cmd/gomote: make "create" create a new group from GOMOTE_GROUP

For golang/go#53956.

Change-Id: I14b2e094f7c21cab3a08d73911e73644e22d0d7b
Reviewed-on: https://go-review.googlesource.com/c/build/+/418788
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
Auto-Submit: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

cmd/gomote/create.go

@@ -144,6 +144,12 @@ func create(args []string) error {
 
 	fs.Usage = func() {
 		fmt.Fprintln(os.Stderr, "create usage: gomote create [create-opts] <type>")
+		fmt.Fprintln(os.Stderr)
+		fmt.Fprintln(os.Stderr, "If there's a valid group specified, new instances are")
+		fmt.Fprintln(os.Stderr, "automatically added to the group. If the group in")
+		fmt.Fprintln(os.Stderr, "$GOMOTE_GROUP doesn't exist, and there's no other group")
+		fmt.Fprintln(os.Stderr, "specified, it will be created and new instances will be")
+		fmt.Fprintln(os.Stderr, "added to that group.")
 		fs.PrintDefaults()
 		fmt.Fprintln(os.Stderr, "\nValid types:")
 		for _, bt := range builders() {
@@ -183,6 +189,12 @@ func create(args []string) error {
 			return err
 		}
 	}
+	if group == nil && os.Getenv("GOMOTE_GROUP") != "" {
+		group, err = doCreateGroup(os.Getenv("GOMOTE_GROUP"))
+		if err != nil {
+			return err
+		}
+	}
 
 	var tmpOutDir string
 	var tmpOutDirOnce sync.Once

cmd/gomote/gomote.go

@@ -228,6 +228,7 @@ func main() {
 			if err == nil {
 				fmt.Fprintf(os.Stderr, "# Using group %q from GOMOTE_GROUP\n", *groupName)
 			}
+			// Note that an invalid group in GOMOTE_GROUP is OK.
 		}
 	}