Add support for MACHINE_ID_FILE environment variable (on linux).

helper.go

@@ -31,6 +31,21 @@ func readFile(filename string) ([]byte, error) {
 	return ioutil.ReadFile(filename)
 }
 
+// readFirstFile tries all the pathnames listed and returns the contents of the first readable file.
+func readFirstFile(pathnames []string) ([]byte, error) {
+	contents := []byte{}
+	var err error
+	for _, pathname := range pathnames {
+		if pathname != "" {
+			contents, err = readFile(pathname)
+			if err == nil {
+				return contents, nil
+			}
+		}
+	}
+	return contents, err
+}
+
 func trim(s string) string {
 	return strings.TrimSpace(strings.Trim(s, "\n"))
 }

id_linux.go

@@ -2,7 +2,12 @@
 
 package machineid
 
+import "os"
+
 const (
+	// the environment variable name pointing to the machine id pathname
+	ENV_VARNAME = "MACHINE_ID_FILE"
+
 	// dbusPath is the default path for dbus machine id.
 	dbusPath = "/var/lib/dbus/machine-id"
 	// dbusPathEtc is the default path for dbus machine id located in /etc.
@@ -15,11 +20,10 @@ const (
 // If there is an error reading the files an empty string is returned.
 // See https://unix.stackexchange.com/questions/144812/generate-consistent-machine-unique-id
 func machineID() (string, error) {
-	id, err := readFile(dbusPath)
-	if err != nil {
-		// try fallback path
-		id, err = readFile(dbusPathEtc)
-	}
+	env_pathname := os.Getenv(ENV_VARNAME)
+	id, err := readFirstFile([]string{
+		env_pathname, dbusPath, dbusPathEtc,
+	})
 	if err != nil {
 		return "", err
 	}