exporter: detect if multi-platform is set

This is required now that the correspondence between platforms and refs
has been broken. Without it, there's no way to detect between the
following cases from the output that they emit:

	buildctl build --opt platform=linux/amd64 --opt attest:sbom
	buildctl build --opt platform=linux/amd64 --opt attest:sbom --opt build-arg:BUILDKIT_MULTI_PLATFORM=true

We'd like the former to produce a flat file structure through the local
exporter, and the latter to produce an explicitly nested file structure
through the same exporter. However, both of these produce Refs instead
of a singular Ref, so we can't just look at the Result to know which
one.

Similar to how we handle the SOURCE_DATE_EPOCH build-arg, we can handle
the multi-platform args at the control API boundary, setting the
explicit option multi-platform in the exporter if it is set. In the
future, we can guide users away from the BUILDKIT_MULTI_PLATFORM args
entirely, and towards the multi-platform exporter option if we want.

Signed-off-by: Justin Chadwell <me@jedevc.com>

control/control.go

@@ -17,6 +17,7 @@ import (
 	controlgateway "github.com/moby/buildkit/control/gateway"
 	"github.com/moby/buildkit/exporter"
 	"github.com/moby/buildkit/exporter/util/epoch"
+	"github.com/moby/buildkit/exporter/util/multiplatform"
 	"github.com/moby/buildkit/frontend"
 	"github.com/moby/buildkit/frontend/attestations"
 	"github.com/moby/buildkit/session"
@@ -304,6 +305,16 @@ func (c *Controller) Solve(ctx context.Context, req *controlapi.SolveRequest) (*
 		}
 	}
 
+	// if multi-platform is set, enable it for the exporter
+	if v, ok := multiplatform.ParseFrontend(req.FrontendAttrs); ok {
+		if _, ok := req.ExporterAttrs[multiplatform.KeyMultiPlatform]; !ok {
+			if req.ExporterAttrs == nil {
+				req.ExporterAttrs = make(map[string]string)
+			}
+			req.ExporterAttrs[multiplatform.KeyMultiPlatform] = v
+		}
+	}
+
 	if req.Exporter != "" {
 		exp, err := w.Exporter(req.Exporter, c.opt.SessionManager)
 		if err != nil {

exporter/containerimage/opts.go

@@ -6,6 +6,7 @@ import (
 
 	cacheconfig "github.com/moby/buildkit/cache/config"
 	"github.com/moby/buildkit/exporter/util/epoch"
+	"github.com/moby/buildkit/exporter/util/multiplatform"
 	"github.com/moby/buildkit/util/compression"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
@@ -34,6 +35,7 @@ type ImageCommitOpts struct {
 	BuildInfoAttrs bool
 	Annotations    AnnotationsGroup
 	Epoch          *time.Time
+	MultiPlatform  *bool
 }
 
 func (c *ImageCommitOpts) Load(opt map[string]string) (map[string]string, error) {
@@ -50,6 +52,11 @@ func (c *ImageCommitOpts) Load(opt map[string]string) (map[string]string, error)
 		return nil, err
 	}
 
+	c.MultiPlatform, opt, err = multiplatform.ParseAttr(opt)
+	if err != nil {
+		return nil, err
+	}
+
 	for k, v := range opt {
 		var err error
 		switch k {

exporter/containerimage/writer.go

@@ -63,11 +63,20 @@ func (ic *ImageWriter) Commit(ctx context.Context, inp *exporter.Source, session
 		return nil, errors.Errorf("unable to export multiple refs, missing platforms mapping")
 	}
 
+	multiPlatform := len(inp.Refs) > 0
+
 	var p exptypes.Platforms
 	if ok && len(platformsBytes) > 0 {
 		if err := json.Unmarshal(platformsBytes, &p); err != nil {
 			return nil, errors.Wrapf(err, "failed to parse platforms passed to exporter")
 		}
+		if len(p.Platforms) > 1 {
+			multiPlatform = true
+		}
+	}
+
+	if opts.MultiPlatform != nil {
+		multiPlatform = *opts.MultiPlatform
 	}
 
 	if opts.Epoch == nil {
@@ -92,8 +101,26 @@ func (ic *ImageWriter) Commit(ctx context.Context, inp *exporter.Source, session
 		}
 	}
 
