123 distroless images with WithJobs(100) took 4s
compared to 11s for the default.

pkg/v1/remote/descriptor.go

@@ -34,11 +34,6 @@ import (
 	"github.com/google/go-containerregistry/pkg/v1/v1util"
 )
 
-var defaultPlatform = v1.Platform{
-	Architecture: "amd64",
-	OS:           "linux",
-}
-
 // ErrSchema1 indicates that we received a schema1 manifest from the registry.
 // This library doesn't have plans to support this legacy image format:
 // https://github.com/google/go-containerregistry/issues/377

pkg/v1/remote/multi_write.go

@@ -26,10 +26,6 @@ import (
 	"golang.org/x/sync/errgroup"
 )
 
-// Parallelism of blob and manifest uploads
-// TODO(jasonhall): Make this an Option.
-const jobs = 4
-
 // MultiWrite writes the given Images or ImageIndexes to the given refs, as
 // efficiently as possible, by deduping shared layer blobs and uploading layers
 // in parallel, then uploading all manifests in parallel.
@@ -96,9 +92,9 @@ func MultiWrite(m map[name.Reference]Taggable, options ...Option) error {
 	}
 
 	// Upload individual blobs and collect any errors.
-	blobChan := make(chan v1.Layer, 2*jobs)
+	blobChan := make(chan v1.Layer, 2*o.jobs)
 	var g errgroup.Group
-	for i := 0; i < jobs; i++ {
+	for i := 0; i < o.jobs; i++ {
 		// Start N workers consuming blobs to upload.
 		g.Go(func() error {
 			for b := range blobChan {
@@ -126,8 +122,8 @@ func MultiWrite(m map[name.Reference]Taggable, options ...Option) error {
 			i   Taggable
 			ref name.Reference
 		}
-		taskChan := make(chan task, 2*jobs)
-		for i := 0; i < jobs; i++ {
+		taskChan := make(chan task, 2*o.jobs)
+		for i := 0; i < o.jobs; i++ {
 			// Start N workers consuming tasks to upload manifests.
 			g.Go(func() error {
 				for t := range taskChan {

pkg/v1/remote/options.go

@@ -16,6 +16,7 @@ package remote
 
 import (
 	"context"
+	"errors"
 	"net/http"
 
 	"github.com/google/go-containerregistry/pkg/authn"
@@ -33,14 +34,23 @@ type options struct {
 	transport http.RoundTripper
 	platform  v1.Platform
 	context   context.Context
+	jobs      int
 }
 
+var defaultPlatform = v1.Platform{
+	Architecture: "amd64",
+	OS:           "linux",
+}
+
+const defaultJobs = 4
+
 func makeOptions(target authn.Resource, opts ...Option) (*options, error) {
 	o := &options{
 		auth:      authn.Anonymous,
 		transport: http.DefaultTransport,
 		platform:  defaultPlatform,
 		context:   context.Background(),
+		jobs:      defaultJobs,
 	}
 
 	for _, option := range opts {
@@ -131,3 +141,18 @@ func WithContext(ctx context.Context) Option {
 		return nil
 	}
 }
+
+// WithJobs is a functional option for setting the parallelism of remote
+// operations performed by a given function. Note that not all remote
+// operations support parallelism.
+//
+// The default value is 4.
+func WithJobs(jobs int) Option {
+	return func(o *options) error {
+		if jobs <= 0 {
+			return errors.New("jobs must be greater than zero")
+		}
+		o.jobs = jobs
+		return nil
+	}
+}

pkg/v1/remote/write.go

@@ -92,6 +92,7 @@ func Write(ref name.Reference, img v1.Image, options ...Option) error {
 			uploaded[h] = true
 		}
 
+		// TODO(#803): Pipe through remote.WithJobs and upload these in parallel.
 		g.Go(func() error {
 			return w.uploadOne(l)
 		})
@@ -501,6 +502,7 @@ func WriteIndex(ref name.Reference, ii v1.ImageIndex, options ...Option) error {
 		context: o.context,
 	}
 
+	// TODO(#803): Pipe through remote.WithJobs and upload these in parallel.
 	for _, desc := range index.Manifests {
 		ref := ref.Context().Digest(desc.Digest.String())
 		exists, err := w.checkExistingManifest(desc.Digest, desc.MediaType)