hack: pin the os release build name

One environment variable to be used when building the pinned release binary:
    RHCOS_BUILD_NAME: a string representing the build name(or rather number e.g. 47.48). If empty, then the latest one will be picked up.
This could just be hardcoded in the hack/build.sh script before making the release branch so that it stays baked in.

hack/build.sh

@@ -43,6 +43,10 @@ release)
 	then
 		LDFLAGS="${LDFLAGS} -X github.com/openshift/installer/pkg/asset/ignition/bootstrap.defaultReleaseImage=${RELEASE_IMAGE}"
 	fi
+	if test -n "${RHCOS_BUILD_NAME}"
+	then
+		LDFLAGS="${LDFLAGS} -X github.com/openshift/installer/pkg/rhcos.buildName=${RHCOS_BUILD_NAME}"
+	fi
 	if test "${SKIP_GENERATION}" != y
 	then
 		go generate ./data

pkg/asset/ignition/bootstrap/bootstrap.go

@@ -35,7 +35,7 @@ const (
 	ignitionUser         = "core"
 )
 
-var t(
+var (
 	defaultReleaseImage = "registry.svc.ci.openshift.org/openshift/origin-release:v4.0"
 )
 

pkg/rhcos/builds.go

@@ -11,10 +11,14 @@ import (
 	"github.com/sirupsen/logrus"
 )
 
-const (
+var (
 	// DefaultChannel is the default RHCOS channel for the cluster.
 	DefaultChannel = "maipo"
 
+	// buildName is the name of the build in the channel that will be picked up
+	// empty string means the first one in the build list (latest) will be used
+	buildName = ""
+
 	baseURL = "https://releases-rhcos.svc.ci.openshift.org/storage/releases"
 )
 
@@ -33,9 +37,13 @@ type metadata struct {
 }
 
 func fetchLatestMetadata(ctx context.Context, channel string) (metadata, error) {
-	build, err := fetchLatestBuild(ctx, channel)
-	if err != nil {
-		return metadata{}, errors.Wrap(err, "failed to fetch latest build")
+	build := buildName
+	var err error
+	if build == "" {
+		build, err = fetchLatestBuild(ctx, channel)
+		if err != nil {
+			return metadata{}, errors.Wrap(err, "failed to fetch latest build")
+		}
 	}
 
 	url := fmt.Sprintf("%s/%s/%s/meta.json", baseURL, channel, build)
@@ -48,7 +56,7 @@ func fetchLatestMetadata(ctx context.Context, channel string) (metadata, error)
 	client := &http.Client{}
 	resp, err := client.Do(req.WithContext(ctx))
 	if err != nil {
-		return metadata{}, errors.Wrap(err, "failed to fetch metadata")
+		return metadata{}, errors.Wrapf(err, "failed to fetch metadata for build %s", build)
 	}
 	defer resp.Body.Close()