manifest: add Checkpoint()/GetCheckpoints()

If Checkpoint() is called on a pipeline, it is marked for
checkpointing. Calling GetCheckpoints() returns the names of all
its pipelines that are marked for checkpointing as a slice of
strings. This can be passed to osbuild by the caller, in which case
the trees produced by each of these pipelines will be checkpointed
to speed up future builds.

Before this can be used in production we need a mechanism for
automatically cleaning up the cache.

internal/manifest/manifest.go

@@ -88,3 +88,13 @@ func (m Manifest) Serialize(packageSets map[string][]rpmmd.PackageSpec) (distro.
 		},
 	)
 }
+
+func (m Manifest) GetCheckpoints() []string {
+	checkpoints := []string{}
+	for _, p := range m.pipelines {
+		if p.getCheckpoint() {
+			checkpoints = append(checkpoints, p.Name())
+		}
+	}
+	return checkpoints
+}

internal/manifest/pipeline.go

@@ -14,6 +14,8 @@ import (
 
 type Pipeline interface {
 	Name() string
+	Checkpoint()
+	getCheckpoint() bool
 	getBuildPackages() []string
 	getPackageSetChain() []rpmmd.PackageSet
 	serializeStart([]rpmmd.PackageSpec)
@@ -27,9 +29,10 @@ type Pipeline interface {
 // A Base represents the core functionality shared between each of the pipeline
 // implementations, and the Base struct must be embedded in each of them.
 type Base struct {
-	manifest *Manifest
-	name     string
-	build    *Build
+	manifest   *Manifest
+	name       string
+	build      *Build
+	checkpoint bool
 }
 
 // Name returns the name of the pipeline. The name must be unique for a given manifest.
@@ -39,6 +42,14 @@ func (p Base) Name() string {
 	return p.name
 }
 
+func (p *Base) Checkpoint() {
+	p.checkpoint = true
+}
+
+func (p Base) getCheckpoint() bool {
+	return p.checkpoint
+}
+
 func (p Base) GetManifest() *Manifest {
 	return p.manifest
 }