[RFC] Support Bazel platform mappings

This is a proof of concept/RFC to support Bazel platforms in skaffold.

Skaffold uses github.com/opencontainers/image-spec/specs-go/v1 and
containerd strings like "linux/amd64" or "linux/arm64/v7" to represent
platforms.

Bazel's platform system uses bazel targets to define platforms
(https://bazel.build/extending/platforms). While there are common
cpu and os definitions available, each Bazel project defines its own
target platforms (!)

Currently, when skaffold invokes bazel, skaffold's platform information
is not passed to Bazel. Bazel defaults to the host platform, unless the
target skaffold is building explicitly specifies a platform.

This change allows a build-level setting for configuring Bazel and
defines a platform mapping from Skaffold platforms to Bazel platforms.
The Bazel artifact builder then passes the --platforms flag to Bazel if applicable.

This allows a Bazel + Skaffold project to flexibly target multiple platforms.

This PR is intended to start a discussion w/ the Skaffold team on whether
this should be supported and if so, how it should be implemented. If there's
interest we can discuss pushing forward from there. This doesn't include
any tests or documentation, and is not intended to be merged as-is.

pkg/skaffold/build/bazel/build.go

@@ -45,7 +45,7 @@ func (b *Builder) Build(ctx context.Context, out io.Writer, artifact *latest.Art
 
 	a := artifact.ArtifactType.BazelArtifact
 
-	tarPath, err := b.buildTar(ctx, out, artifact.Workspace, a)
+	tarPath, err := b.buildTar(ctx, out, artifact.Workspace, a, matcher)
 	if err != nil {
 		return "", err
 	}
@@ -58,7 +58,7 @@ func (b *Builder) Build(ctx context.Context, out io.Writer, artifact *latest.Art
 
 func (b *Builder) SupportedPlatforms() platform.Matcher { return platform.All }
 
-func (b *Builder) buildTar(ctx context.Context, out io.Writer, workspace string, a *latest.BazelArtifact) (string, error) {
+func (b *Builder) buildTar(ctx context.Context, out io.Writer, workspace string, a *latest.BazelArtifact, matcher platform.Matcher) (string, error) {
 	if !strings.HasSuffix(a.BuildTarget, ".tar") {
 		return "", errors.New("the bazel build target should end with .tar, see https://github.com/bazelbuild/rules_docker#using-with-docker-locally")
 	}
@@ -67,6 +67,18 @@ func (b *Builder) buildTar(ctx context.Context, out io.Writer, workspace string,
 	args = append(args, a.BuildArgs...)
 	args = append(args, a.BuildTarget)
 
+	if b.bazelCfg != nil {
+		platformMappings := b.bazelCfg.PlatformMappings
+		for _, mapping := range platformMappings {
+			m, err := platform.Parse([]string{mapping.Platform})
+			if err == nil {
+				if matcher.Intersect(m).IsNotEmpty() {
+					args = append(args, fmt.Sprintf("--platforms=%s", mapping.BazelPlatformTarget))
+				}
+			}
+		}
+	}
+
 	if output.IsColorable(out) {
 		args = append(args, "--color=yes")
 	} else {

pkg/skaffold/build/bazel/types.go

@@ -16,20 +16,25 @@ limitations under the License.
 
 package bazel
 
-import "github.com/GoogleContainerTools/skaffold/v2/pkg/skaffold/docker"
+import (
+	"github.com/GoogleContainerTools/skaffold/v2/pkg/skaffold/docker"
+	"github.com/GoogleContainerTools/skaffold/v2/pkg/skaffold/schema/latest"
+)
 
 // Builder is an artifact builder that uses Bazel
 type Builder struct {
 	localDocker docker.LocalDaemon
 	cfg         docker.Config
 	pushImages  bool
+	bazelCfg    *latest.BazelBuildConfig
 }
 
