Refactor the way we annotate and set the config of the final image

Change the way we set up the configuration of the final image in order
to support the reuse of the base image's config. In that way, we can
maintain the env variables, labels and other information from the base
image without overwriting them.

Also, improve the readability of the code and create smaller helper
functions with a particular purpose.

Signed-off-by: Charalampos Mainas <cmainas@nubificus.co.uk>

Author: cmainas

cmd/main.go

@@ -16,21 +16,16 @@ package main
 
 import (
 	"context"
-	"encoding/json"
 	"flag"
 	"fmt"
 	"os"
-	"runtime"
-	"strings"
 
 	"bunny/hops"
 
 	"github.com/moby/buildkit/client/llb"
-	"github.com/moby/buildkit/exporter/containerimage/exptypes"
 	"github.com/moby/buildkit/frontend/gateway/client"
 	"github.com/moby/buildkit/frontend/gateway/grpcclient"
 	"github.com/moby/buildkit/util/appcontext"
-	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
 )
 
 const (
@@ -105,40 +100,6 @@ func readFileFromLLB(ctx context.Context, c client.Client, filename string) ([]b
 	return fileBytes, nil
 }
 
-func annotateRes(annots map[string]string, res *client.Result) (*client.Result, error) {
-	ref, err := res.SingleRef()
-	if err != nil {
-		return nil, fmt.Errorf("Failed te get reference build result: %v", err)
-	}
-
-	config := ocispecs.Image{
-		Platform: ocispecs.Platform{
-			Architecture: runtime.GOARCH,
-			OS:           "linux",
-		},
-		RootFS: ocispecs.RootFS{
-			Type: "layers",
-		},
-		Config: ocispecs.ImageConfig{
-			WorkingDir: "/",
-			Cmd:        strings.Fields(annots["com.urunc.unikernel.cmdline"]),
-			Labels:     annots,
-		},
-	}
-
-	imageConfig, err := json.Marshal(config)
-	if err != nil {
-		return nil, fmt.Errorf("Failed to marshal image config: %v", err)
-	}
-	res.AddMeta(exptypes.ExporterImageConfigKey, imageConfig)
-	for annot, val := range annots {
-		res.AddMeta(exptypes.AnnotationManifestKey(nil, annot), []byte(val))
-	}
-	res.SetRef(ref)
-
-	return res, nil
-}
-
 func bunnyBuilder(ctx context.Context, c client.Client) (*client.Result, error) {
 	// Get the Build options from buildkit
 	buildOpts := c.BuildOpts().Opts
@@ -167,21 +128,33 @@ func bunnyBuilder(ctx context.Context, c client.Client) (*client.Result, error)
 		return nil, fmt.Errorf("Could not create LLB definition: %v", err)
 	}
 
+	rc := &hops.ResultAndConfig{}
+
 	// Pass LLB to buildkit
-	result, err := c.Solve(ctx, client.SolveRequest{
+	rc.Res, err = c.Solve(ctx, client.SolveRequest{
 		Definition: dt.ToPB(),
 	})
 	if err != nil {
 		return nil, fmt.Errorf("Failed to resolve LLB: %v", err)
 	}
 
-	// Add annotations and Labels in output image
-	result, err = annotateRes(packInst.Annots, result)
+	// Get the OCI Image config of the base Image if there is any
+	err = rc.GetBaseConfig(ctx, c, packInst.Config.BaseRef, packInst.Config.Monitor)
+	if err != nil {
+		return nil, fmt.Errorf("Failed to get OCI config of base image: %v", err)
+	}
+
+	// Set some default values in the Image config
+	// and add cmdline and Labels
+	rc.UpdateConfig(packInst.Annots)
+
+	// Apply annotations and the new config to the solver's result
+	err = rc.ApplyConfig(packInst.Annots)
 	if err != nil {
 		return nil, fmt.Errorf("Failed to annotate final image: %v", err)
 	}
 
