fix: data lost caused by Longhorn CSI plugin doing a wrong re-encryption of volume in rare race condition (backport #3566)

[mergify]

longhorn/longhorn#10416

RCA

This happens with encrypted volume only

While Longhorn CSI plugin is doing NodeStageVolume, if the Longhorn engine crash right before GetDiskFormat and recover quickly after that, the following race condition can happen:

1. When engine is crash, the engine removes the Longhorn device at /dev/longhorn/volume-name
2. Because the device path is gone, getDiskFormat returns empty value for diskFormat and nil error. Ref

   longhorn-manager/vendor/k8s.io/mount-utils/mount_linux.go

   Lines 687 to 693 in 5f9ec86

   	if exit.ExitStatus() == 2 {
   	// Disk device is unformatted.
   	// For `blkid`, if the specified token (TYPE/PTTYPE, etc) was
   	// not found, or no (specified) devices could be identified, an
   	// exit code of 2 is returned.
   	return "", nil
   	}

3. Now engine is recovered, so it recreates the device at /dev/longhorn/volume-name
4. However, due to diskFormat having empty value, Longhorn re-encrypts the volume which wipes out the all data. Ref

   longhorn-manager/csi/node_server.go

   Lines 513 to 517 in 5f9ec86

   	if diskFormat == "" {
   	if err := crypto.EncryptVolume(devicePath, passphrase, cryptoParams); err != nil {
   	return nil, status.Error(codes.Internal, err.Error())
   	}
   	}

5. Finally, Longhorn cannot detect a filesystem for the volume any more so it reformat the filesystem
6. The root cause of the issue is that getDiskFormat is using blkid. The blkid command cannot differentiate 2 cases: the device doesn't exist VS the device doesn't have filesystem inside it

Proposal

Use cryptsetup isLuks instead. It can differentiate between:

1. The device is already encrypted -> return exit code 0
2. The device is not encrypted -> return exit code 1
3. All other errors -> return exit code 4

Ref: https://gitlab.com/cryptsetup/cryptsetup/-/blob/main/FAQ.md?plain=1#L2848

With this proposal, we can safely make decision of only doing encryption in the 2nd case

Additional documentation or context

We were discussing of the below ideas with @derekbit and @shuo-wu but it seems cannot remove the race condition 100%

Idea 1: Check if the device path exist before doing GetDiskFormat

_, err := os.stat(devicePath)
if err != nil {
    return err
}

formatMounter.GetDiskFormat(devicePath)

-> Cannot avoid the race condition as the device can still disappear before GetDiskFormat and re-appear after GetDiskFormat

Idea 2: Check if the device path exist after doing GetDiskFormat

formatMounter.GetDiskFormat(devicePath)

_, err := os.stat(devicePath)
if err != nil {
    return err
}

-> Cannot avoid the race condition as the device can still disappear before GetDiskFormat and re-appear after GetDiskFormat

---

This is an automatic backport of pull request #3566 done by [Mergify](https://mergify.com).

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