#!/usr/bin/ruby
#
# Program to create a snapshot and/or rotate a directory of backup snapshots 
# using btrfs subvolume commands.

require 'trollop'
require 'time'

def error(message)
	STDERR.print "*** #{message}\n"
	exit 1
end

def verbose(message)
	print "#{Time.now}: #{message}\n" if @verbose
end

# Icky way to find out free disc space on our mount
#
class DiskFree
	def initialize(mount)
		@mount = mount
	end

	def total
		all[2]
	end

	def used
		all[3]
	end

	def available
		all[4]
	end

	def fraction_used
		disk_device, disk_fs, disk_total, disk_used, disk_available, *rest = all
		disk_used.to_f / disk_available
	end

	protected

	def all
		disk_device, disk_fs, disk_total, disk_used, disk_available, *rest = 
			df.
			split("\n")[1].
			split(/\s+/).
			map { |i| /^[0-9]+$/.match(i) ? i.to_i : i }
	end

	def df
		`/bin/df -T -P -B1 #{@mount}`
	end
end

# Represent a directory full of backups where "current"  is a subvolume
# which is snapshotted to frozen backup directories called e.g. 
# "yyyy-mm-ddThh:mm+zzzz".
#
class BackupDirectory
	attr_reader :dir

	def initialize(dir)
		@dir = Dir.new(dir)
		current
	end

	# Return total amount of free space in backup directory (bytes)
	#
	def free
		df = DiskFree.new(@dir.path)
		df.total - df.used
	end

	# Return an array of Times representing the current list of 
	# snapshots.
	#
	def snapshot_times
		@dir.entries.map do |entry|
			begin
				Time.parse(entry)
			rescue ArgumentError => error
				nil
			end
		end.
		compact.
		sort
	end

	# What order to remove snapshots in to regain disk space?
	#
	# Order backups by their closeness to defined backup times, which are
	# listed in a set order (i.e. today's backup is more important than yesterday's).
	#
	BACKUP_IMPORTANCE = [0, 1, 2, 3, 7, 14, 21, 28, 56, 112]
	def snapshot_times_by_importance
		now = Time.now
		snapshot_times_unsorted = snapshot_times
		snapshot_times_sorted = []
		while !snapshot_times_unsorted.empty?
			BACKUP_IMPORTANCE.each do |days|
				target_time = now + (days*86400)
				closest = snapshot_times_unsorted.inject(nil) do |best, time|
					if best.nil? || (time-target_time).abs < (best-target_time).abs
						time
					else
						best
					end
				end
				break unless closest
				snapshot_times_sorted << snapshot_times_unsorted.delete(closest)
			end
		end
		snapshot_times_sorted
	end

	# Returns the size of the given snapshot (runs du, may be slow)
	#
	# Would much prefer to take advantage of this feature:
	#   http://dustymabe.com/2013/09/22/btrfs-how-big-are-my-snapshots/
	# but it's not currently in Debian/wheezy.
	#
	def snapshot_size(time=snapshot_times.latest)
		`du -s -b #{snapshot_path(time)}`.to_i
	end

	def average_snapshot_size(number=10)
		snapshot_times.sort[0..number].inject(0) { |time, total| snapshot_size(time) } / number
	end

	# Create a new snapshot of 'current'
	#
	def new_snapshot!
		system_no_error("btrfs subvolume snapshot -r #{current.path} #{snapshot_path}")
	end

	def delete_snapshot!(time)
		system_no_error("btrfs subvolume delete #{snapshot_path(time)}")
	end

	def current
		Dir.new("#{dir.path}/current")
	end

	def snapshot_path(time=Time.now)
		"#{dir.path}/#{time.strftime("%Y-%m-%dT%H:%M%z")}"
	end

	protected

	def system_no_error(*args)
		raise RuntimeError.new("Command failed: "+args.join(" ")) unless
		  system(*args)
	end
end

opts = Trollop::options do

    opt :root, "Backups directory (must be a btrfs subvolume)",
    	:type => :string

    opt :snapshot, "Take a new snapshot"

    opt :prune, "Prune old backups",
    	:type => :string

    opt :list, "List backups (by 'age' or 'importance')",
    	:type => :string

    opt :verbose, "Print diagnostics"

end

@root = opts[:root]
@verbose = opts[:verbose]
@do_snapshot = opts[:snapshot]
@do_list = opts[:list]
@do_prune = opts[:prune]

error("Must specify snapshot, prune or list") unless @do_snapshot || @do_prune || @do_list

error("--root not readable") unless File.directory?(@root)

@backups = BackupDirectory.new(@root)

def get_snapshots_by(method)
	if method == 'importance'
		@backups.snapshot_times_by_importance.reverse # least important first
	elsif method == 'age'
		@backups.snapshot_times
	else
		raise ArgumentError.new("Unknown snapshot sort method #{method}")
	end
end

if @do_snapshot
	last_snapshot_time = @backups.snapshot_times.last
	error("Last snapshot was less than six hours ago") unless 
		!last_snapshot_time || 
		Time.now - @backups.snapshot_times.last >= 6*60*60 # FIXME: make configurable

	verbose "Making new snapshot"
	@backups.new_snapshot! 
end

if @do_list
	list = get_snapshots_by(@do_list)
	print "Backups in #{@root} by #{@do_list}:\n"
	list.each_with_index do |time, index|
		print "#{sprintf('% 3d',index)}: #{time}\n"
	end
end

if @do_prune
	verbose "Counting last 10 backups"
	target_free_space = 1.5 * @backups.average_snapshot_size(10)
	verbose "Want to ensure we have #{target_free_space}"

	if @backups.free >= target_free_space
		verbose "(we have #{@backups.free} so no action needed)"
	else
		list = get_snapshots_by(@do_prune)

		while @backups.free < target_free_space && !list.empty?
			to_delete = list.pop
			verbose "Deleting #{to_delete}"
			@backups.delete_snapshot!(to_delete)
			verbose "Leaves us with #{@backups.free}"
		end
	end
end

verbose "Finished"