-	return result, nil
+	return rc.Res, nil
 }
 
 func main() {

hops/image_config.go

@@ -0,0 +1,119 @@
+// Copyright (c) 2023-2025, Nubificus LTD
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package hops
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"runtime"
+	"strings"
+
+	"github.com/distribution/reference"
+	"github.com/moby/buildkit/client/llb/sourceresolver"
+	"github.com/moby/buildkit/exporter/containerimage/exptypes"
+	"github.com/moby/buildkit/frontend/gateway/client"
+	ocispecs "github.com/opencontainers/image-spec/specs-go/v1"
+)
+
+type ResultAndConfig struct {
+	// The result
+	Res *client.Result
+	// The OCI config of the final image
+	OCIConfig ocispecs.Image
+}
+
+func (rc *ResultAndConfig) GetBaseConfig(ctx context.Context, c client.Client, ref string, mon string) error {
+	if ref == "" || ref == "scratch" {
+		return nil
+	}
+
+	baseRef, err := reference.ParseNormalizedNamed(ref)
+	if err != nil {
+		return fmt.Errorf("Failed to parse image name %s: %v", ref, err)
+	}
+	baseImageName := reference.TagNameOnly(baseRef).String()
+
+	plat := ocispecs.Platform{
+		Architecture: runtime.GOARCH,
+	}
+	if strings.HasPrefix(ref, unikraftHub) {
+		// Define the platform to qemu/amd64 so we can pull unikraft images
+		plat.OS = mon
+	} else {
+		plat.OS = "linux"
+	}
+	_, _, config, err := c.ResolveImageConfig(ctx, baseImageName,
+		sourceresolver.Opt{
+			LogName:  "resolving image metadata for " + baseImageName,
+			Platform: &plat,
+		})
+	if err != nil {
+		return fmt.Errorf("Failed to get image config from %s: %v", baseImageName, err)
+	}
+
+	err = json.Unmarshal(config, &rc.OCIConfig)
+	if err != nil {
+		return fmt.Errorf("Failed to unmarshal image config of %ss: %v", baseImageName, err)
+	}
+
+	return nil
+}
+
+func (rc *ResultAndConfig) UpdateConfig(annots map[string]string) {
+	plat := ocispecs.Platform{
+		Architecture: runtime.GOARCH,
+		OS:           "linux",
+	}
+	rfs := ocispecs.RootFS{
+		Type: "layers",
+	}
+
+	// Overwrite platform and rootfs to remove unikraft specific platform
+	// and initialize empty configs.
+	rc.OCIConfig.Platform = plat
+	rc.OCIConfig.RootFS = rfs
+	// Overwrite Cmd and entrypoint based on the values of bunnyfile
+	rc.OCIConfig.Config.Cmd = strings.Fields(annots["com.urunc.unikernel.cmdline"])
+	rc.OCIConfig.Config.Entrypoint = []string{}
+
+	if rc.OCIConfig.Config.Labels == nil {
+		rc.OCIConfig.Config.Labels = make(map[string]string)
+	}
+	for k, v := range annots {
+		rc.OCIConfig.Config.Labels[k] = v
+	}
+}
+
+func (rc *ResultAndConfig) ApplyConfig(annots map[string]string) error {
+	res := rc.Res
+	ref, err := res.SingleRef()
+	if err != nil {
+		return fmt.Errorf("Failed te get reference build result: %v", err)
+	}
+
+	imageConfig, err := json.Marshal(rc.OCIConfig)
+	if err != nil {
+		return fmt.Errorf("Failed to marshal image config: %v", err)
+	}
+	res.AddMeta(exptypes.ExporterImageConfigKey, imageConfig)
+	for annot, val := range annots {
+		res.AddMeta(exptypes.AnnotationManifestKey(nil, annot), []byte(val))
+	}
+	res.SetRef(ref)
+
+	rc.Res = res
+	return nil
+}