Unix signal driver signals writable interest without ever writing

[Janrupf]

Version
tokio: 1.20.1

Platform
(any Unix with signal feature enabled)

Description

tokio/tokio/src/signal/unix/driver.rs

Line 79 in 2099d0b

Interest::READABLE | Interest::WRITABLE,

Here the WRITABLE interest is set, but the driver never writes this descriptor.

Why this is a problem
With the current mio implementation on Unix systems this won't make a difference. Since mio currently always uses edge triggered polling, the writable interest is signaled once at most and then never again.

However, while implementing a poll syscall based selector in mio, one CPU core got stuck at 100% spinning through the signal driver. The reason for this happening is that the socket used there is always writable (after all, it only gets written on events and the buffer should probably never fill). This causes the poll syscall to fall through and immediately mark the descriptor as writable, causing an infinite spin loop.

Removing the WRITABLE interest solves the problem, as the READABLE interest is signaled correctly. As far as I can tell this does not break anything, as the receiver never gets written.

[ipetkov]

Thanks for the report @Janrupf! You're totally right that the driver does not actually care for write-readiness since it uses read events to wake up