ReolveImageConfig: Only fetch best matching config

Before this change, all platforms that loosely match the provided
platform will be fetched even though we only care about 1 of them.
As an example when linux/amd64 is requested it will also fetch linux/386
because it is a compatible architecture.
This means extra round trips to the registry, potentially even for
content that doesn't exist in the remote.

This is especially a problem when resolve mode is prefer-local because
we'll have the index locally but most likely only one manifest.
In this case we'll end up reaching out to the registry to fetch the
other manifests unncessarily.

With this change instead of fetching all matching platforms it chooses
only the best matching platform.

Signed-off-by: Brian Goff <[email protected]>

util/imageutil/config.go

@@ -4,6 +4,7 @@ import (
  "context"
  "encoding/json"
  "fmt"
+ "sort"
  "strings"
  "sync"
  "time"
@@ -213,10 +214,25 @@ func childrenConfigHandler(provider content.Provider, platform platforms.MatchCo
  }
 
  if platform != nil {
- for _, d := range index.Manifests {
- if d.Platform == nil || platform.Match(*d.Platform) {
- descs = append(descs, d)
- }
+ if len(index.Manifests) > 0 {
+ // Here we want to select only the best manifest for the platform.
+ // We do not want every platform or even every matching platform.
+ // Whatever is selected here will, depending on resolve mode, be checked for locally and then fetched if it is not present.
+ // Selecting only the best match means we are making fewer fewer round trips with the content store and the registry.
+ // It also means we may be able to avoid talking to the registry at all.
+ // This is important for cases where the image doesn't actually exist in the registry (yet) but we have a local copy.
+ sort.SliceStable(index.Manifests, func(i, j int) bool {
+ mi := index.Manifests[i]
+ if mi.Platform == nil {
+ return false
+ }
+ mj := index.Manifests[j]
+ if mj.Platform == nil {
+ return true
+ }
+ return platform.Less(*mi.Platform, *mj.Platform)
+ })
+ descs = append(descs, index.Manifests[0])
  }
  } else {
  descs = append(descs, index.Manifests...)

