Largely parallel to what we do for Writer.

Yes, there is a fair amount of duplicated code now for the outgoing
("sink") stuff in sw and sc, that should be factored out (to
vbahelper, probably).

Change-Id: I8df4a81c3b9043e8d6b0b206e3c04660205987c7
(cherry picked from commit b9ef6b66)