daemon: use btrfs(1) to get btrfs labels

blkid(1) (or actually, libblkid) seems to handle filesystem labels up
to 127 characters. Considering that btrfs labels can be up to 255
characters, this means long labels are not read correctly (i.e. get
truncated) by blkid.

Get the filesystem type, and if btrfs is available invoke
`btrfs filesystem` to get the label of btrfs filesystems.

Related to RHBZ#1164708.

daemon/blkid.c

@@ -26,6 +26,7 @@
 
 #include "daemon.h"
 #include "actions.h"
+#include "optgroups.h"
 
 GUESTFSD_EXT_CMD(str_blkid, blkid);
 
@@ -76,6 +77,13 @@ do_vfs_type (const mountable_t *mountable)
 char *
 do_vfs_label (const mountable_t *mountable)
 {
+  CLEANUP_FREE char *type = do_vfs_type (mountable);
+
+  if (type) {
+    if (STREQ (type, "btrfs") && optgroup_btrfs_available ())
+      return btrfs_get_label (mountable->device);
+  }
+
   return get_blkid_tag (mountable->device, "LABEL");
 }
 

daemon/btrfs.c

@@ -44,6 +44,30 @@ optgroup_btrfs_available (void)
   return prog_exists (str_btrfs) && filesystem_available ("btrfs") > 0;
 }
 
+char *
+btrfs_get_label (const char *device)
+{
+  int r;
+  CLEANUP_FREE char *err = NULL;
+  char *out = NULL;
+  size_t len;
+
+  r = command (&out, &err, str_btrfs, "filesystem", "label",
+               device, NULL);
+  if (r == -1) {
+    reply_with_error ("%s", err);
+    free (out);
+    return NULL;
+  }
+
+  /* Trim trailing \n if present. */
+  len = strlen (out);
+  if (len > 0 && out[len-1] == '\n')
+    out[len-1] = '\0';
+
+  return out;
+}
+
 /* Takes optional arguments, consult optargs_bitmask. */
 int
 do_btrfs_filesystem_resize (const char *filesystem, int64_t size)

daemon/daemon.h

@@ -262,6 +262,9 @@ extern char *debug_bmap (const char *subcmd, size_t argc, char *const *const arg
 extern char *debug_bmap_file (const char *subcmd, size_t argc, char *const *const argv);
 extern char *debug_bmap_device (const char *subcmd, size_t argc, char *const *const argv);
 
+/*-- in btrfs.c --*/
+extern char *btrfs_get_label (const char *device);
+
 /* ordinary daemon functions use these to indicate errors
  * NB: you don't need to prefix the string with the current command,
  * it is added automatically by the client-side RPC stubs.