Attempt to trigger automounts to find the ESP

The ESP may be an automount partition, so try touching a file in each
candidate location so as to trigger an automounts.

This is the same way systemd attempts to find it:
https://github.com/systemd/systemd/blob/f565b86/src/shared/bootspec.c#L1014-L1018

I've also changed the function to return an error if no ESP is found.
The previous behaviour (an empty string) just results in a crash later
on.

When no ESP is found, the `bundle` command will have no default for the
`esp` flag. Passing an empty string to it as a default results in no
value being show in the output of `--help`.

This seemed like the most reasonable compromise instead of panicking.

Fixes #78

bundles.go

@@ -74,15 +74,18 @@ func GetEfistub() string {
 	return ""
 }
 
-func NewBundle() *Bundle {
-	esp := GetESP()
+func NewBundle() (bundle *Bundle, err error) {
+	esp, err := GetESP()
+	if err != nil {
+		return
+	}
 
 	stub := GetEfistub()
 	if stub == "" {
 		panic("No EFISTUB file found. Please install systemd-boot or gummiboot!")
 	}
 
-	return &Bundle{
+	bundle = &Bundle{
 		Output:         "",
 		IntelMicrocode: "",
 		AMDMicrocode:   "",
@@ -94,6 +97,8 @@ func NewBundle() *Bundle {
 		EFIStub:        stub,
 		ESP:            esp,
 	}
+
+	return
 }
 
 func GenerateBundle(bundle *Bundle) (bool, error) {

cmd/sbctl/bundle.go

@@ -40,7 +40,10 @@ var bundleCmd = &cobra.Command{
 				os.Exit(1)
 			}
 		}
-		bundle := sbctl.NewBundle()
+		bundle, err := sbctl.NewBundle()
+		if err != nil {
+			return err
+		}
 		output, err := filepath.Abs(args[0])
 		if err != nil {
 			return err
@@ -81,7 +84,11 @@ var bundleCmd = &cobra.Command{
 }
 
 func bundleCmdFlags(cmd *cobra.Command) {
-	esp := sbctl.GetESP()
+	esp, err := sbctl.GetESP()
+	if err != nil {
+		logging.Warn("Failed to find ESP: %s", err)
+	}
+
 	f := cmd.Flags()
 	f.StringVarP(&amducode, "amducode", "a", "", "AMD microcode location")
 	f.StringVarP(&intelucode, "intelucode", "i", "", "Intel microcode location")

cmd/sbctl/list-bundles.go

@@ -35,7 +35,10 @@ var listBundlesCmd = &cobra.Command{
 					isSigned = false
 					logging.NotOk("Not Signed")
 				}
-				esp := sbctl.GetESP()
+				esp, err := sbctl.GetESP()
+				if err != nil {
+					return err
+				}
 				logging.Print("\tESP Location:\t%s\n", esp)
 				logging.Print("\tOutput:\t\t└─%s\n", strings.TrimPrefix(s.Output, esp))
 				logging.Print("\tEFI Stub Image:\t  └─%s\n", s.EFIStub)

cmd/sbctl/verify.go

@@ -18,7 +18,10 @@ var verifyCmd = &cobra.Command{
 		if err := sbctl.CanVerifyFiles(); err != nil {
 			return err
 		}
-		espPath := sbctl.GetESP()
+		espPath, err := sbctl.GetESP()
+		if err != nil {
+			return err
+		}
 		logging.Print("Verifying file database and EFI images in %s...\n", espPath)
 		if err := sbctl.SigningEntryIter(func(file *sbctl.SigningEntry) error {
 			sbctl.AddChecked(file.OutputFile)

sbctl.go

@@ -24,16 +24,29 @@ type LsblkRoot struct {
 	Blockdevices []LsblkEntry `json:"blockdevices"`
 }
 
+var espLocations = []string{
+	"/boot",
+	"/boot/efi",
+	"/efi",
+}
+var ErrNoESP = errors.New("failed to find EFI system partition")
+
 // Slightly more advanced check
-func GetESP() string {
+func GetESP() (string, error) {
 
 	for _, env := range []string{"SYSTEMD_ESP_PATH", "ESP_PATH"} {
 		envEspPath, found := os.LookupEnv(env)
 		if found {
-			return envEspPath
+			return envEspPath, nil
 		}
 	}
 
+	for _, location := range espLocations {
+		// "Touch" a file inside all candiadate locations to trigger an
+		// automount if there's an automount partition.
+		os.Stat(fmt.Sprintf("%s/does-not-exist", location))
+	}
+
 	out, err := exec.Command(
 		"lsblk",
 		"--json",
@@ -80,10 +93,10 @@ func GetESP() string {
 			continue
 		}
 
-		return entryToCheck.Mountpoint
+		return entryToCheck.Mountpoint, nil
 	}
 
-	return ""
+	return "", ErrNoESP
 }
 
 func Sign(file, output string, enroll bool) error {