[PATCH 0/12] push progress reporting and keepalives

If you push a large number of objects, the server side may have to chew CPU for a long time processing the input (delta resolution, connectivity check, and whatever any hooks choose to do). During this time, you get no feedback that anything is happening, unless the hooks feel like writing something to stderr. For a repository the size of linux.git, a full push from scratch can take several minutes.

This is annoying and confusing to the user, who wonders if the connection has hung. But it can also cause problems on systems that have other timeouts (e.g., firewalls dropping TCP sessions, or web proxies dropping requests that produce no response within a certain time).

This patch series adds two new features:

 1. Progress reporting for the CPU-intensive parts of receive-pack.

 2. A keepalive mechanism similar to what we use in upload-pack
    (basically sending zero-length packets on sideband 1 while the client
    is waiting for us to speak).

Both are enabled for any client which speaks the sideband protocol. Existing versions of git handle the new behavior just fine (the progress reporting is easy, because they were expecting stderr messages anyway; the keepalive works because the demuxer just relays zero bytes back to send-pack).

I also tested with both JGit and libgit2 clients, and both seem to handle the zero-length packets just fine.

There are unfortunately no automated tests, as it's hard to simulate the effect. My manual testing involved inserting "sleep" statements into index-pack (and hooks with manual sleeps), and then using "strace" to confirm that the keepalives were sent.

  [01/12]: check_everything_connected: always pass --quiet to rev-list
  [02/12]: rev-list: add optional progress reporting
  [03/12]: check_everything_connected: convert to argv_array
  [04/12]: check_everything_connected: use a struct with named options
  [05/12]: check_connected: relay errors to alternate descriptor
  [06/12]: check_connected: add progress flag
  [07/12]: clone: use a real progress meter for connectivity check
  [08/12]: index-pack: add flag for showing delta-resolution progress
  [09/12]: receive-pack: turn on index-pack resolving progress
  [10/12]: receive-pack: relay connectivity errors to sideband
  [11/12]: receive-pack: turn on connectivity progress
  [12/12]: receive-pack: send keepalives during quiet periods

-Peff