Add core ko builder implementation

This adds the `Build()` method for building artifacts using ko. It
supports both publishing the resulting image to a registry, and
sideloading it to the local Docker daemon.

The Skaffold docker client is used in the ko builder. This ensures that
any Minikube config set by Skaffold is used. The ko builder uses the
docker client when sideloading images to the docker daemon.

The `temporary.go` file contains the structs intended to be added to the
schema.

The additions to the design proposal explain image naming for the ko
builder, specifically how container images are named and how Go import
paths are resolved when using the proposed ko builder.

This commit includes ko builder unit tests for
non-current-working-directory workspace dirs. These verify
that the ko builder works even if the context specified in
`skaffold.yaml` differs from the current working directory.

This implementation is still missing the following features:

- integration test
- dependencies (for file watching)
- insecure registries
- debug mode
- support for `go` flags and environment variables (based on
  ko-build/ko#340)
- actually plumbing the builder into the Skaffold CLI and API :-)

Tracking: #6041

docs/design_proposals/ko-builder.md

@@ -83,6 +83,135 @@ The ko builder supports and enhances these Skaffold
   installing additional tools or keeping toolchain versions in sync across
   local development and CI/CD.
 
+## Background: ko image names and Go import paths
+
+Ko uses Go import paths to build images. The
+[`ko publish`](https://github.com/google/ko#build-an-image) command takes a
+required positional argument, which can be either a local file path or a Go
+import path. If the argument is a local file path (as per
+[`go/build.IsLocalImport()`](https://pkg.go.dev/go/build#IsLocalImport))
+then, ko resolves the local file path to a Go import path (see
+[`github.com/google/ko/pkg/build`](https://github.com/google/ko/blob/ab4d264103bd4931c6721d52bfc9d1a2e79c81d1/pkg/build/gobuild.go#L261)).
+
+The import path must be of the package than contains the `main()` function.
+For instance, to build Skaffold using ko, from the repository root directory:
+
+```sh
+ko publish ./cmd/skaffold
+```
+
+or
+
+```sh
+ko publish github.com/GoogleContainerTools/skaffold/cmd/skaffold
+```
+
+When the ko CLI is used to
+[populate the image name in templated Kubernetes resource files](https://github.com/google/ko#kubernetes-integration),
+only the Go import path option can be used, and the import path must be
+prefixed by the `ko://` scheme, e.g.,
+`ko://github.com/GoogleContainerTools/skaffold/cmd/skaffold`.
+
+Ko determines the image name from the container image registry (provided by the
+`KO_DOCKER_REPO` environment variable) and the Go import path. The Go import
+path is appended in one of these ways:
+
+- The last path segment (e.g., `skaffold`), followed by a hyphen and a MD5
+  hash. This is the default behavior of the `ko publish` command.
+
+- The last path segment (e.g., `skaffold`) only, if `ko publish` is invoked
+  with the `-B` or `--base-import-paths` flag.
+
+- The full import path, lowercased (e.g.,
+  `github.com/googlecontainertools/skaffold/cmd/skaffold`), if `ko publish` is
+  invoked with the `-P` or `--preserve-import-paths` flag. This is the option
+  used by projects such as Knative (see the
+  [`release.sh` script](https://github.com/knative/serving/blob/v0.24.0/vendor/knative.dev/hack/release.sh#L98))
+  and Tekton
+  (see the pipeline in
+  [publish.yaml](https://github.com/tektoncd/pipeline/blob/v0.25.0/tekton/publish.yaml#L137)).
+
+- No import path (just `KO_DOCKER_REPO`), if `ko publish` is invoked with the
+  `--bare` flag.
+
+## Supporting existing Skaffold users
+
+The Skaffold ko builder follows the existing Skaffold image naming logic. This
+means that the image naming behavior doesn't change for existing Skaffold users
+who migrate from other builders to the ko builder.
+
+The ko builder achieves this by using ko's
+[`Bare`](https://github.com/google/ko/blob/ab4d264103bd4931c6721d52bfc9d1a2e79c81d1/pkg/commands/options/publish.go#L60)
+naming option.
+
+By using this option, the image name is not tied to the Go import path. If the
+Skaffold
+[default repo](https://skaffold.dev/docs/environment/image-registries/) value
+is `gcr.io/k8s-skaffold` and tne value of the `image` field in `skaffold.yaml`
+is `skaffold`, the resulting image name will be `gcr.io/k8s-skaffold/skaffold`.
+
+It is still necessary to resolve the Go import path for the underlying ko
+implementation. To do so, the ko builder determines the import path of the
+current
+[`context`](https://skaffold.dev/docs/references/yaml/#build-artifacts-context)
+(a.k.a.
+[`Workspace`](https://github.com/GoogleContainerTools/skaffold/blob/v1.27.0/pkg/skaffold/schema/latest/v1/config.go#L832))
+directory.
+
+By specifying different `context` directories for each `artifact` in
+`skaffold.yaml`, the ko builder supports building multiple artifacts in the
+same Skaffold config, such as in the
+[microservices example](https://github.com/GoogleContainerTools/skaffold/tree/v1.27.0/examples/microservices).
+
+## Supporting existing ko users
+
+To support existing ko users moving to Skaffold, the ko builder also supports
+`image` names in `skaffold.yaml` that use the Go import path, prefixed by the
+`ko://` scheme. Examples of such image references in Kubernetes manifest files
+can be seen in projects such as
+[Knative](https://github.com/knative/serving/blob/main/config/core/deployments/activator.yaml#L41)
+and
+[Tekton](https://github.com/tektoncd/pipeline/blob/v0.25.0/config/controller.yaml#L66).
+
+In the case of `ko://`-prefixed image names, the Skaffold ko builder
+constructs the image name by:
+
+1.  Removing the `ko://` scheme prefix.
+2.  Transforming the import path to a valid image name using the function
+    [`SanitizeImageName()`](https://github.com/GoogleContainerTools/skaffold/blob/v1.27.0/pkg/skaffold/docker/reference.go#L83)
+    (from the package
+    `github.com/GoogleContainerTools/skaffold/pkg/skaffold/docker`).
+3.  Combining the Skaffold default repo with the transformed import path as per
+    existing Skaffold image naming logic.
+
+This will result in image names that match those produced by the `ko` CLI when
+using the `-P` or `--preserve-import-paths` flag. For example, if the Skaffold
+default repo is `gcr.io/k8s-skaffold` and the `image` name in `skaffold.yaml`
+is `ko://github.com/GoogleContainerTools/skaffold/cmd/skaffold`, the resulting
+image name will be
+`gcr.io/k8s-skaffold/github.com/googlecontainertools/skaffold/cmd/skaffold`.
+
+Real-world examples of image names that follow this naming convention can be
+found in the Tekton and Knative release manifests. For instance, view the
+images in the Knative Serving release YAMLs:
+
+```sh
+curl -sL https://github.com/knative/serving/releases/download/v0.24.0/serving-core.yaml | grep 'image: '
+```
+
+If the `image` field in `skaffold.yaml` starts with the `ko://` scheme prefix,
+the Skaffold ko builder uses the Go import path that follows the prefix. If the
+`image` name in `skaffold.yaml` does _not_ start with `ko://`, then the ko
+builder determines the Go import path from the artifact `context` directory.
+
+Users who want to build an artifact where the `main()` function is _not_ in the
+`context` directory must specify the full import path in the image name. For
+instance, to build Skaffold itself using the Skaffold ko builder, for a
+`context` directory of `.` (the default), the `image` name must be
+`ko://github.com/GoogleContainerTools/skaffold/cmd/skaffold`.
+Image names that start with relative path references such as `./cmd/skaffold`
+are _not_ supported by Skaffold.
+
 ## Design
 
