Merge branch 'btrfs-no-subvolume' into calamares

FIXES #1837

src/modules/fstab/main.py

@@ -258,7 +258,8 @@ class FstabGenerator(object):
         if mount_point == "/":
             self.root_is_ssd = is_ssd
 
-        if filesystem == "btrfs" and "subvol" in partition:
+        # If there's a set-and-not-empty subvolume set, add it
+        if filesystem == "btrfs" and partition.get("subvol",None):
             options = "subvol={},".format(partition["subvol"]) + options
 
         if has_luks:

src/modules/mount/main.py

@@ -191,6 +191,8 @@ def mount_partition(root_mount_point, partition, partitions):
         libcalamares.globalstorage.insert("btrfsSubvolumes", btrfs_subvolumes)
         # Create the subvolumes that are in the completed list
         for s in btrfs_subvolumes:
+            if not s["subvolume"]:
+                continue
             subprocess.check_call(["btrfs", "subvolume", "create",
                                    root_mount_point + s["subvolume"]])
             if s["mountPoint"] == "/":

src/modules/mount/mount.conf

@@ -42,15 +42,24 @@ extraMountsEfi:
       mountPoint: /sys/firmware/efi/efivars
 
 # Btrfs subvolumes to create if root filesystem is on btrfs volume.
-# If mountpoint is mounted already to another partition, it is ignored.
+# If *mountpoint* is mounted already to another partition, it is ignored.
 # Separate subvolume for swapfile is handled separately and automatically.
+#
+# It is possible to prevent subvolume creation -- this is likely only relevant
+# for the root (/) subvolume -- by giving an empty string as a subvolume
+# name. In this case no subvolume will be created. When using snapper as
+# a rollback mechanism, it is recommended to **not** create a subvolume
+# for root.
 
 btrfsSubvolumes:
     - mountPoint: /
       subvolume: /@
+      # As an alternative:
+      #
+      # subvolume: ""
     - mountPoint: /home
       subvolume: /@home
     - mountPoint: /var/cache
       subvolume: /@cache
     - mountPoint: /var/log
-      subvolume: /@log
+      subvolume: /@log