/lib/udev/rules.d/64-btrfs-dm.rules is in btrfs-progs 4.15.1-1build1.
This file is owned by root:root, with mode 0o644.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 | SUBSYSTEM!="block", GOTO="btrfs_end"
KERNEL!="dm-[0-9]*", GOTO="btrfs_end"
ACTION!="add|change", GOTO="btrfs_end"
ENV{ID_FS_TYPE}!="btrfs", GOTO="btrfs_end"
# Once the device mapper symlink is created, tell btrfs about it
# so we get the friendly name in /proc/mounts (and tools that read it)
ENV{DM_NAME}=="?*", RUN{builtin}+="btrfs ready /dev/mapper/$env{DM_NAME}"
LABEL="btrfs_end"
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