Fallback to manifest list when no platform match

In some cases, in fact many in the wild, an image may have the incorrect
platform on the image config.
This can lead to failures to run an image, particularly when a user
specifies a `--platform`.
Typically what we see in the wild is a manifest list with an an entry
for, as an example, linux/arm64 pointing to an image config that has
linux/amd64 on it.

This change falls back to looking up the manifest list for an image to
see if the manifest list shows the image as the correct one for that
platform.

In order to accomplish this we need to traverse the leases associated
with an image. Each image, if pulled with Docker 20.10, will have the
manifest list stored in the containerd content store with the resource
assigned to a lease keyed on the image ID.
So we look up the lease for the image, then look up the assocated
resources to find the manifest list, then check the manifest list for a
platform match, then ensure that manifest referes to our image config.

This is only used as a fallback when a user specified they want a
particular platform and the image config that we have does not match
that platform.

Signed-off-by: Brian Goff <cpuguy83@gmail.com>

daemon/images/image.go

@@ -1,15 +1,24 @@
 package images // import "github.com/docker/docker/daemon/images"
 
 import (
+	"context"
+	"encoding/json"
 	"fmt"
+	"io"
+	"io/ioutil"
 
+	"github.com/containerd/containerd/content"
+	c8derrdefs "github.com/containerd/containerd/errdefs"
+	"github.com/containerd/containerd/images"
+	"github.com/containerd/containerd/leases"
 	"github.com/containerd/containerd/platforms"
-	"github.com/pkg/errors"
-
 	"github.com/docker/distribution/reference"
 	"github.com/docker/docker/errdefs"
 	"github.com/docker/docker/image"
+	digest "github.com/opencontainers/go-digest"
 	specs "github.com/opencontainers/image-spec/specs-go/v1"
+	"github.com/pkg/errors"
+	"github.com/sirupsen/logrus"
 )
 
 // ErrImageDoesNotExist is error returned when no image can be found for a reference.
@@ -28,6 +37,115 @@ func (e ErrImageDoesNotExist) Error() string {
 // NotFound implements the NotFound interface
 func (e ErrImageDoesNotExist) NotFound() {}
 
+type manifestList struct {
+	Manifests []specs.Descriptor `json:"manifests"`
+}
+
+type manifest struct {
+	Config specs.Descriptor `json:"config"`
+}
+
+func (i *ImageService) manifestMatchesPlatform(img *image.Image, platform specs.Platform) bool {
+	ctx := context.TODO()
+	logger := logrus.WithField("image", img.ID).WithField("desiredPlatform", platforms.Format(platform))
+
+	ls, leaseErr := i.leases.ListResources(context.TODO(), leases.Lease{ID: imageKey(img.ID().Digest())})
+	if leaseErr != nil {
+		logger.WithError(leaseErr).Error("Error looking up image leases")
+		return false
+	}
+
+	comparer := platforms.Only(platform)
+
+	var (
+		ml manifestList
+		m  manifest
+	)
+
+	makeRdr := func(ra content.ReaderAt) io.Reader {
+		return io.LimitReader(io.NewSectionReader(ra, 0, ra.Size()), 1e6)
+	}
+
+	for _, r := range ls {
+		logger := logger.WithField("resourceID", r.ID).WithField("resourceType", r.Type)
+		logger.Debug("Checking lease resource for platform match")
+		if r.Type != "content" {
+			continue
+		}
+
+		ra, err := i.content.ReaderAt(ctx, specs.Descriptor{Digest: digest.Digest(r.ID)})
+		if err != nil {
+			if c8derrdefs.IsNotFound(err) {
+				continue
+			}
+			logger.WithError(err).Error("Error looking up referenced manifest list for image")
+			continue
+		}
+
+		data, err := ioutil.ReadAll(makeRdr(ra))
+		ra.Close()
+
+		if err != nil {
+			logger.WithError(err).Error("Error reading manifest list for image")
+			continue
+		}
+
+		ml.Manifests = nil
+
+		if err := json.Unmarshal(data, &ml); err != nil {
+			logger.WithError(err).Error("Error unmarshalling content")
+			continue
+		}
+
+		for _, md := range ml.Manifests {
+			switch md.MediaType {
+			case specs.MediaTypeImageManifest, images.MediaTypeDockerSchema2Manifest:
+			default:
+				continue
+			}
+
+			p := specs.Platform{
+				Architecture: md.Platform.Architecture,
+				OS:           md.Platform.OS,
+				Variant:      md.Platform.Variant,
+			}
+			if !comparer.Match(p) {
+				logger.WithField("otherPlatform", platforms.Format(p)).Debug("Manifest is not a match")
+				continue
+			}
+
+			// Here we have a platform match for the referenced manifest, let's make sure the manifest is actually for the image config we are using.
+
+			ra, err := i.content.ReaderAt(ctx, specs.Descriptor{Digest: md.Digest})
+			if err != nil {
+				logger.WithField("otherDigest", md.Digest).WithError(err).Error("Could not get reader for manifest")
+				continue
+			}
+
+			data, err := ioutil.ReadAll(makeRdr(ra))
+			ra.Close()
+			if err != nil {
+				logger.WithError(err).Error("Error reading manifest for image")
+				continue
+			}
+
+			if err := json.Unmarshal(data, &m); err != nil {
+				logger.WithError(err).Error("Error desserializing manifest")
+				continue
+			}
+
+			if m.Config.Digest == img.ID().Digest() {
+				logger.WithField("manifestDigest", md.Digest).Debug("Found matching manifest for image")
+				return true
+			}
+
+			logger.WithField("otherDigest", md.Digest).Debug("Skipping non-matching manifest")
+		}
+	}
+
+	return false
+}
+
 // GetImage returns an image corresponding to the image referred to by refOrID.
 func (i *ImageService) GetImage(refOrID string, platform *specs.Platform) (retImg *image.Image, retErr error) {
 	defer func() {
@@ -44,6 +162,19 @@ func (i *ImageService) GetImage(refOrID string, platform *specs.Platform) (retIm
 		// Note that `platforms.Only` will fuzzy match this for us
 		// For example: an armv6 image will run just fine an an armv7 CPU, without emulation or anything.
 		if !platforms.Only(p).Match(imgPlat) {
+			// Sometimes image variant is not populated due to legacy builders
+			// We still should support falling back here.
+			if imgPlat.OS == platform.OS && imgPlat.Architecture == platform.Architecture && imgPlat.Variant == "" {
+				logrus.WithField("image", refOrID).WithField("platform", platforms.Format(p)).Debug("Image platform cpu variant is not populated, but otherwise it matches what was requested")
+				return
+			}
+
+			// In some cases the image config can actually be wrong (e.g. classic `docker build` may not handle `--platform` correctly)
+			// So we'll look up the manifest list that coresponds to this imaage to check if at least the manifest list says it is the correct image.
+			if i.manifestMatchesPlatform(retImg, p) {
+				return
+			}
+
 			// This allows us to tell clients that we don't have the image they asked for
 			// Where this gets hairy is the image store does not currently support multi-arch images, e.g.:
 			//   An image `foo` may have a multi-arch manifest, but the image store only fetches the image for a specific platform