Add option to configure default platforms

docs/configuration.md

@@ -83,6 +83,30 @@ The `ldflags` default value is `[]`.
 only the `env`, `flags` and `ldflags` fields are currently supported. Also, the
 templating support is currently limited to using environment variables only.
 
+
+### Setting default platforms
+
+By default, `ko` builds images based on the platform it runs on. If your target platform differs from your build platform you can specify the build platform:
+
+**As a parameter**
+See [Multi-Platform Images](./features/multi-platform.md).
+
+**In .ko.yaml**
+Add this to your `.ko.yaml` file:
+
+```yaml
+defaultPlatforms:
+- linux/arm64
+- linux/amd64
+```
+
+You can also use the `KO_DEFAULTPLATFORMS` environment variable to set the default platforms, which overrides the YAML configuration:
+
+```shell
+KO_DEFAULTPLATFORMS=linux/arm64,linux/amd64
+```
+
+
 ## Naming Images
 
 `ko` provides a few different strategies for naming the image it pushes, to

pkg/commands/options/build.go

@@ -44,6 +44,9 @@ type BuildOptions struct {
 	// BaseImageOverrides stores base image overrides for import paths.
 	BaseImageOverrides map[string]string
 
+	// DefaultPlatforms defines the default platforms when Platforms is not explicitly defined
+	DefaultPlatforms []string
+
 	// WorkingDirectory allows for setting the working directory for invocations of the `go` tool.
 	// Empty string means the current working directory.
 	WorkingDirectory string
@@ -129,6 +132,11 @@ func (bo *BuildOptions) LoadConfig() error {
 		}
 	}
 
+	dp := v.GetStringSlice("defaultPlatforms")
+	if len(dp) > 0 {
+		bo.DefaultPlatforms = dp
+	}
+
 	if bo.BaseImage == "" {
 		ref := v.GetString("defaultBaseImage")
 		if _, err := name.ParseReference(ref); err != nil {

pkg/commands/options/build_test.go

@@ -16,6 +16,7 @@ package options
 
 import (
 	"os"
+	"reflect"
 	"strings"
 	"testing"
 
@@ -38,6 +39,34 @@ func TestDefaultBaseImage(t *testing.T) {
 	}
 }
 
+func TestDefaultPlatformsAll(t *testing.T) {
+	allBo := &BuildOptions{
+		WorkingDirectory: "testdata/config",
+	}
+	err := allBo.LoadConfig()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	wantDefaultPlatformsAll := []string{"all"} // matches value in ./testdata/config/.ko.yaml
+	if !reflect.DeepEqual(allBo.DefaultPlatforms, wantDefaultPlatformsAll) {
+		t.Fatalf("wanted DefaultPlatforms %s, got %s", wantDefaultPlatformsAll, allBo.DefaultPlatforms)
+	}
+
+	multipleBo := &BuildOptions{
+		WorkingDirectory: "testdata/multiple-platforms",
+	}
+	err = multipleBo.LoadConfig()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	wantDefaultPlatformsMultiple := []string{"linux/arm64", "linux/amd64"} // matches value in ./testdata/multiple-platforms/.ko.yaml
+	if !reflect.DeepEqual(multipleBo.DefaultPlatforms, wantDefaultPlatformsMultiple) {
+		t.Fatalf("wanted DefaultPlatforms %s, got %s", wantDefaultPlatformsMultiple, multipleBo.DefaultPlatforms)
+	}
+}
+
 func TestBuildConfigWithWorkingDirectoryAndDirAndMain(t *testing.T) {
 	bo := &BuildOptions{
 		WorkingDirectory: "testdata/paths",

pkg/commands/options/testdata/config/.ko.yaml

@@ -1 +1,2 @@
 defaultBaseImage: alpine
+defaultPlatforms: all

pkg/commands/options/testdata/multiple-platforms/.ko.yaml

@@ -0,0 +1,4 @@
+defaultBaseImage: alpine
+defaultPlatforms:
+- linux/arm64
+- linux/amd64

pkg/commands/resolver.go

@@ -58,6 +58,10 @@ func gobuildOptions(bo *options.BuildOptions) ([]build.Option, error) {
 		return nil, err
 	}
 
+	if len(bo.Platforms) == 0 && len(bo.DefaultPlatforms) > 0 {
+		bo.Platforms = bo.DefaultPlatforms
+	}
+
 	if len(bo.Platforms) == 0 {
 		envPlatform := "linux/amd64"
 
@@ -78,7 +82,7 @@ func gobuildOptions(bo *options.BuildOptions) ([]build.Option, error) {
 		// Make sure these are all unset
 		for _, env := range []string{"GOOS", "GOARCH", "GOARM"} {
 			if s, ok := os.LookupEnv(env); ok {
-				return nil, fmt.Errorf("cannot use --platform with %s=%q", env, s)
+				return nil, fmt.Errorf("cannot use --platform or defaultPlatforms in .ko.yaml or env KO_DEFAULTPLATFORMS combined with %s=%q", env, s)
 			}
 		}
 	}