[$] Toward race-free process signaling

Signals have existed in Unix systems for years, despite the generalconsensus that they are an example of a baddesign. Extensions and new ways of using signals pop up from time totime, fixing the issues that have been found. A notable addition was theintroduction of signalfd()nearly 10 years ago. Recently, the kernel developers have discussed how to avoidrace conditions related to process-ID (PID) recycling, which occurs when aprocess terminates and another one is assigned the same PID. A process that failsto notice that its target has exited may try to send a signal to the wrongrecipient, with potentially grave consequences. A patch set from ChristianBrauner is trying to solve the issue by adding signaling via file descriptors.