Fix image copying when using disk URIs

v0.20, that added userspace disk URI support, did not implement the
support for image copying. This made URIs useful only in conjunction
with the null backend. In order to overcome this and until we have a
better solution we'll map the URIs to NBD block devices.

snf-image-host/common.sh.in

@@ -31,6 +31,7 @@ KVM="kvm"
 MD5SUM="md5sum"
 BLOCKDEV="blockdev"
 DD="dd"
+QEMU_NBD="qemu-nbd"
 
 # Temporary use stderr as monitoring file descriptor.
 # `create' will overwrite this
@@ -234,6 +235,42 @@ losetup_disk() {
     fi
 }
 
+map_uri() {
+    # Use qemu-nbd to map a URI to a block device
+    local disk bdev max i rc
+
+    disk="$1"
+
+    modprobe nbd # just to be on the safe side
+
+    if [ -f /sys/module/nbd/parameters/nbds_max ]; then
+        max=$(cat /sys/module/nbd/parameters/nbds_max)
+    else
+        log_error "NBD module is not loaded correctly"
+        exit 1
+    fi
+
+    for ((i=0; i<max; i++)); do
+        set +e
+        $QEMU_NBD -c "/dev/nbd$i" "$disk"
+        rc=$?
+        set -e
+        if [ $rc -eq 0 ]; then
+            add_cleanup $QEMU_NBD -d "/dev/nbd$i"
+            break
+        else
+            log_warning "Unable to map \`$disk' to /dev/nbd$i"
+        fi
+    done
+
+    if [ $i -eq $max ]; then
+        log_error "Could not find suitable NBD device to map: \`$disk'"
+        exit 1
+    fi
+
+    echo "/dev/nbd$i"
+}
+
 find_disk() {
     local idx path uri
 
@@ -245,10 +282,11 @@ find_disk() {
     # Here we check if we have a valid disk path (block device or regular file)
     if [ -b "$path" -o -f "$path" ]; then
         echo $path
-    # Otherwise we choose the userspace URI and
-    # hope that snf-image can handle it
     elif [ -n "$uri" ]; then
-        echo $uri
+        #TODO: Mapping a userspace disk URI to a block device is not the best
+        # thing to do, but until we implement another way to copy the content
+        # of the image into the VM's disk this must be done.
+        map_uri $uri
     else
         log_error "Disk information not exported by Ganeti"
         exit 1