disk: tweak ensure{Btrfs,LVM}() to be more linear

[mvo5]

Tiny drive-by commit to make ensure{Btrfs,LVM}() more linear to read. I.e. avoid nested if/else when returning and do error checks early.

Best viewed side-by-side as the diff makes it hard to follow.

[edit: moved to draft as I have not inspected how much testing this function has, I'm pretty confident in the change but would really like to double check the test situation as well]
[edit2: played around a bit more with the tests and it seems it's well covered so moving out of draft again]

[achilleas-k]

🤏 tiny nitpick

[achilleas-k]

Let's make this a package constant alongside the others.

[mvo5]

I can push it here:

diff --git a/pkg/disk/disk.go b/pkg/disk/disk.go
index fdfd027cb..25da6d657 100644
--- a/pkg/disk/disk.go
+++ b/pkg/disk/disk.go
@@ -70,6 +70,9 @@ const (
 
        // Partition type ID for ESP on dos
        DosESPID = "ef00"
+
+       // Partition type ID for LVM partitions on dos
+       DosLvmID = "8e"
 )
 
 // pt type -> type -> ID mapping for convenience
diff --git a/pkg/disk/partition_table.go b/pkg/disk/partition_table.go
index d91df09bd..947631b37 100644
--- a/pkg/disk/partition_table.go
+++ b/pkg/disk/partition_table.go
@@ -733,7 +733,7 @@ func (pt *PartitionTable) ensureLVM() error {
        if pt.Type == PT_GPT {
                part.Type = LVMPartitionGUID
        } else {
-               part.Type = "8e"
+               part.Type = DosLvmID
        }
 
        return nil

but what is curious is that https://github.com/osbuild/images/blob/main/pkg/disk/disk.go#L82 uses a different DOS id for lvm it seems?

[achilleas-k]

I think 83 is still valid as it covers any generic Linux partition type but according to https://tldp.org/HOWTO/Partition-Mass-Storage-Definitions-Naming-HOWTO/x190.html you're right, 8e is specific for LVM, so let's add 8e and add it to the ID map.