 // NewArtifactBuilder returns a new bazel artifact builder
-func NewArtifactBuilder(localDocker docker.LocalDaemon, cfg docker.Config, pushImages bool) *Builder {
+func NewArtifactBuilder(localDocker docker.LocalDaemon, cfg docker.Config, pushImages bool, bazelCfg *latest.BazelBuildConfig) *Builder {
 	return &Builder{
 		localDocker: localDocker,
 		cfg:         cfg,
 		pushImages:  pushImages,
+		bazelCfg:    bazelCfg,
 	}
 }

pkg/skaffold/build/local/types.go

@@ -58,6 +58,7 @@ type Builder struct {
 	localPruner        *pruner
 	artifactStore      build.ArtifactStore
 	sourceDependencies graph.SourceDependenciesCache
+	bazelCfg           *latest.BazelBuildConfig
 }
 
 type Config interface {
@@ -80,7 +81,7 @@ type BuilderContext interface {
 }
 
 // NewBuilder returns an new instance of a local Builder.
-func NewBuilder(ctx context.Context, bCtx BuilderContext, buildCfg *latest.LocalBuild) (*Builder, error) {
+func NewBuilder(ctx context.Context, bCtx BuilderContext, buildCfg *latest.LocalBuild, bazelCfg *latest.BazelBuildConfig) (*Builder, error) {
 	localDocker, err := docker.NewAPIClient(ctx, bCtx)
 	if err != nil {
 		return nil, fmt.Errorf("getting docker client: %w", err)
@@ -120,6 +121,7 @@ func NewBuilder(ctx context.Context, bCtx BuilderContext, buildCfg *latest.Local
 		muted:              bCtx.Muted(),
 		artifactStore:      bCtx.ArtifactStore(),
 		sourceDependencies: bCtx.SourceDependenciesResolver(),
+		bazelCfg:           bazelCfg,
 	}, nil
 }
 
@@ -136,7 +138,7 @@ func newPerArtifactBuilder(b *Builder, a *latest.Artifact) (artifactBuilder, err
 		return dockerbuilder.NewArtifactBuilder(b.localDocker, b.cfg, b.local.UseDockerCLI, b.local.UseBuildkit, b.pushImages, b.artifactStore, b.sourceDependencies), nil
 
 	case a.BazelArtifact != nil:
-		return bazel.NewArtifactBuilder(b.localDocker, b.cfg, b.pushImages), nil
+		return bazel.NewArtifactBuilder(b.localDocker, b.cfg, b.pushImages, b.bazelCfg), nil
 
 	case a.JibArtifact != nil:
 		return jib.NewArtifactBuilder(b.localDocker, b.cfg, b.pushImages, b.skipTests, b.artifactStore), nil

pkg/skaffold/runner/builder.go

@@ -53,7 +53,7 @@ func GetBuilder(ctx context.Context, r *runcontext.RunContext, s build.ArtifactS
 	switch {
 	case p.Build.LocalBuild != nil:
 		log.Entry(context.TODO()).Debug("Using builder: local")
-		builder, err := local.NewBuilder(ctx, bCtx, p.Build.LocalBuild)
+		builder, err := local.NewBuilder(ctx, bCtx, p.Build.LocalBuild, p.Build.Bazel)
 		if err != nil {
 			return nil, err
 		}

pkg/skaffold/schema/latest/config.go

@@ -240,6 +240,8 @@ type BuildConfig struct {
 	// Example: `["linux/amd64", "linux/arm64"]`.
 	Platforms []string `yaml:"platforms,omitempty"`
 
+	Bazel *BazelBuildConfig `yaml:"bazel,omitempty"`
+
 	BuildType `yaml:",inline"`
 }
 
@@ -331,6 +333,15 @@ type TaggerComponent struct {
 	Component TagPolicy `yaml:",inline" yamltags:"skipTrim"`
 }
 
+type BazelBuildConfig struct {
+	PlatformMappings []BazelPlatformMapping `yaml:"platforms,omitempty"`
+}
+
+type BazelPlatformMapping struct {
+	Platform            string `yaml:"platform,omitempty"`
+	BazelPlatformTarget string `yaml:"target,omitempty"`
+}
+
 // BuildType contains the specific implementation and parameters needed
 // for the build step. Only one field should be populated.
 type BuildType struct {