fix(ko): Fall back to build configs in .ko.yaml

docs/content/en/docs/pipeline-stages/builders/ko.md

@@ -212,9 +212,18 @@ Useful tips for existing `ko` users:
   [`default-repo` functionality]({{< relref "/docs/environment/image-registries" >}}).
   The ko builder does _not_ read the `KO_DOCKER_REPO` environment variable.
 
-- The ko builder supports reading base image configuration from the `.ko.yaml`
-  file. If you already configure your base images using this file, you do not
-  need to repeat this configuration in `skaffold.yaml`.
+- The ko builder supports reading
+  [base image configuration](https://github.com/google/ko#overriding-base-images)
+  from the `.ko.yaml` file. If you already configure your base images using
+  this file, you do not need to specify the `fromImage` field for the
+  artifact in `skaffold.yaml`.
+
+- The ko builder supports reading
+  [build configs](https://github.com/google/ko#overriding-go-build-settings)
+  from the `.ko.yaml` file if `skaffold.yaml` does not specify any of the build
+  config fields (`dir`, `main`, `env`, `flags`, and `ldflags`). If you already
+  specify these fields in `.ko.yaml`, you do not need to repeat them in
+  `skaffold.yaml`.
 
 - When you use the Skaffold ko builder, Skaffold takes care of replacing the
   image placeholder name in your Kubernetes manifest files using its

pkg/skaffold/build/ko/builder.go

@@ -40,23 +40,13 @@ func (b *Builder) newKoBuilder(ctx context.Context, a *latestV1.Artifact) (build
 }
 
 func buildOptions(a *latestV1.Artifact, runMode config.RunMode) (*options.BuildOptions, error) {
-	koImportpath, err := getImportPath(a)
+	buildconfig, err := buildConfig(a)
 	if err != nil {
-		return nil, fmt.Errorf("could not determine import path: %v", err)
+		return nil, fmt.Errorf("could not create ko build config: %v", err)
 	}
-	importpath := strings.TrimPrefix(koImportpath, build.StrictScheme)
 	return &options.BuildOptions{
-		BaseImage: a.KoArtifact.BaseImage,
-		BuildConfigs: map[string]build.Config{
-			importpath: {
-				ID:      a.ImageName,
-				Dir:     ".",
-				Env:     a.KoArtifact.Env,
-				Flags:   a.KoArtifact.Flags,
-				Ldflags: a.KoArtifact.Ldflags,
-				Main:    a.KoArtifact.Main,
-			},
-		},
+		BaseImage:            a.KoArtifact.BaseImage,
+		BuildConfigs:         buildconfig,
 		ConcurrentBuilds:     1, // we could plug in Skaffold's max builds here, but it'd be incorrect if users build more than one artifact
 		DisableOptimizations: runMode == config.RunModes.Debug,
 		Labels:               labels(a),
@@ -66,6 +56,49 @@ func buildOptions(a *latestV1.Artifact, runMode config.RunMode) (*options.BuildO
 	}, nil
 }
 
+// buildConfig creates the ko build config map based on the artifact config.
+// A map entry is only required if the artifact config specifies fields that need to be part of ko build configs.
+// If none of these are specified, we can provide an empty `BuildConfigs` map.
+// In this case, ko falls back to build configs provided in `.ko.yaml`, or to the default zero config.
+func buildConfig(a *latestV1.Artifact) (map[string]build.Config, error) {
+	buildconfigs := map[string]build.Config{}
+	if koArtifactSpecifiesBuildConfig(*a.KoArtifact) {
+		koImportpath, err := getImportPath(a)
+		if err != nil {
+			return nil, fmt.Errorf("could not determine import path of image %s: %v", a.ImageName, err)
+		}
+		importpath := strings.TrimPrefix(koImportpath, build.StrictScheme)
+		buildconfigs[importpath] = build.Config{
+			ID:      a.ImageName,
+			Dir:     ".",
+			Env:     a.KoArtifact.Env,
+			Flags:   a.KoArtifact.Flags,
+			Ldflags: a.KoArtifact.Ldflags,
+			Main:    a.KoArtifact.Main,
+		}
+	}
+	return buildconfigs, nil
+}
+
+func koArtifactSpecifiesBuildConfig(k latestV1.KoArtifact) bool {
+	if k.Dir != "" && k.Dir != "." {
+		return true
+	}
+	if k.Main != "" && k.Main != "." {
+		return true
+	}
+	if len(k.Env) != 0 {
+		return true
+	}
+	if len(k.Flags) != 0 {
+		return true
+	}
+	if len(k.Ldflags) != 0 {
+		return true
+	}
+	return false
+}
+
 func labels(a *latestV1.Artifact) []string {
 	var labels []string
 	for k, v := range a.KoArtifact.Labels {

