Fixed a memory leak caused by the singleton idle callback failing to close correctly. The problem was that the close function requires running inside a callback in the event loop, but we were trying to close the idle watcher after the loop returned from run. The fix was to just call run again to process this callback. There is an additional tweak to move the initialization logic fully into bootstrap, so tasks that do not ever call run do not have problems destructing.
libuv handles are tied to the event loop that created them. In order to perform IO, the handle must be on the thread with its home event loop. Thus, when as task wants to do IO it must first go to the IO handle's home event loop and pin itself to the corresponding scheduler while the IO action is in flight. Once the IO action completes, the task is unpinned and either returns to its home scheduler if it is a pinned task, or otherwise stays on the current scheduler.
Making new blocking IO implementations (i.e. files) thread safe is rather simple. Add a home field to the IO handle's struct in uvio and implement the HomingIO trait. Wrap every IO call in the HomingIO.home_for_io method, which will take care of the scheduling.
I'm not sure if this remains thread safe in the presence of asynchronous IO at the libuv level. If we decide to do that, then this set up should be revisited.
Instead of a furious storm of idle callbacks we just have one. This is a major performance gain - around 40% on my machine for the ping pong bench.
Also in this PR is a cleanup commit for the scheduler code. Was previously up as a separate PR, but bors load + imminent merge hell led me to roll them together. Was #8549.
Each IO handle has a home event loop, which created it.
When a task wants to use an IO handle, it must first make sure it is on that home event loop.
It uses the scheduler handle in the IO handle to send itself there before starting the IO action.
Once the IO action completes, the task restores its previous home state.
If it is an AnySched task, then it will be executed on the new scheduler.
If it has a normal home, then it will return there before executing any more code after the IO action.
.with_c_str() is a replacement for the old .as_c_str(), to avoid
unnecessary boilerplate.
Replace all usages of .to_c_str().with_ref() with .with_c_str().
