Add support for specifying GCP instances

- Specifying 'gcp:<instance-name>' for the host argument is now
  supported for indicating a Google Cloud compute instance

Author: sreya

main.go

@@ -74,7 +74,7 @@ Environment variables:
 More info: https://github.com/cdr/sshcode
 
 Arguments:
-%vHOST is passed into the ssh command.
+%vHOST is passed into the ssh command. Valid formats are '<ip-address>' or 'gcp:<instance-name>'. 
 %vDIR is optional.
 
 %v`,

sshcode.go

@@ -32,6 +32,14 @@ type options struct {
 func sshCode(host, dir string, o options) error {
 	flog.Info("ensuring code-server is updated...")
 
+	host, extraSSHFlags, err := parseIP(host)
+	if err != nil {
+		return xerrors.Errorf("failed to parse host IP: %w", err)
+	}
+	if extraSSHFlags != "" {
+		o.sshFlags = strings.Join([]string{extraSSHFlags, o.sshFlags}, " ")
+	}
+
 	dlScript := downloadScript(codeServerPath)
 
 	// Downloads the latest code-server and allows it to be executed.
@@ -41,7 +49,7 @@ func sshCode(host, dir string, o options) error {
 	sshCmd.Stdout = os.Stdout
 	sshCmd.Stderr = os.Stderr
 	sshCmd.Stdin = strings.NewReader(dlScript)
-	err := sshCmd.Run()
+	err = sshCmd.Run()
 	if err != nil {
 		return xerrors.Errorf("failed to update code-server: \n---ssh cmd---\n%s\n---download script---\n%s: %w",
 			sshCmdStr,
@@ -341,3 +349,55 @@ func ensureDir(path string) error {
 
 	return nil
 }
+
+// parseIP parses the host to a valid IP address. If 'gcp:' is prefixed to the
+// host then a lookup is done using gcloud to determine the external IP and any
+// additional SSH arguments that should be used for ssh commands.
+func parseIP(host string) (ip string, additionalFlags string, err error) {
+	host = strings.TrimSpace(host)
+	switch {
+	case strings.HasPrefix(host, "gcp:"):
+		instance := strings.TrimPrefix(host, "gcp:")
+		return parseGCPSSHCmd(instance)
+	default:
+		if net.ParseIP(host) == nil {
+			return "", "", xerrors.New("host argument is not a valid IP address")
+		}
+		return host, "", nil
+	}
+}
+
+// parseGCPSSHCmd parses the IP address and flags used by 'gcloud' when
+// ssh'ing to an instance.
+func parseGCPSSHCmd(instance string) (ip, sshFlags string, err error) {
+	dryRunCmd := fmt.Sprintf("gcloud compute ssh --dry-run %v", instance)
+
+	out, err := exec.Command("sh", "-c", dryRunCmd).CombinedOutput()
+	if err != nil {
+		return "", "", xerrors.Errorf("%s: %w", out, err)
+	}
+
+	toks := strings.Split(string(out), " ")
+	if len(toks) < 2 {
+		return "", "", xerrors.Errorf("unexpected output for '%v' command, %s", dryRunCmd, out)
+	}
+
+	// Slice off the '/usr/bin/ssh' prefix and the '<user>@<ip>' suffix.
+	sshFlags = strings.Join(toks[1:len(toks)-1], " ")
+
+	// E.g. [email protected].
+	userIP := toks[len(toks)-1]
+	toks = strings.Split(userIP, "@")
+	// Assume the '<user>@' is missing.
+	if len(toks) < 2 {
+		ip = strings.TrimSpace(toks[0])
+	} else {
+		ip = strings.TrimSpace(toks[1])
+	}
+
+	if net.ParseIP(ip) == nil {
+		return "", "", xerrors.Errorf("parsed invalid ip address %v", ip)
+	}
+
+	return ip, sshFlags, nil
+}

sshcode_test.go

@@ -261,7 +261,7 @@ func waitForSSHCode(t *testing.T, port string, timeout time.Duration) {
 	}
 }
 
-// fakeRSAKey isn't used for anything other than the trashh ssh
+// fakeRSAKey isn't used for anything other than the trassh ssh
 // server.
 const fakeRSAKey = `-----BEGIN RSA PRIVATE KEY-----
 MIIEpQIBAAKCAQEAsbbGAxPQeqti2OgdzuMgJGBAwXe/bFhQTPuk0bIvavkZwX/a