Specify a richer scheme to run the migration with

config.yml.example

@@ -50,5 +50,9 @@ log_level: INFO
 # incurs an extra API call on every gitlab-shell command.
 audit_usernames: false
 
-# Feature flag: go or ruby
-experimental: false
+# Migration to Go: anything listed here has two implementations. Use these flags
+# to try the new implementations out, or to revert to the old behaviour if there
+# problems arise.
+migration:
+  enabled: false
+  features: []

go/cmd/gitlab-shell/main.go

@@ -9,28 +9,41 @@ import (
 	"gitlab.com/gitlab-org/gitlab-shell/go/internal/config"
 )
 
-func experiment() {
-	fmt.Println("Experiment! nothing works!")
-	os.Exit(1)
+func migrate(_cfg *config.Config, _args []string) (int, bool) {
+	// TODO: decide whether to handle the request in Go or not
+	return 0, false
 }
 
-func main() {
+// rubyExec will never return. It either replaces the current process with a
+// Ruby interpreter, or outputs an error and kills the process.
+func execRuby() {
 	root := filepath.Dir(os.Args[0])
-	ruby := filepath.Join(root, "gitlab-shell-ruby")
 
+	rubyCmd := filepath.Join(root, "gitlab-shell-ruby")
+	rubyArgs := os.Args[1:]
+	rubyEnv := os.Environ()
+
+	execErr := syscall.Exec(rubyCmd, rubyArgs, rubyEnv)
+	if execErr != nil {
+		fmt.Fprintf(os.Stderr, "Failed to exec(%q): %v\n", rubyCmd, execErr)
+		os.Exit(1)
+	}
+}
+
+func main() {
+	// Fall back to Ruby in case of problems reading the config, but issue a
+	// warning as this isn't something we can sustain indefinitely
 	config, err := config.New()
 	if err != nil {
-		fmt.Println(err)
-		os.Exit(1)
+		fmt.Fprintln(os.Stderr, "Failed to read config, falling back to gitlab-shell-ruby")
+		execRuby()
 	}
 
-	if config.Experimental {
-		experiment()
-	} else {
-		execErr := syscall.Exec(ruby, os.Args, os.Environ())
-		if execErr != nil {
-			fmt.Fprintf(os.Stderr, "Failed to exec(%q): %v\n", ruby, execErr)
-			os.Exit(1)
-		}
+	// Try to handle the command with the Go implementation
+	if exitCode, done := migrate(config, os.Args[1:]); done {
+		os.Exit(exitCode)
 	}
+
+	// Since a migration has not handled the command, fall back to Ruby to do so
+	execRuby()
 }

go/internal/config/config.go

@@ -13,11 +13,16 @@ const (
 	logFile    = "gitlab-shell.log"
 )
 
+type MigrationConfig struct {
+	Enabled  bool     `yaml:"enabled"`
+	Features []string `yaml:"features"`
+}
+
 type Config struct {
-	RootDir      string
-	LogFile      string `yaml:"log_file"`
-	LogFormat    string `yaml:"log_format"`
-	Experimental bool   `yaml:"experimental"`
+	RootDir   string
+	LogFile   string          `yaml:"log_file"`
+	LogFormat string          `yaml:"log_format"`
+	Migration MigrationConfig `yaml:"migration"`
 }
 
 func New() (*Config, error) {

go/internal/config/config_test.go

@@ -2,22 +2,28 @@ package config
 
 import (
 	"fmt"
+	"strings"
 	"testing"
 )
 
 func TestConfigLogFile(t *testing.T) {
 	testRoot := "/foo/bar"
 	testCases := []struct {
-		yaml         string
-		path         string
-		format       string
-		experimental bool
+		yaml      string
+		path      string
+		format    string
+		migration MigrationConfig
 	}{
 		{path: "/foo/bar/gitlab-shell.log", format: "text"},
 		{yaml: "log_file: my-log.log", path: "/foo/bar/my-log.log", format: "text"},
 		{yaml: "log_file: /qux/my-log.log", path: "/qux/my-log.log", format: "text"},
 		{yaml: "log_format: json", path: "/foo/bar/gitlab-shell.log", format: "json"},
-		{yaml: "experimental: true", path: "/foo/bar/gitlab-shell.log", format: "text", experimental: true},
+		{
+			yaml:      "migration:\n  enabled: true\n  features:\n    - foo\n    - bar",
+			path:      "/foo/bar/gitlab-shell.log",
+			format:    "text",
+			migration: MigrationConfig{Enabled: true, Features: []string{"foo", "bar"}},
+		},
 	}
 
 	for _, tc := range testCases {
@@ -27,8 +33,12 @@ func TestConfigLogFile(t *testing.T) {
 				t.Fatal(err)
 			}
 
-			if cfg.Experimental != tc.experimental {
-				t.Fatalf("expected %v, got %v", tc.experimental, cfg.Experimental)
+			if cfg.Migration.Enabled != tc.migration.Enabled {
+				t.Fatalf("migration.enabled: expected %v, got %v", tc.migration.Enabled, cfg.Migration.Enabled)
+			}
+
+			if strings.Join(cfg.Migration.Features, ":") != strings.Join(tc.migration.Features, ":") {
+				t.Fatalf("migration.features: expected %#v, got %#v", tc.migration.Features, cfg.Migration.Features)
 			}
 
 			if cfg.LogFile != tc.path {