util/imageutil/config_test.go

@@ -0,0 +1,183 @@
+package imageutil
+
+import (
+ "bytes"
+ "context"
+ "encoding/json"
+ "fmt"
+ "io"
+ "testing"
+
+ "github.com/containerd/containerd/content"
+ "github.com/containerd/containerd/platforms"
+ "github.com/containerd/containerd/remotes"
+ digest "github.com/opencontainers/go-digest"
+ ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
+ "github.com/stretchr/testify/require"
+)
+
+func TestConfigMultiplatform(t *testing.T) {
+ ctx := context.Background()
+
+ cc := &testCache{}
+
+ pAmd64 := platforms.MustParse("linux/amd64")
+ cfgDescAmd64 := cc.Add(t, ocispecs.Image{Platform: pAmd64}, ocispecs.MediaTypeImageConfig, nil)
+ mfstAmd64 := ocispecs.Manifest{MediaType: ocispecs.MediaTypeImageManifest, Config: cfgDescAmd64}
+
+ // Add linux/amd64 to the cache
+ descAmd64 := cc.Add(t, mfstAmd64, mfstAmd64.MediaType, &pAmd64)
+
+ // Make a compatible manifest but do not add to the cache
+ p386 := platforms.MustParse("linux/386")
+ cfgDesc386 := cc.Add(t, ocispecs.Image{Platform: p386}, ocispecs.MediaTypeImageConfig, nil)
+ mfst386 := ocispecs.Manifest{MediaType: ocispecs.MediaTypeImageManifest, Config: cfgDesc386}
+ _, desc386 := makeDesc(t, mfst386, mfst386.MediaType, &p386)
+
+ idx := ocispecs.Index{
+ MediaType: ocispecs.MediaTypeImageIndex,
+ Manifests: []ocispecs.Descriptor{desc386, descAmd64},
+ }
+
+ idxDesc := cc.Add(t, idx, idx.MediaType, nil)
+ r := &testResolver{cc: cc, resolve: func(ctx context.Context, ref string) (string, ocispecs.Descriptor, error) {
+ return ref, idxDesc, nil
+ }}
+
+ // Now we should be able to get the amd64 config without fetching anything from the remote
+ // If it tries to fetch from the remote this will error out.
+ const ref = "example.com/test:latest"
+ _, _, dt, err := Config(ctx, ref, r, cc, nil, &pAmd64, nil)
+ require.NoError(t, err)
+
+ var cfg ocispecs.Image
+ require.NoError(t, json.Unmarshal(dt, &cfg))
+ // Validate this is the amd64 config
+ require.True(t, platforms.OnlyStrict(cfg.Platform).Match(pAmd64))
+}
+
+type testCache struct {
+ content.Manager
+ content map[digest.Digest]content.ReaderAt
+}
+
+func (testCache) Info(ctx context.Context, dgst digest.Digest) (content.Info, error) {
+ return content.Info{}, nil
+}
+
+func (testCache) Update(context.Context, content.Info, ...string) (content.Info, error) {
+ return content.Info{}, nil
+}
+
+func (*testCache) Writer(ctx context.Context, opts ...content.WriterOpt) (content.Writer, error) {
+ // This needs to be implemented because the content helpers will open a writer to use as a lock
+ return nopWriter{}, nil
+}
+
+type nopWriter struct{}
+
+func (nopWriter) Write(p []byte) (int, error) {
+ return len(p), nil
+}
+
+func (nopWriter) Close() error {
+ return nil
+}
+
+func (nopWriter) Commit(ctx context.Context, size int64, expected digest.Digest, opts ...content.Opt) error {
+ return nil
+}
+
+func (nopWriter) Status() (content.Status, error) {
+ return content.Status{}, nil
+}
+
+func (nopWriter) Truncate(size int64) error {
+ return nil
+}
+
+func (nopWriter) Digest() digest.Digest {
+ panic("not implemented")
+}
+
+func makeDesc(t *testing.T, item any, mt string, p *ocispecs.Platform) ([]byte, ocispecs.Descriptor) {
+ t.Helper()
+
+ dt, err := json.Marshal(item)
+ require.NoError(t, err)
+
+ return dt, ocispecs.Descriptor{
+ MediaType: mt,
+ Digest: digest.FromBytes(dt),
+ Size: int64(len(dt)),
+ Platform: p,
+ }
+}
+
+func (c *testCache) Add(t *testing.T, item any, mt string, p *ocispecs.Platform) ocispecs.Descriptor {
+ if c.content == nil {
+ c.content = make(map[digest.Digest]content.ReaderAt)
+ }
+
+ dt, desc := makeDesc(t, item, mt, p)
+ c.content[desc.Digest] = &sectionNopCloser{io.NewSectionReader(bytes.NewReader(dt), 0, int64(len(dt)))}
+ return desc
+}
+
+type sectionNopCloser struct {
+ *io.SectionReader
+}
+
+func (*sectionNopCloser) Close() error {
+ return nil
+}
+
+func (t *testCache) ReaderAt(ctx context.Context, desc ocispecs.Descriptor) (content.ReaderAt, error) {
+ ra, ok := t.content[desc.Digest]
+ if !ok {
+ return nil, fmt.Errorf("not found")
+ }
+ return ra, nil
+}
+
+type testResolver struct {
+ resolve func(ctx context.Context, ref string) (string, ocispecs.Descriptor, error)
+ cc ContentCache
+}
+
+type fetcherFunc func(context.Context, ocispecs.Descriptor) (io.ReadCloser, error)
+
+func (f fetcherFunc) Fetch(ctx context.Context, desc ocispecs.Descriptor) (io.ReadCloser, error) {
+ return f(ctx, desc)
+}
+
+func (r *testResolver) Fetcher(ctx context.Context, ref string) (remotes.Fetcher, error) {
+ return fetcherFunc(func(ctx context.Context, desc ocispecs.Descriptor) (io.ReadCloser, error) {
+ ra, err := r.cc.ReaderAt(ctx, desc)
+ if err != nil {
+ return nil, err
+ }
+ return &raReader{ReaderAt: ra, Closer: ra}, nil
+ }), nil
+}
+
+type raReader struct {
+ io.ReaderAt
+ io.Closer
+
+ pos int
+}
+
+func (r *raReader) Read(p []byte) (int, error) {
+ n, err := r.ReaderAt.ReadAt(p, int64(r.pos))
+ r.pos += n
+ return n, err
+}
+
+func (r *testResolver) Resolve(ctx context.Context, ref string) (string, ocispecs.Descriptor, error) {
+ return r.resolve(ctx, ref)
+}
+
+func (r *testResolver) Pusher(context.Context, string) (remotes.Pusher, error) {
+ panic("not implemented")
+}