/lib/udev/rules.d/64-btrfs.rules is in udev 232-25+deb9u8.
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 11 12 13 | # do not edit this file, it will be overwritten on update
SUBSYSTEM!="block", GOTO="btrfs_end"
ACTION=="remove", GOTO="btrfs_end"
ENV{ID_FS_TYPE}!="btrfs", GOTO="btrfs_end"
# let the kernel know about this btrfs filesystem, and check if it is complete
IMPORT{builtin}="btrfs ready $devnode"
# mark the device as not ready to be used by the system
ENV{ID_BTRFS_READY}=="0", ENV{SYSTEMD_READY}="0"
LABEL="btrfs_end"
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