dom0: block-snapshot: add prepare and cleanup actions

"prepare" and "cleanup" actions can be used to setup device manually - not from
udev. This is used by qvm-revert-template-changes.

common/block-snapshot

@@ -5,7 +5,12 @@
 # This creates dm-snapshot device on given arguments
 
 dir=$(dirname "$0")
-. "$dir/block-common.sh"
+if [ "$1" = "prepare" ] || [ "$1" = "cleanup" ]; then
+  . "$dir/xen-hotplug-common.sh"
+  command=$1
+else
+  . "$dir/block-common.sh"
+fi
 
 shopt -s nullglob
 
@@ -143,10 +148,58 @@ case "$command" in
         ;;
     esac
     ;;
-  remove)
+  prepare)
+    t=$2
     case $t in
       snapshot|origin)
-        node=$(cat "$HOTPLUG_STORE-node")
+        p=$3
+        base=${p/:*/}
+        cow=${p/*:/}
+
+        if [ -L "$base" ]; then
+          base=$(readlink -f "$base") || fatal "$base link does not exist."
+        fi
+
+        if [ -L "$cow" ]; then
+          cow=$(readlink -f "$cow") || fatal "$cow link does not exist."
+        fi
+
+        # first ensure that snapshot device exists (to write somewhere changes from snapshot-origin)
+        dm_devname=$(get_dm_snapshot_name "$base" "$cow")
+
+        claim_lock "block"
+
+        # prepare snapshot device
+        create_dm_snapshot $dm_devname "$base" "$cow"
+
+        if [ "$t" == "snapshot" ]; then
+          #that's all for snapshot, store name of prepared device
+          echo "/dev/mapper/$dm_devname"
+        elif [ "$t" == "origin" ]; then
+          # for origin - prepare snapshot-origin device and store its name
+          dm_devname=origin-$(stat -c '%D:%i' "$base")
+          create_dm_snapshot_origin $dm_devname "$base"
+          echo "/dev/mapper/$dm_devname"
+        fi
+
+        release_lock "block"
+        exit 0
+        ;;
+    esac
+    ;;
+  remove|cleanup)
+    if [ "$command" = "cleanup" ]; then
+      t=$2
+    else
+      t=$(cat $HOTPLUG_STORE-type)
+    fi
+    case $t in
+      snapshot|origin)
+        if [ "$command" = "cleanup" ]; then
+          node=$3
+        else
+          node=$(cat "$HOTPLUG_STORE-node")
+        fi
 
         if [ -z "$node" ]; then
           fatal "No device node to remove"
@@ -186,11 +239,13 @@ case "$command" in
               dmsetup remove $snap
             fi
           done
-          # Commit template changes
-          domain=$(cat "$HOTPLUG_STORE-domain")
-          if [ "$domain" ]; then
-            # Dont stop on errors
-            /usr/bin/qvm-template-commit "$domain" || true
+          if [ "$command" = "remove" ]; then
+            # Commit template changes
+            domain=$(cat "$HOTPLUG_STORE-domain")
+            if [ "$domain" ]; then
+              # Dont stop on errors
+              /usr/bin/qvm-template-commit "$domain" || true
+            fi
           fi
         fi