Support services with scale > 1 for the tar watch sync.
Add a "lossy" multi-writer specific to pipes that writes the
tar data to each `io.PipeWriter`, which is connected to `stdin`
for the `tar` process being exec'd in the container.
The data is written serially to each writer. This could be
adjusted to do concurrent writes but that will rapidly increase
the I/O load, so is not done here - in general, 99% of the
time you'll be developing (and thus using watch/sync) with a
single replica of a service.
If a write fails, the corresponding `io.PipeWriter` is removed
from the active set and closed with an error.
This means that a single container copy failing won't stop
writes to the others that are succeeding. Of course, they will
be in an inconsistent state afterwards still, but that's a
different problem.
Signed-off-by: Milas Bowman <milas.bowman@docker.com>
Matthieu MOREL
Milas Bowman