pkg/skaffold/build/ko/builder_test.go

@@ -28,8 +28,6 @@ import (
 	"github.com/GoogleContainerTools/skaffold/testutil"
 )
 
-// Most of the test cases set the artifact image name to a `ko://`-prefixed import path.
-// This speeds up the tests, as the code under test doesn't need to repeatedly determine the import path of this package.
 func TestBuildOptions(t *testing.T) {
 	tests := []struct {
 		description              string
@@ -48,7 +46,6 @@ func TestBuildOptions(t *testing.T) {
 					KoArtifact: &latestV1.KoArtifact{},
 				},
 			},
-			wantImportPath: "github.com/GoogleContainerTools/skaffold/pkg/skaffold/build/ko", // this package
 		},
 		{
 			description: "base image",
@@ -60,7 +57,6 @@ func TestBuildOptions(t *testing.T) {
 				},
 				ImageName: "ko://example.com/foo",
 			},
-			wantImportPath: "example.com/foo",
 		},
 		{
 			description: "empty platforms",
@@ -72,7 +68,6 @@ func TestBuildOptions(t *testing.T) {
 				},
 				ImageName: "ko://example.com/foo",
 			},
-			wantImportPath: "example.com/foo",
 		},
 		{
 			description: "multiple platforms",
@@ -84,8 +79,7 @@ func TestBuildOptions(t *testing.T) {
 				},
 				ImageName: "ko://example.com/foo",
 			},
-			wantPlatform:   "linux/amd64,linux/arm64",
-			wantImportPath: "example.com/foo",
+			wantPlatform: "linux/amd64,linux/arm64",
 		},
 		{
 			description: "workspace",
@@ -97,7 +91,6 @@ func TestBuildOptions(t *testing.T) {
 				Workspace: "my-app-subdirectory",
 			},
 			wantWorkingDirectory: "my-app-subdirectory",
-			wantImportPath:       "example.com/foo",
 		},
 		{
 			description: "source dir",
@@ -136,7 +129,6 @@ func TestBuildOptions(t *testing.T) {
 			},
 			runMode:                  config.RunModes.Debug,
 			wantDisableOptimizations: true,
-			wantImportPath:           "example.com/foo",
 		},
 		{
 			description: "labels",
@@ -151,8 +143,7 @@ func TestBuildOptions(t *testing.T) {
 				},
 				ImageName: "ko://example.com/foo",
 			},
-			wantLabels:     []string{"foo=bar", "frob=baz"},
-			wantImportPath: "example.com/foo",
+			wantLabels: []string{"foo=bar", "frob=baz"},
 		},
 	}
 	for _, test := range tests {
@@ -170,7 +161,7 @@ func TestBuildOptions(t *testing.T) {
 			t.CheckDeepEqual(test.wantLabels, bo.Labels,
 				cmpopts.SortSlices(func(x, y string) bool { return x < y }),
 				cmpopts.EquateEmpty())
-			if len(bo.BuildConfigs) != 1 {
+			if test.wantImportPath != "" && len(bo.BuildConfigs) != 1 {
 				t.Fatalf("expected exactly one build config, got %d", len(bo.BuildConfigs))
 			}
 			for importpath := range bo.BuildConfigs {