api: add mountable_device and mountable_subvolume

These two functions allow the user to split the mountable strings
into a device and a subvolume if any. See this thread on the mailing
list for the rationale:

https://www.redhat.com/archives/libguestfs/2016-February/msg00247.html

generator/actions.ml

@@ -1289,6 +1289,32 @@ for a filesystem to be shared between operating systems.
 Please read L<guestfs(3)/INSPECTION> for more details.
 See also C<guestfs_inspect_get_mountpoints>." };
 
+  { defaults with
+    name = "mountable_device"; added = (1, 33, 15);
+    style = RString "device", [Mountable "mountable"], [];
+    shortdesc = "extract the device part of a mountable";
+    longdesc = "\
+Returns the device name of a mountable. In quite a lot of
+cases, the mountable is the device name.
+
+However this doesn't apply for btrfs subvolumes, where the
+mountable is a combination of both the device name and the
+subvolume path (see also C<guestfs_mountable_subvolume> to
+extract the subvolume path of the mountable if any)." };
+
+  { defaults with
+    name = "mountable_subvolume"; added = (1, 33, 15);
+    style = RString "subvolume", [Mountable "mountable"], [];
+    shortdesc = "extract the subvolume part of a mountable";
+    longdesc = "\
+Returns the subvolume path of a mountable. Btrfs subvolumes
+mountables are a combination of both the device name and the
+subvolume path (see also C<guestfs_mountable_device> to extract
+the device of the mountable).
+
+If the mountable does not represent a btrfs subvolume, then
+this function fails and the C<errno> is set to C<EINVAL>." };
+
   { defaults with
     name = "set_network"; added = (1, 5, 4);
     style = RErr, [Bool "network"], [];

po/POTFILES

@@ -346,6 +346,7 @@ src/libvirt-is-version.c
 src/listfs.c
 src/lpj.c
 src/match.c
+src/mountable.c
 src/osinfo.c
 src/private-data.c
 src/proto.c

src/Makefile.am

@@ -121,6 +121,7 @@ libguestfs_la_SOURCES = \
 	listfs.c \
 	lpj.c \
 	match.c \
+	mountable.c \
 	osinfo.c \
 	private-data.c \
 	proto.c \

src/mountable.c

@@ -0,0 +1,52 @@
+/* libguestfs
+ * Copyright (C) 2016 SUSE LLC
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Lesser General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Lesser General Public License for more details.
+ *
+ * You should have received a copy of the GNU Lesser General Public
+ * License along with this library; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
+ */
+
+#include <config.h>
+#include <string.h>
+#include <errno.h>
+
+#include "guestfs.h"
+#include "guestfs-internal.h"
+#include "guestfs-internal-actions.h"
+
+
+char *
+guestfs_impl_mountable_device (guestfs_h *g, const char *mountable)
+{
+  CLEANUP_FREE_INTERNAL_MOUNTABLE struct guestfs_internal_mountable *mnt = NULL;
+
+  mnt = guestfs_internal_parse_mountable (g, mountable);
+  if (mnt == NULL)
+    return NULL;
+
+  return safe_strdup (g, mnt->im_device);
+}
+
+char *
+guestfs_impl_mountable_subvolume (guestfs_h *g, const char *mountable)
+{
+  CLEANUP_FREE_INTERNAL_MOUNTABLE struct guestfs_internal_mountable *mnt = NULL;
+
+  mnt = guestfs_internal_parse_mountable (g, mountable);
+  if (mnt == NULL || STREQ (mnt->im_volume, "")) {
+    guestfs_int_error_errno (g, EINVAL, "not a btrfs subvolume identifier");
+    return NULL;
+  }
+
+  return safe_strdup (g, mnt->im_volume);
+}