We've done this a number of times already where we're getting
exclusive access to either the out direction of a connection, or we
try to lock out the read side while we are responding to a previous
request. They usually are really cumbersome because we reach around to
the other direction to stop it from proceeding, or we flag our
exclusive access somewhere, and we always need to know whom to notify.
PR ElementsProject/lightning#1970 adds two new instances of this:
- Streaming a JSON response requires that nothing else should write
while the stream is active.
- We also want to stop reading new requests while we are responding
to one.
To remove the complexity of having to know whom to stop and notify
when we're done, this adds a simple `io_lock` primitive that can be
used to get exclusive access to a connection. This inverts the
requirement for notifications, since everybody registers interest in
the lock and they get notified if the lock holder releases it.
Rusty Russell
Christian Decker