Comment # 10 on bug 1068647 from Martin Wilck

It's even more funny. The problem seems to occur if and only if I'm running in
a graphical environment. 

# systemctl isolate multi-user.target
# btrfs prop set -ts / ro true; btrfs send --no-data -v / >/dev/null; \
  btrfs prop set -ts / ro false
BTRFS_IOC_SEND returned 0
## (this can be repeated with same result)

# btrfs isolate graphical.target
# btrfs prop set -ts / ro true; btrfs send --no-data -v / >/dev/null; \
  btrfs prop set -ts / ro false
ERROR: send ioctl failed with -2: no such file or directory
## (this can also be repeated with same result)

# systemctl isolate multi-user.target
# btrfs prop set -ts / ro true; btrfs send --no-data -v / >/dev/null; \
  btrfs prop set -ts / ro false
BTRFS_IOC_SEND returned 0
## (this can be repeated with same result)