CoreFoundation thread safety

[madsmtm]

Some types like CFRunLoop, CFRunLoopTimer, CFRunLoopSource etc. are thread safe, and should be safe for us to mark Send + Sync as well, except for the fact that any callbacks set via CFRunLoopAddSource/CFRunLoopAddObserver/CFRunLoopAddTimer are not necessarily guaranteed to be thread safe.

Can we assume that that is a guarantee, and then mark the types as Send + Sync?

See also servo/core-foundation-rs#649 and servo/core-foundation-rs#448.

Difficulty: Some objects are toll-free bridged (#693) with the Objective-C variants, and those are usually not thread-safe.

[madsmtm]

I've marked a bunch of immutable CF types as thread-safe in 576190c.

Regarding CFRunLoop, I'm actually leaning a bit towards not marking thread-unsafe, since while modifying it is (probably) thread-safe, the sources/observers/timers are run on the same thread as the run loop, and thus registering those are usually not thread-safe. Instead, we could make CFRunLoopGetMain unsafe / require MainThreadMarker? This might also allow our wrappers to accept non-Send + Sync callbacks.

CFRunLoopSource would still need to be thread-safe though, that's kind of the entire point of it.

CFRunLoopTimer cannot be Send + Sync, since it's toll-free bridged with NSTimer (which isn't).

Unsure about CFRunLoopObserver.