The printing of the error message on stack overflow had two sometimes false
assumptions previously. The first is that a local task was always available (it
called Local::take) and the second is that it used println! instead of
manually writing.
The first assumption isn't necessarily true because while stack overflow will
likely only be detected in situations that a local task is available, it's not
guaranteed to always be in TLS. For example, during a println! call a task
may be blocking, causing it to be unavailable. By using Local::try_take(), we
can be resilient against these occurrences.
The second assumption could lead to odd behavior because the stdout logger can
be overwritten to run arbitrary code. Currently this should be possible, but the
utility is much diminished because a stack overflow translates to an abort()
instead of a failure.
This extracts everything related to green scheduling from libstd and introduces
a new libgreen crate. This mostly involves deleting most of std::rt and moving
it to libgreen.
Along with the movement of code, this commit rearchitects many functions in the
scheduler in order to adapt to the fact that Local::take now *only* works on a
Task, not a scheduler. This mostly just involved threading the current green
task through in a few locations, but there were one or two spots where things
got hairy.
There are a few repercussions of this commit:
* tube/rc have been removed (the runtime implementation of rc)
* There is no longer a "single threaded" spawning mode for tasks. This is now
encompassed by 1:1 scheduling + communication. Convenience methods have been
introduced that are specific to libgreen to assist in the spawning of pools of
schedulers.
These two attributes are no longer useful now that Rust has decided to leave
segmented stacks behind. It is assumed that the rust task's stack is always
large enough to make an FFI call (due to the stack being very large).
There's always the case of stack overflow, however, to consider. This does not
change the behavior of stack overflow in Rust. This is still normally triggered
by the __morestack function and aborts the whole process.
C stack overflow will continue to corrupt the stack, however (as it did before
this commit as well). The future improvement of a guard page at the end of every
rust stack is still unimplemented and is intended to be the mechanism through
which we attempt to detect C stack overflow.
Closes#8822Closes#10155
