Extend the logic for linux

The machine id is looked in "canonical" locations. If not value is found, one is generated and persisted.
The machine id is looked in:
  - the file pointed by the `MACHINE_ID_FILE` env var
  - `/var/lib/dbus/machine-id`
  - `/etc/machine-id`
  - `$HOME/.config/machine-id`

If no such file is found, a random uuid is generated and persisted in the first
writable file among `$MACHINE_ID_FILE`, `/var/lib/dbus/machine-id`, `/etc/machine-id`, `$HOME/.config/machine-id`.

If there is an error reading _all_ the files an empty string is returned.

The logic implemented is a variation of the one described in denisbrodbeck#5 (comment)

helper.go

@@ -31,6 +31,10 @@ func readFile(filename string) ([]byte, error) {
 	return ioutil.ReadFile(filename)
 }
 
+func writeFile(filename string, data []byte) error {
+	return ioutil.WriteFile(filename, data, 0644)
+}
+
 // readFirstFile tries all the pathnames listed and returns the contents of the first readable file.
 func readFirstFile(pathnames []string) ([]byte, error) {
 	contents := []byte{}
@@ -46,6 +50,20 @@ func readFirstFile(pathnames []string) ([]byte, error) {
 	return contents, err
 }
 
+// writeFirstFile writes to the first file that "works" between all pathnames listed.
+func writeFirstFile(pathnames []string, data []byte) error {
+	var err error
+	for _, pathname := range pathnames {
+		if pathname != "" {
+			err = writeFile(pathname, data)
+			if err == nil {
+				return nil
+			}
+		}
+	}
+	return err
+}
+
 func trim(s string) string {
 	return strings.TrimSpace(strings.Trim(s, "\n"))
 }

id_linux.go

@@ -2,7 +2,10 @@
 
 package machineid
 
-import "os"
+import (
+	"os"
+	"path"
+)
 
 const (
 	// the environment variable name pointing to the machine id pathname
@@ -14,16 +17,41 @@ const (
 	// Some systems (like Fedora 20) only know this path.
 	// Sometimes it's the other way round.
 	dbusPathEtc = "/etc/machine-id"
+
+	// this returns a random uuid each time it's read
+	linuxRandomUuid = "/proc/sys/kernel/random/uuid"
 )
 
-// machineID returns the uuid specified at `/var/lib/dbus/machine-id` or `/etc/machine-id`.
-// 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
+// machineID returns the uuid specified in the "canonical" locations. If not such value is found, one is generated and persisted.
+// The machine id is looked in:
+//   - the file pointed by the `MACHINE_ID_FILE` env var
+//   - `/var/lib/dbus/machine-id`
+//   - `/etc/machine-id`
+//   - `$HOME/.config/machine-id`
+//
+// If no such file is found, a random uuid is generated and persisted in the first
+// writable file among `$MACHINE_ID_FILE`, `/var/lib/dbus/machine-id`, `/etc/machine-id`, `$HOME/.config/machine-id`.
+//
+// If there is an error reading _all_ the files an empty string is returned.
+// The logic implemented is a variation of the one described in https://github.com/denisbrodbeck/machineid/issues/5#issuecomment-523803164
+// See also https://unix.stackexchange.com/questions/144812/generate-consistent-machine-unique-id
 func machineID() (string, error) {
 	env_pathname := os.Getenv(ENV_VARNAME)
+
+	home := os.Getenv("HOME")
+	userMachineId := path.Join(home, ".config", "machine-id")
+
 	id, err := readFirstFile([]string{
-		env_pathname, dbusPath, dbusPathEtc,
+		env_pathname, dbusPath, dbusPathEtc, userMachineId,
 	})
+	if err != nil {
+		id, err = readFile(uuid)
+		if err == nil {
+			writeFirstFile([]string{
+				env_pathname, dbusPathEtc, dbusPath, userMachineId,
+			}, id)
+		}
+	}
 	if err != nil {
 		return "", err
 	}