Add the coordinator token subcommand for joining workers

coordinator/cmd/coordinator/main.go

@@ -46,6 +46,12 @@ func main() {
 				os.Exit(1)
 			}
 			return
+		case "token":
+			if err := runToken(args[1:]); err != nil {
+				fmt.Fprintln(os.Stderr, "token:", err)
+				os.Exit(1)
+			}
+			return
 		}
 	}
 	showVersion := flag.Bool("version", false, "print the build version and exit")

coordinator/cmd/coordinator/token_cmd.go

@@ -0,0 +1,33 @@
+package main
+
+import (
+	"flag"
+	"fmt"
+	"os"
+	"path/filepath"
+)
+
+// runToken implements `coordinator token`: prints the worker auth token of a
+// `coordinator serve` instance, so a scientist can join a worker without
+// hunting through the data directory. The file itself is what serve created.
+func runToken(args []string) error {
+	flags := flag.NewFlagSet("token", flag.ContinueOnError)
+	flags.Usage = func() {
+		_, _ = fmt.Fprintf(flags.Output(), "usage: coordinator token [options]\n")
+		_, _ = fmt.Fprintf(flags.Output(), "Prints the WORKER_AUTH_TOKEN of this coordinator's serve instance.\n\n")
+		flags.PrintDefaults()
+	}
+	var dataDir = flags.String("data-dir", defaultDataDir(), "data directory (default: ~/.scimesh)")
+	if err := flags.Parse(args); err != nil {
+		return err
+	}
+	if flags.NArg() > 0 {
+		return fmt.Errorf("token takes no positional arguments")
+	}
+	token, err := os.ReadFile(filepath.Join(*dataDir, "worker.token"))
+	if err != nil {
+		return fmt.Errorf("no worker token found in %s — start the coordinator with `coordinator serve` first", *dataDir)
+	}
+	fmt.Print(string(token))
+	return nil
+}