cmd/bootstrapswarm: allow hostname flag to be left empty on GCE

CL 526618 changed the 'on GCE' path of bootstrapswarm to forcibly use
its own internal hostname computation rather than the value of the
hostname flag, dropping "(required)" from its description. But the flag
is still required when not on GCE, and even on GCE bootstrapswarm would
exit fatally if os.Getenv("HOSTNAME") happened to be the empty string.

Make all of this more precise. Spotted while investigating why the
swarming bot wouldn't come up in sid and wanted to eliminate this
possibility, even though it turned out to be due to a Python 3.13
incompatibility as described in CL 665016 and not this.

Change-Id: Ibf65057290e9d8cb79a2800fdc982d35d7edf68b
Reviewed-on: https://go-review.googlesource.com/c/build/+/666375
LUCI-TryBot-Result: Go LUCI <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Michael Pratt <[email protected]>

Author: dmitshur

cmd/bootstrapswarm/bootstrapswarm.go

@@ -11,15 +11,15 @@
 // within the user's home directory.
 //
 // Requirements:
-// - Python3 installed and in the calling user's PATH.
+//   - Python 3 installed and available in PATH (or equivalent) under the name python3 (or python3.exe on Windows).
 //
 // Not on GCE: bootstrapswarm will read the token file and retrieve the
 // the luci machine token. It will use that token to authenticate and
 // download the swarming bot. It will then start the swarming bot in a
 // directory within the user's home directory.
 //
 // Requirements:
-//   - Python3 installed and in the calling user's PATH.
+//   - Python 3 installed and available in PATH (or equivalent) under the name python3 (or python3.exe on Windows).
 //   - luci_machine_tokend running as root in a cron job.
 //     See https://chromium.googlesource.com/infra/luci/luci-go/+/main/tokenserver.
 //     Further instructions can be found at https://go.dev/wiki/DashboardBuilders.
@@ -48,7 +48,7 @@ import (
 )
 
 var (
-	hostname = flag.String("hostname", os.Getenv("HOSTNAME"), "Hostname of machine to bootstrap")
+	hostname = flag.String("hostname", os.Getenv("HOSTNAME"), "Hostname of machine to bootstrap (required when not on GCE)")
 	swarming = flag.String("swarming", "chromium-swarm.appspot.com", "Swarming server to connect to")
 )
 
@@ -58,7 +58,8 @@ func main() {
 		flag.PrintDefaults()
 	}
 	flag.Parse()
-	if *hostname == "" {
+	if *hostname == "" && !metadata.OnGCE() {
+		fmt.Fprintln(os.Stderr, "Hostname is required when not on GCE.")
 		flag.Usage()
 		os.Exit(2)
 	}
@@ -83,7 +84,7 @@ func bootstrap(ctx context.Context, hostname string) error {
 
 		// Override the hostname flag with the GCE hostname. This is a hard
 		// requirement for LUCI, so there's no point in trying anything else.
-		fullHost, err := metadata.Hostname()
+		fullHost, err := metadata.HostnameWithContext(ctx)
 		if err != nil {
 			return fmt.Errorf("retrieving hostname: %w", err)
 		}