Creates a source that triggers if this is cancelled and calls its callback of type CancellableSourceFunc.
This is primarily useful for attaching to another (non-cancellable) source with add_child_source to add cancellability to it.
For convenience, you can call this with a null Cancellable, in which case the source will never trigger.
| this |
a Cancellable, or null |
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the new SocketSource. |