sync: kill git fetch process before SSH control master process

If the SSH control master process is killed while an active git
fetch is using its network socket, the underlying SSH client may
not realize the connection was broken.  This can lead to both the
client and the server waiting indefinitely for network messages
which will never be sent.

Work around the problem by keeping track of any processes that use
the tunnels we establish.  If we are about to kill any of the SSH
control masters that we started, ensure the clients using them are
successfully killed first.

Change-Id: Ida6c124dcb0c6a26bf7dd69cba2fbdc2ecd5b2fc
Signed-off-by: Shawn O. Pearce <sop@google.com>
2 files changed