later-squash! RAID: when a MD superblock is found on a disk, offer us…

…er to assemble RAID

This commit is kept as separate until decided this is the way to go, but
would be better squashed.

Introduce installer feature flags, and hide the RAID-assembling
feature behind the `raid-assemble` flag.

constants.py

@@ -181,3 +181,7 @@ def error_string(error, logname, with_hd):
     'var/lib/misc/ran-network-init',
     'var/lib/misc/ran-storage-init',
 ]
+
+# optional features
+FEATURES_DIR = "/etc/xensource/features"
+HAS_RAID_ASSEMBLE = os.path.exists(os.path.join(FEATURES_DIR, "raid-assemble"))

doc/features.txt

@@ -0,0 +1,18 @@
+Features flags
+==============
+
+Some host-installer features are not enabled by default, and
+downstream installers can activate them by creating a file in
+/etc/xensource/features/ in their installer filesystem.
+
+Currently available feature flags are:
+
+  raid-assemble
+
+    Detect Linux software-RAID (a.k.a "md") superblocks in disks, adds
+    a choice for the user to activate software RAID volumes, and do
+    not offer the user the ability to upgrade or restore a system on a
+    software-RAID device.
+
+    This only impacts the UI, the <assemble-raid/> answerfile
+    construct does not need this feature flag.

tui/installer/screens.py

@@ -229,7 +229,7 @@ def get_installation_type(answers):
     # RAID members, for filtering out from upgradable-products and
     # backups, and to decide whether to propose to activate existing RAID.
     raid_members = []
-    if "assemble-raid" not in answers:
+    if constants.HAS_RAID_ASSEMBLE and "assemble-raid" not in answers:
         for disk in diskutil.getQualifiedDiskList():
             rv, out = util.runCmd2([ 'mdadm', '--examine', disk ], with_stdout=True)
             if rv == 0 and re.search("Array UUID :", out):