feat(featureflags): add build-kernel-version flag for new builds

packages/api/internal/handlers/deprecated_template_request_build.go

@@ -160,10 +160,11 @@
 	templateID api.TemplateID,
 	body api.TemplateBuildRequest,
 ) (*template.RegisterBuildResponse, *api.APIError) {
-	// TODO(ENG-3852): Stop sending the firecracker version from the API. The orchestrator
-	// now resolves its own FC version via the FirecrackerVersions feature flag
-	// (see packages/orchestrator/pkg/template/build/builder.go)
+	// TODO(ENG-3852): Stop sending the firecracker/kernel versions from the API.
+	// The orchestrator resolves them itself via the BuildFirecrackerVersion /
+	// BuildKernelVersion feature flags (see packages/orchestrator/pkg/template/server/create_template.go).
 	firecrackerVersion := a.featureFlags.StringFlag(ctx, featureflags.BuildFirecrackerVersion)
+	kernelVersion := a.featureFlags.StringFlag(ctx, featureflags.BuildKernelVersion)
 
 	var alias *string
 	var tags []string
@@ -206,7 +207,7 @@
 		CpuCount:           body.CpuCount,
 		MemoryMB:           body.MemoryMB,
 		Version:            templates.TemplateV1Version,
-		KernelVersion:      a.config.DefaultKernelVersion,
+		KernelVersion:      kernelVersion,
 		FirecrackerVersion: firecrackerVersion,
 	}
 

packages/api/internal/handlers/template_request_build_v3.go

@@ -115,10 +115,11 @@ func requestTemplateBuild(ctx context.Context, c *gin.Context, a *APIStore, body
 	}
 	span.End()
 
-	// TODO(ENG-3852): Stop sending the firecracker version from the API. The orchestrator
-	// now resolves its own FC version via the FirecrackerVersions feature flag
-	// (see packages/orchestrator/pkg/template/build/builder.go)
+	// TODO(ENG-3852): Stop sending the firecracker/kernel versions from the API.
+	// The orchestrator resolves them itself via the BuildFirecrackerVersion /
+	// BuildKernelVersion feature flags (see packages/orchestrator/pkg/template/server/create_template.go).
 	firecrackerVersion := a.featureFlags.StringFlag(ctx, featureflags.BuildFirecrackerVersion)
+	kernelVersion := a.featureFlags.StringFlag(ctx, featureflags.BuildKernelVersion)
 	buildReq := template.RegisterBuildData{
 		ClusterID:          clusters.WithClusterFallback(team.ClusterID),
 		TemplateID:         templateID,
@@ -129,7 +130,7 @@ func requestTemplateBuild(ctx context.Context, c *gin.Context, a *APIStore, body
 		CpuCount:           body.CpuCount,
 		MemoryMB:           body.MemoryMB,
 		Version:            templates.TemplateV2LatestVersion,
-		KernelVersion:      a.config.DefaultKernelVersion,
+		KernelVersion:      kernelVersion,
 		FirecrackerVersion: firecrackerVersion,
 	}
 

packages/orchestrator/pkg/template/server/create_template.go

@@ -57,7 +57,7 @@ func (s *ServerStore) TemplateCreate(ctx context.Context, templateRequest *templ
 		featureflags.TeamContext(cfg.GetTeamID()),
 	)
 
-	kernelVersion := featureflags.DefaultKernelVersion
+	kernelVersion := s.featureFlags.StringFlag(ctx, featureflags.BuildKernelVersion)
 	firecrackerVersion := s.featureFlags.StringFlag(ctx, featureflags.BuildFirecrackerVersion)
 
 	fcInfo, err := fcversion.New(firecrackerVersion)

packages/shared/pkg/featureflags/client.go

@@ -110,7 +110,12 @@ func (c *Client) IntFlag(ctx context.Context, flag IntFlag, contexts ...ldcontex
 }
 
 func (c *Client) StringFlag(ctx context.Context, flag StringFlag, contexts ...ldcontext.Context) string {
-	return getFlag(ctx, c.ld, c.ld.StringVariationCtx, flag, c.allContexts(contexts))
+	v := getFlag(ctx, c.ld, c.ld.StringVariationCtx, flag, c.allContexts(contexts))
+	if v == "" && flag.FallbackWhenEmpty() {
+		return flag.Fallback()
+	}
+
+	return v
 }
 
 type typedFlag[T any] interface {

packages/shared/pkg/featureflags/flags.go

@@ -214,8 +214,9 @@ var (
 )
 
 type StringFlag struct {
-	name     string
-	fallback string
+	name              string
+	fallback          string
+	fallbackWhenEmpty bool
 }
 
 func (f StringFlag) Key() string {
@@ -230,6 +231,12 @@ func (f StringFlag) Fallback() string {
 	return f.fallback
 }
 
+// FallbackWhenEmpty reports whether the fallback should also be used when LD
+// returns an empty string (e.g. flag defined in LD without a value).
+func (f StringFlag) FallbackWhenEmpty() bool {
+	return f.fallbackWhenEmpty
+}
+
 func NewStringFlag(name string, fallback string) StringFlag {
 	flag := StringFlag{name: name, fallback: fallback}
 	builder := launchDarklyOfflineStore.Flag(flag.name).ValueForAll(ldvalue.String(fallback))
@@ -238,7 +245,15 @@ func NewStringFlag(name string, fallback string) StringFlag {
 	return flag
 }
 
-// This is currently not configurable via feature flags.
+// NewStringFlagFallbackOnEmpty is like NewStringFlag but also returns the
+// fallback when LD evaluates the flag to an empty string.
+func NewStringFlagFallbackOnEmpty(name string, fallback string) StringFlag {
+	flag := NewStringFlag(name, fallback)
+	flag.fallbackWhenEmpty = true
+
+	return flag
+}
+
 const (
 	DefaultKernelVersion = "vmlinux-6.1.158"
 )
@@ -261,6 +276,7 @@ var FirecrackerVersionMap = map[string]string{
 // BuildIoEngine Sync is used by default as there seems to be a bad interaction between Async and a lot of io operations.
 var (
 	BuildFirecrackerVersion     = NewStringFlag("build-firecracker-version", env.GetEnv("DEFAULT_FIRECRACKER_VERSION", DefaultFirecrackerVersion))
+	BuildKernelVersion          = NewStringFlagFallbackOnEmpty("build-kernel-version", env.GetEnv("DEFAULT_KERNEL_VERSION", DefaultKernelVersion))
 	BuildIoEngine               = NewStringFlag("build-io-engine", "Sync")
 	DefaultPersistentVolumeType = NewStringFlag("default-persistent-volume-type", "")
 	BuildNodeInfo               = NewJSONFlag("preferred-build-node", ldvalue.Null())