add download progress polling thread

doc/design/assistant/progressbars.mdwn

@@ -5,10 +5,24 @@ for it, in the terminal.
 Something better is needed for the [[webapp]]. There needs to be a
 way for the web app to know what the current progress is of all transfers.
 
-To get this info for downloads, git-annex can watch the file as it arrives
-and use its size.
-
-TODO: What about uploads? Will i have to parse rsync's progresss output?
-Feed it via a named pipe? Ugh. Check into librsync.
-
 This is one of those potentially hidden but time consuming problems.
+
+## downloads
+
+* Watch temp file as it's coming in and use its size.
+  This is the only option for some special remotes (ie, non-rsync).
+  Can either poll every .5 seconds or so to check file size, or
+  could use inotify. Implemented.
+* Feed rsync output into a parser and parse out a progress value. Ugly,
+  failure prone, but potentially the least CPU-expensive option.
+* Use librsync. Note: It's not wire-compatiable with the actual rsync
+  command.
+* Set up a FIFO, have rsync read from or write to that, and the FIFO
+  feeder/reader then can update the transfer info. Generic enough to
+  work for most (all?) special remotes, but also the most expensive option,
+  involving another copy through memory of the whole file contents.
+
+## uploads
+
+Cannot use temp file, as we're not receiving it. Rsync progress parser,
+librsync, and FIFO all work.