-	if len(inp.Refs) == 0 {
-		remotes, err := ic.exportLayers(ctx, opts.RefCfg, session.NewGroup(sessionID), inp.Ref)
+	if !multiPlatform {
+		if len(p.Platforms) > 1 {
+			return nil, errors.Errorf("unable to export without multi-platform, multiple platforms passed to exporter")
+		}
+
+		var ref cache.ImmutableRef
+		if inp.Ref != nil {
+			ref = inp.Ref
+		} else if len(p.Platforms) > 0 {
+			p := p.Platforms[0]
+			if _, ok := inp.Attestations[p.ID]; ok {
+				return nil, errors.Errorf("unable to export attestations without multi-platform")
+			}
+			ref = inp.Refs[p.ID]
+		} else if len(inp.Refs) == 1 {
+			for _, ref = range inp.Refs {
+			}
+		}
+
+		remotes, err := ic.exportLayers(ctx, opts.RefCfg, session.NewGroup(sessionID), ref)
 		if err != nil {
 			return nil, err
 		}
@@ -112,7 +139,7 @@ func (ic *ImageWriter) Commit(ctx context.Context, inp *exporter.Source, session
 			return nil, errors.Errorf("index annotations not supported for single platform export")
 		}
 
-		mfstDesc, configDesc, err := ic.commitDistributionManifest(ctx, opts, inp.Ref, inp.Metadata[exptypes.ExporterImageConfigKey], &remotes[0], annotations, inp.Metadata[exptypes.ExporterInlineCache], dtbi, opts.Epoch, session.NewGroup(sessionID))
+		mfstDesc, configDesc, err := ic.commitDistributionManifest(ctx, opts, ref, inp.Metadata[exptypes.ExporterImageConfigKey], &remotes[0], annotations, inp.Metadata[exptypes.ExporterInlineCache], dtbi, opts.Epoch, session.NewGroup(sessionID))
 		if err != nil {
 			return nil, err
 		}

exporter/local/export.go

@@ -11,6 +11,7 @@ import (
 	"github.com/moby/buildkit/exporter"
 	"github.com/moby/buildkit/exporter/containerimage/exptypes"
 	"github.com/moby/buildkit/exporter/util/epoch"
+	"github.com/moby/buildkit/exporter/util/multiplatform"
 	"github.com/moby/buildkit/session"
 	"github.com/moby/buildkit/session/filesync"
 	"github.com/moby/buildkit/solver/result"
@@ -46,10 +47,16 @@ func (e *localExporter) Resolve(ctx context.Context, opt map[string]string) (exp
 		return nil, err
 	}
 
+	multiPlatform, _, err := multiplatform.ParseAttr(opt)
+	if err != nil {
+		return nil, err
+	}
+
 	i := &localExporterInstance{
 		localExporter: e,
 		opts: CreateFSOpts{
-			Epoch: tm,
+			Epoch:         tm,
+			MultiPlatform: multiPlatform,
 		},
 	}
 
@@ -93,6 +100,8 @@ func (e *localExporterInstance) Export(ctx context.Context, inp *exporter.Source
 		return nil, err
 	}
 
+	isMap := len(inp.Refs) > 0
+
 	platformsBytes, ok := inp.Metadata[exptypes.ExporterPlatformsKey]
 	if len(inp.Refs) > 0 && !ok {
 		return nil, errors.Errorf("unable to export multiple refs, missing platforms mapping")
@@ -103,8 +112,17 @@ func (e *localExporterInstance) Export(ctx context.Context, inp *exporter.Source
 		if err := json.Unmarshal(platformsBytes, &p); err != nil {
 			return nil, errors.Wrapf(err, "failed to parse platforms passed to exporter")
 		}
+		if len(p.Platforms) > 1 {
+			isMap = true
+		}
+	}
+
+	if e.opts.MultiPlatform != nil {
+		isMap = *e.opts.MultiPlatform
+	}
+	if !isMap && len(p.Platforms) > 1 {
+		return nil, errors.Errorf("unable to export multiple platforms without map")
 	}
-	isMap := len(p.Platforms) > 1
 
 	now := time.Now().Truncate(time.Second)
 

