Add a new option, --data-source [Fixes GH#10, GH#17]

Specify a different block device to read from.  Not something I'd be
inclined to use myself, but when two people take the time to submit PRs for
the same issue, clearly there's demand for it.

Author: mpalmer

bin/lvmsync

@@ -61,6 +61,10 @@ def main()
 			options[:stdout] = true
 		end
 
+		opts.on("-r", "--data-source", "Read data blocks from a block device other than the snapshot origin") do |v|
+			options[:source] = v
+		end
+
 		opts.on("-V", "--version", "Print version of lvmsync") do |v|
 			begin
 				puts "lvmsync #{GVB.version}"
@@ -189,8 +193,6 @@ def run_client(opts)
 		exit 1
 	end
 
-	$stderr.puts "Origin device: #{lv.origin.path}" if opts[:verbose]
-
 	# Since, in principle, we're not supposed to be reading from snapshot
 	# devices directly, the kernel makes no attempt to make the device's read
 	# cache stay in sync with the actual state of the device.  As a result,
@@ -200,8 +202,11 @@ def run_client(opts)
 
 	snapback = opts[:snapback] ? "--snapback #{opts[:snapback]}" : ''
 
+	source = opts[:source] || lv.origin.path
+	$stderr.puts "Data source: #{source}" if opts[:verbose]
+
 	if opts[:stdout]
-		dump_changes(lv, $stdout, opts)
+		dump_changes(lv, source, $stdout, opts)
 	else
 		verbose = opts[:verbose] ? '-v' : ''
 		server_cmd = if desthost
@@ -216,7 +221,7 @@ def run_client(opts)
 		Open3.popen3(server_cmd) do |stdin_fd, stdout_fd, stderr_fd, wait_thr|
 			fds = [stdout_fd, stderr_fd]
 
-			dump_changes(lv, stdin_fd, opts) do
+			dump_changes(lv, source, stdin_fd, opts) do
 				# Remember that this fires between *every* block sent to the
 				# receiver, so don't do anything particularly slow in here!
 				until (active_fds = IO.select(fds, [], [], 0)).nil?
@@ -257,23 +262,23 @@ def run_client(opts)
 	end
 end
 
-def dump_changes(lv, outfd, opts)
+def dump_changes(snapshot, source, outfd, opts)
 	outfd.puts PROTOCOL_VERSION
 
 	start_time = Time.now
 	xfer_count = 0
 	xfer_size  = 0
 	total_size = 0
-	change_count = lv.changes.length
+	change_count = snapshot.changes.length
 
-	File.open(lv.origin.path, 'r') do |origindev|
-		lv.changes.each do |r|
+	File.open(source, 'r') do |origindev|
+		snapshot.changes.each do |r|
 			xfer_count += 1
 			chunk_size = r.last - r.first + 1
 			xfer_size  += chunk_size
 
 			$stderr.puts "Sending chunk #{r.to_s}..." if opts[:verbose]
-			$stderr.puts "Seeking to #{r.first} in #{lv.origin.path}" if opts[:verbose]
+			$stderr.puts "Seeking to #{r.first} in #{source}" if opts[:verbose]
 
 			origindev.seek(r.first, IO::SEEK_SET)