basic lxc-start-ephemeral support on overlayfs

TODO:
* support aufs
* support tmpfs
* documentation
* tests

lib/lxc.rb

@@ -1,2 +1,3 @@
 require 'lxc/version'
 require 'lxc/lxc'
+require 'lxc/start_ephemeral'

lib/lxc/start_ephemeral.rb

@@ -0,0 +1,98 @@
+module LXC
+  class << self
+    def start_ephemeral(original_container_name, target_container_name, opts={})
+      orig = LXC::Container.new(original_container_name)
+      dest = LXC::Container.new(target_container_name)
+      raise "#{original_container_name} is not present. Exiting.." unless orig.defined?
+      raise "#{target_container_name} is already present. Exiting.." if dest.defined?
+
+      dest_path = File.join(LXC.global_config_item('lxc.lxcpath'), target_container_name)
+      Dir.mkdir(dest_path, 0770)
+      overlay_dirs = [[orig.config_item("lxc.rootfs"), File.join(dest_path, "rootfs")]]
+
+      dest.load_config(orig.config_file_name)
+
+      dest.set_config_item("lxc.utsname", dest.name)
+      dest.set_config_item("lxc.rootfs", File.join(dest_path, "rootfs"))
+
+      dest.config_item("lxc.network").each_with_index do |network_type, index|
+        dest.set_config_item("lxc.network.#{index}.hwaddr", random_mac) if dest.config_item("lxc.network.#{index}.hwaddr")
+      end
+
+      File.open(File.join(dest_path, 'pre-mount'), 'w+', 0755) do |pre_mount|
+        pre_mount.puts "#!/bin/sh"
+        pre_mount.puts %Q{LXC_DIR="#{dest_path}"}
+        pre_mount.puts %Q{LXC_BASE="#{orig.name}"}
+        pre_mount.puts %Q{LXC_NAME="#{dest.name}"}
+        overlay_dirs.each_with_index do |entry, count|
+          tmpdir = File.join(dest_path, 'tmpfs')
+          pre_mount.puts "mkdir -p #{tmpdir}"
+          deltdir = File.join(tmpdir, "/delta#{count}")
+          workdir = File.join(tmpdir, "/work#{count}")
+          pre_mount.puts "mkdir -p #{deltdir} #{entry[1]} #{workdir if new_overlay?}"
+          pre_mount.puts "getfacl -a #{entry[0]} | setfacl --set-file=- #{deltdir} || true"
+          pre_mount.puts "getfacl -a #{entry[0]} | setfacl --set-file=- #{entry[1]} || true"
+
+          if new_overlay?
+            pre_mount.puts "mount -n -t overlay -oupperdir=#{deltdir},lowerdir=#{entry[0]},workdir=#{workdir} none #{entry[1]}"
+          else
+            pre_mount.puts "mount -n -t overlayfs -oupperdir=#{deltdir},lowerdir=#{entry[0]} none #{entry[1]}"
+          end
+        end
+
+        opts[:bdir].each do |host_entry, container_entry|
+          if Dir.exists?(host_entry)
+            src_path = File.absolute_path(host_entry)
+            dst_path = File.join(dest_path, 'rootfs', container_entry)
+            pre_mount.puts "mkdir -p #{dst_path}"
+            pre_mount.puts "mount -n --bind #{src_path} #{dst_path}"
+          else
+            raise "Couldn't locate #{host_entry} on the host"
+          end
+        end
+
+        pre_mount.puts %Q{[ -e $LXC_DIR/configured ] && exit 0}
+        pre_mount.puts %Q{for file in $LXC_DIR/rootfs/etc/hostname \\}
+        pre_mount.puts %Q{  $LXC_DIR/rootfs/etc/hosts \\}
+        pre_mount.puts %Q{  $LXC_DIR/rootfs/etc/sysconfig/network \\}
+        pre_mount.puts %Q{  $LXC_DIR/rootfs/etc/sysconfig/network-scripts/ifcfg-eth0; do}
+        pre_mount.puts %Q{    [ -f "$file" ] && sed -i -e "s/$LXC_BASE/$LXC_NAME/" $file}
+        pre_mount.puts %Q{done}
+        pre_mount.puts %Q{touch $LXC_DIR/configured}
+      end
+
+      dest.set_config_item("lxc.hook.pre-mount", File.join(dest_path, "pre-mount"))
+
+      File.open(File.join(dest_path, 'post-stop'), 'w+', 0755) do |post_stop|
+        post_stop.puts %Q{[ -d #{dest_path} ] && rm -Rf "#{dest_path}"}
+      end
+
+      dest.set_config_item("lxc.hook.post-stop", File.join(dest_path, "post-stop"))
+
+      dest.save_config
+
+      dest.start(daemonize: opts[:daemonize])
+
+      if !dest.wait(:running, 5)
+        dest.stop
+        dest.destroy if dest.defined?
+        raise "The container '#{dest.name}' failed to start."
+      end
+    end
+
+    private
+
+    def new_overlay?
+      @new_overlay ||= File.readlines('/proc/filesystems').include?("nodev\toverlay\n")
+    end
+
+    def random_mac
+      mac = [0x00, 0x16, 0x3e,
+             SecureRandom.random_number(0x7f),
+             SecureRandom.random_number(0xff),
+             SecureRandom.random_number(0xff)
+      ]
+      mac.map {|number| number.to_s(16) }.join(':')
+    end
+  end
+end