refactor: separate workload class from placement driver

internal/runnerbootstrap/executor.go

@@ -17,7 +17,7 @@ import (
 type ExecutorConfig struct {
 	AssignmentID    string
 	Capability      string
-	Backend         taskassignment.Backend
+	Placement       taskassignment.Placement
 	FacadeURL       string
 	FacadeSPIFFEID  string
 	WorkloadAPIAddr string
@@ -71,7 +71,7 @@ func RunExecutor(ctx context.Context, config ExecutorConfig) error {
 		defer os.RemoveAll(workDir)
 	}
 	registration, err := RegistrationJSON(
-		config.AssignmentID, config.Capability, "http://"+listener.Addr().String(), config.Backend,
+		config.AssignmentID, config.Capability, "http://"+listener.Addr().String(), config.Placement,
 	)
 	if err != nil {
 		return err
@@ -144,13 +144,13 @@ func waitForFacade(ctx context.Context, endpoint string) error {
 // the assignment-scoped values injected by the backend. The Workload API
 // address follows SPIFFE_ENDPOINT_SOCKET through go-spiffe when not set here.
 func ExecutorConfigFromEnvironment() (ExecutorConfig, error) {
-	backend := taskassignment.Backend(os.Getenv(EnvBackend))
-	if backend != taskassignment.BackendPod && backend != taskassignment.BackendVM {
-		return ExecutorConfig{}, fmt.Errorf("invalid %s %q", EnvBackend, backend)
+	placement := taskassignment.Placement{Class: taskassignment.WorkloadClass(os.Getenv(EnvWorkloadClass)), Driver: taskassignment.Driver(os.Getenv(EnvDriver))}
+	if err := placement.Validate(); err != nil {
+		return ExecutorConfig{}, err
 	}
 	config := ExecutorConfig{
 		AssignmentID: os.Getenv(EnvAssignmentID), Capability: os.Getenv(EnvCapability),
-		Backend: backend, FacadeURL: os.Getenv(EnvFacadeURL), FacadeSPIFFEID: os.Getenv(EnvFacadeID),
+		Placement: placement, FacadeURL: os.Getenv(EnvFacadeURL), FacadeSPIFFEID: os.Getenv(EnvFacadeID),
 		RunnerBinary: os.Getenv("GITEA_RUNNER_BINARY"), RunnerConfig: os.Getenv("GITEA_RUNNER_CONFIG_FILE"),
 		ListenAddress: "127.0.0.1:0",
 		Stdout:        os.Stdout, Stderr: os.Stderr,