 Adding the ko builder requires making config changes to the Skaffold schema.
@@ -196,11 +325,11 @@ Adding the ko builder requires making config changes to the Skaffold schema.
 Example basic config, this will be sufficient for many users:
 
 ```yaml
-apiVersion: skaffold/v2beta15
+apiVersion: skaffold/v2beta19
 kind: Config
 build:
   artifacts:
-  - image: ko://github.com/GoogleContainerTools/skaffold/examples/ko
+  - image: skaffold-example-ko
     ko: {}
 ```
 
@@ -210,7 +339,7 @@ The value of the `image` field is the Go import path of the app entry point,
 A more comprehensive example config:
 
 ```yaml
-apiVersion: skaffold/v2beta15
+apiVersion: skaffold/v2beta19
 kind: Config
 build:
   artifacts:
@@ -313,8 +442,6 @@ maps directly to this value.
     <https://github.com/google/ko#why-are-my-images-all-created-in-1970> and
     <https://reproducible-builds.org/docs/source-date-epoch/>.
 
-
-
 ### Open questions
 
 1.  Should we default dependency paths to `{"go.mod", "**.go"}` instead of
@@ -327,10 +454,8 @@ maps directly to this value.
 
 2.  Add a Google Cloud Build (`gcb`) support for the ko builder?
 
-    Other builders that support `gcb` have default public builder images.
-    The image `gcr.io/tekton-releases/ko-ci` is public, but do we want to
-    rely on it? Once ko is embedded in Skaffold, we could use
-    `gcr.io/k8s-skaffold/skaffold` as a default image.`
+    By embedding ko as a module, there is no need for a ko-specific Skaffold
+    builder image.
 
     __Not Yet Resolved__
 
@@ -347,6 +472,7 @@ maps directly to this value.
     file?
 
     Suggest yes, to make Skaffold a compelling choice for Go developers.
+
     __Not Yet Resolved__
 
 ## Approach
@@ -479,13 +605,13 @@ The steps roughly outlined:
     Example `skaffold.yaml` supported at this stage:
 
     ```yaml
-    apiVersion: skaffold/v2beta18
+    apiVersion: skaffold/v2beta19
     kind: Config
     build:
       artifacts:
       - image: skaffold-ko
         ko:
-          fromImage: gcr.io/distroless/static-debian10:nonroot
+          fromImage: gcr.io/distroless/base:nonroot
           dependencies:
             paths:
             - go.mod
@@ -498,8 +624,8 @@ The steps roughly outlined:
           - linux/arm64
     ```
 
-3.  After [google/ko#340](https://github.com/google/ko/pull/340) is merged,
-    implement Skaffold config support for additional ko config options:
+3.  Implement Skaffold config support for additional ko config options added in
+    [google/ko#340](https://github.com/google/ko/pull/340):
 
     -   `args`, e.g., `-v`, `-trimpath`
     -   `asmflags`

go.mod

@@ -41,7 +41,7 @@ require (
 	github.com/google/go-containerregistry v0.5.1
 	github.com/google/go-containerregistry/pkg/authn/k8schain v0.0.0-20210216200643-d81088d9983e // indirect
 	github.com/google/go-github v17.0.0+incompatible
-	github.com/google/ko v0.8.4-0.20210615195035-ee2353837872
+	github.com/google/ko v0.8.4-0.20210715141624-56282bf645ea
 	github.com/google/uuid v1.1.2
 	github.com/grpc-ecosystem/grpc-gateway v1.14.8
 	github.com/heroku/color v0.0.6