/usr/share/initramfs-tools/hooks/btrfs is in btrfs-progs 4.15.1-1build1.
This file is owned by root:root, with mode 0o755.
The actual contents of the file can be viewed below.
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33  | #!/bin/sh
set -e
PREREQ=""
prereqs()
{
	echo "${PREREQ}"
}
case "${1}" in
	prereqs)
		prereqs
		exit 0
		;;
esac
. /usr/share/initramfs-tools/hook-functions
if [ -x /bin/btrfs ]
then
	copy_exec /bin/btrfs /bin
	copy_exec /bin/btrfs-zero-log
	if [ ! -x /usr/share/initramfs-tools/hooks/fsck ] && [ ! -x /etc/initramfs-tools/hooks/fsck ]
	then
	    copy_exec /bin/fsck.btrfs /bin
	fi
	LIBC_DIR=$(ldd /bin/btrfs | sed -nr 's#.* => (/lib.*)/libc\.so\.[0-9.-]+ \(0x[[:xdigit:]]+\)$#\1#p')
	find -L "$LIBC_DIR" -maxdepth 1 -name 'libgcc_s.*' -type f | while read so; do
		copy_exec "$so"
	done
fi
 |