exporter/local/fs.go

@@ -26,6 +26,7 @@ import (
 type CreateFSOpts struct {
 	Epoch             *time.Time
 	AttestationPrefix string
+	MultiPlatform     *bool
 }
 
 func CreateFS(ctx context.Context, sessionID string, k string, ref cache.ImmutableRef, refs map[string]cache.ImmutableRef, attestations []result.Attestation, defaultTime time.Time, opt CreateFSOpts) (fsutil.FS, func() error, error) {

exporter/tar/export.go

@@ -13,6 +13,7 @@ import (
 	"github.com/moby/buildkit/exporter/containerimage/exptypes"
 	"github.com/moby/buildkit/exporter/local"
 	"github.com/moby/buildkit/exporter/util/epoch"
+	"github.com/moby/buildkit/exporter/util/multiplatform"
 	"github.com/moby/buildkit/session"
 	"github.com/moby/buildkit/session/filesync"
 	"github.com/moby/buildkit/solver/result"
@@ -48,12 +49,18 @@ func New(opt Opt) (exporter.Exporter, error) {
 func (e *localExporter) Resolve(ctx context.Context, opt map[string]string) (exporter.ExporterInstance, error) {
 	li := &localExporterInstance{localExporter: e}
 
-	tm, _, err := epoch.ParseAttr(opt)
+	tm, opt, err := epoch.ParseAttr(opt)
 	if err != nil {
 		return nil, err
 	}
 	li.opts.Epoch = tm
 
+	multiPlatform, opt, err := multiplatform.ParseAttr(opt)
+	if err != nil {
+		return nil, err
+	}
+	li.opts.MultiPlatform = multiPlatform
+
 	for k, v := range opt {
 		switch k {
 		case preferNondistLayersKey:
@@ -126,6 +133,8 @@ func (e *localExporterInstance) Export(ctx context.Context, inp *exporter.Source
 		}, nil
 	}
 
+	isMap := len(inp.Refs) > 0
+
 	platformsBytes, ok := inp.Metadata[exptypes.ExporterPlatformsKey]
 	if len(inp.Refs) > 0 && !ok {
 		return nil, errors.Errorf("unable to export multiple refs, missing platforms mapping")
@@ -136,8 +145,17 @@ func (e *localExporterInstance) Export(ctx context.Context, inp *exporter.Source
 		if err := json.Unmarshal(platformsBytes, &p); err != nil {
 			return nil, errors.Wrapf(err, "failed to parse platforms passed to exporter")
 		}
+		if len(p.Platforms) > 1 {
+			isMap = true
+		}
+	}
+
+	if e.opts.MultiPlatform != nil {
+		isMap = *e.opts.MultiPlatform
+	}
+	if !isMap && len(p.Platforms) > 1 {
+		return nil, errors.Errorf("unable to export multiple platforms without map")
 	}
-	isMap := len(p.Platforms) > 1
 
 	var fs fsutil.FS
 

exporter/util/multiplatform/parse.go

@@ -0,0 +1,45 @@
+package multiplatform
+
+import (
+	"strconv"
+
+	"github.com/pkg/errors"
+)
+
+const (
+	frontendMultiPlatform    = "multi-platform"
+	frontendMultiPlatformArg = "build-arg:BUILDKIT_MULTI_PLATFORM"
+
+	KeyMultiPlatform = "multi-platform"
+)
+
+func ParseFrontend(opt map[string]string) (string, bool) {
+	if v, ok := opt[frontendMultiPlatform]; ok {
+		return v, true
+	}
+	if v, ok := opt[frontendMultiPlatformArg]; ok {
+		return v, true
+	}
+	return "", false
+}
+
+func ParseAttr(opt map[string]string) (*bool, map[string]string, error) {
+	rest := make(map[string]string, len(opt))
+
+	var multiPlatform *bool
+
+	for k, v := range opt {
+		switch k {
+		case KeyMultiPlatform:
+			b, err := strconv.ParseBool(v)
+			if err != nil {
+				return nil, nil, errors.Errorf("invalid boolean value %s", v)
+			}
+			multiPlatform = &b
+		default:
+			rest[k] = v
+		}
+	}
+
+	return multiPlatform, rest, nil
+}