[RFC PATCHv1 0/2] Parallel git submodule fetching

This is a new approach to making git submodules faster without using threads. We'll directly run commands in parallel without an intermediate thread pool.

patch 1 is fixing an error, which was hard to come by in a parallel world, but is not much of a problem in a single-childed world.

The second patch works fine, (I'd love to hear feedback on the percieved change of output) though it has still two edges I want to tackle:

* The fcntl call to make the pipes nonblocking is not checked for errors.
* We also need to communicate back if one process cannot spawn or such,
  i.e. the reporting for all subprocesses combined needs to added. I think
  about adding another callback which is called to finish up a child process.

Earlier attempts used a threading pool, which made the conversion very easy as you don't need to touch lots of existing code as you would just replace the run_command(..) call by an add_task(..). The threading pool would abstract away the whole parallelism and its problems. It would also have the backpressure to the main program, add_task would just not return if there was no place left in the queue.

[1] http://www.spinics.net/lists/git/msg258435.html

Any feedback welcome! Thanks, Stefan

Jonathan Nieder (1):
  Sending "Fetching submodule <foo>" output to stderr

Stefan Beller (1):
  fetch: fetch submodules in parallel

 Documentation/fetch-options.txt |   7 ++
 builtin/fetch.c                 |   6 +-
 builtin/pull.c                  |   6 ++
 run-command.c                   | 144 ++++++++++++++++++++++++++++++++++++++++
 run-command.h                   |  29 ++++++++
 strbuf.c                        |  31 +++++++++
 strbuf.h                        |   1 +
 submodule.c                     |  99 +++++++++++++++++----------
 submodule.h                     |   2 +-
 t/t0061-run-command.sh          |  16 +++++
 t/t5526-fetch-submodules.sh     |  70 ++++++++++++-------
 test-run-command.c              |  23 +++++++
 12 files changed, 373 insertions(+), 61 deletions(-)