filesystem: fall back to path-only links if UUID cannot be determined

This is needed to allow creating protector links to btrfs filesystems.

Update #339

Author: ebiggers

filesystem/mountpoint.go

@@ -545,12 +545,19 @@ func (mnt *Mount) getFilesystemUUID() (string, error) {
 }
 
 // makeLink creates the contents of a link file which will point to the given
-// filesystem.  This will be a string of the form "UUID=<uuid>\nPATH=<path>\n".
-// An error is returned if the filesystem's UUID cannot be determined.
+// filesystem.  This will normally be a string of the form
+// "UUID=<uuid>\nPATH=<path>\n".  If the UUID cannot be determined, the UUID
+// portion will be omitted.
 func makeLink(mnt *Mount) (string, error) {
 	uuid, err := mnt.getFilesystemUUID()
 	if err != nil {
-		return "", &ErrMakeLink{mnt, err}
+		// The UUID could not be determined.  This happens for btrfs
+		// filesystems, as the device number found via
+		// /dev/disk/by-uuid/* for btrfs filesystems differs from the
+		// actual device number of the mounted filesystem.  Just rely
+		// entirely on the fallback to mountpoint path.
+		log.Print(err)
+		return fmt.Sprintf("%s=%s\n", pathToken, mnt.Path), nil
 	}
 	return fmt.Sprintf("%s=%s\n%s=%s\n", uuidToken, uuid, pathToken, mnt.Path), nil
 }

filesystem/mountpoint_test.go

@@ -411,6 +411,40 @@ func TestGetMountFromLink(t *testing.T) {
 	}
 }
 
+// Test that makeLink() is including the expected information in links.
+func TestMakeLink(t *testing.T) {
+	mnt, err := getTestMount(t)
+	if err != nil {
+		t.Skip(err)
+	}
+	link, err := makeLink(mnt)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	// Normally, both UUID and PATH should be included.
+	if !strings.Contains(link, "UUID=") {
+		t.Fatal("Link doesn't contain UUID")
+	}
+	if !strings.Contains(link, "PATH=") {
+		t.Fatal("Link doesn't contain PATH")
+	}
+
+	// Without a valid device number, only PATH should be included.
+	mntCopy := *mnt
+	mntCopy.DeviceNumber = 0
+	link, err = makeLink(&mntCopy)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if strings.Contains(link, "UUID=") {
+		t.Fatal("Link shouldn't contain UUID")
+	}
+	if !strings.Contains(link, "PATH=") {
+		t.Fatal("Link doesn't contain PATH")
+	}
+}
+
 // Test that old filesystem links that contain a UUID only still work.
 func TestGetMountFromLegacyLink(t *testing.T) {
 	mnt, err := getTestMount(t)
@@ -456,6 +490,16 @@ func TestGetMountFromLinkFallback(t *testing.T) {
 		t.Fatal("Link doesn't point to the same Mount")
 	}
 
+	// only PATH given at all (should succeed)
+	link = fmt.Sprintf("PATH=%s\n", mnt.Path)
+	linkedMnt, err = getMountFromLink(link)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if linkedMnt != mnt {
+		t.Fatal("Link doesn't point to the same Mount")
+	}
+
 	// only UUID valid (should succeed)
 	link = fmt.Sprintf("UUID=%s\nPATH=%s\n", goodUUID, badPath)
 	if linkedMnt, err = getMountFromLink(link); err != nil {