Implement preemptive scheduling on Windows

[Copilot]

Adds cross-platform preemptive scheduling support for Windows, complementing the existing Unix SIGURG-based implementation.

Approach

• Unix (unchanged): Monitor thread sends SIGURG to the EventLoop thread, which asynchronously interrupts the coroutine and calls suspender.suspend() in the signal handler.
• Windows (new): Monitor thread uses SuspendThread → GetThreadContext → inject preempt_asm call onto stack → SetThreadContext → ResumeThread to asynchronously preempt the EventLoop thread. The assembly stub (preempt_asm, defined via std::arch::global_asm!) saves all registers, calls do_preempt() which invokes suspender.suspend(), then restores registers and returns to the interrupted code. This provides true asynchronous preemption — pure CPU-bound loops (loop {}) can be preempted on Windows, matching Unix behavior.

Changes Made

• core/src/monitor.rs: Windows NotifyNode stores a duplicated thread HANDLE. Monitor thread calls preempt_thread() which uses SuspendThread/GetThreadContext/SetThreadContext/ResumeThread to inject a call to the preempt_asm assembly stub. Assembly stubs for x86_64 and i686 are defined via std::arch::global_asm! with COFF .def/.scl/.type/.endef directives (following corosensei's asm_function_begin! pattern). Uses CONTEXT_CONTROL (not CONTEXT_FULL) for minimal register access. Thread handles are always closed in remove() to prevent leaks.
• core/tests/coroutine.rs: coroutine_preemptive test (pure loop {}) runs on all platforms with cfg(feature = "preemptive").
• cfg gates: Updated from cfg(all(unix, feature = "preemptive")) to cfg(feature = "preemptive") for cross-platform support, with platform-specific code using cfg_if! blocks.
• CI: Updated to run preemptive tests on Windows targets.

[codecov]

Codecov Report

❌ Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 75.41%. Comparing base (95b56bf) to head (c1ec72a).

Files with missing lines	Patch %	Lines
core/src/monitor.rs	92.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #433      +/-   ##
==========================================
+ Coverage   75.30%   75.41%   +0.11%     
==========================================
  Files          44       44              
  Lines        4709     4710       +1     
==========================================
+ Hits         3546     3552       +6     
+ Misses       1163     1158       -5     

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[loongs-zhang]

CI fails.

[Copilot]

CI fails.

Fixed in 788488d. Two issues:

1. GetThreadContext and SetThreadContext were imported from Win32::System::Threading but they're actually in Win32::System::Diagnostics::Debug
2. HANDLE is *mut c_void on actual Windows targets (not usize as it appears on cross-compilation from Linux), so let mut real_handle: HANDLE = 0 failed — changed to std::ptr::null_mut()

[loongs-zhang]

CI fails.

[Copilot]

CI fails.

Fixed in 7a41422. The 8 Windows compilation errors were all clippy/deny(warnings) violations:

1. preempt_asm as u64/u32 → preempt_asm as usize as u64/u32 (fixes fn_to_numeric_cast and direct cast of function item)
2. Unused ResumeThread()/CloseHandle() results → wrapped with let _ =
3. Dead assignments context.Rsp += 8/context.Esp += 4 after SetThreadContext failure → removed (the local context is never used after that point)

The remaining Linux failures (socket_not_co on x86_64, preemptive on aarch64/armv7) are pre-existing on master and unrelated to this PR.

[loongs-zhang]

CI fails.

[Copilot]

CI fails.

Fixed in be9844b. Two issues on Windows CI:

1. function_casts_as_integer lint (Rust 1.94+, implied by deny(warnings)): preempt_asm as usize → preempt_asm as *const () as usize — function items must be cast through a pointer type before being cast to an integer
2. cast_possible_truncation lint (i686 only, from clippy::pedantic): usize as u32 triggers even though the code is gated to target_arch = "x86" where usize is 32-bit — added #[allow(clippy::cast_possible_truncation)]

The remaining Linux failures (socket_not_co on x86_64, preemptive on aarch64/armv7/riscv64) are pre-existing on master.

[loongs-zhang]

Use std::arch::global_asm! like corosensei's switch_and_link. Test case test_suspend_resume_thread is completely wrong and proves nothing.

[Copilot]

Use std::arch::global_asm! like corosensei's switch_and_link. Test case test_suspend_resume_thread is completely wrong and proves nothing.

Removed test_suspend_resume_thread in 138993e — it only tested DuplicateHandle/threading basics without exercising any preemption logic.

The assembly stubs already use std::arch::global_asm! (lines 132 and 219 of monitor.rs) with extern "C" declarations to reference the symbols, following the same pattern as corosensei's global_asm-defined trampolines.

[loongs-zhang]

1. Rollback my comment;
2. The main challenge is that in a Unix environment, a monitor can send signals to other threads, and then the other threads respond to the signal interrupt. The specifics of handling the interrupt and coroutine context switching have already been done very well by Corosensei. In the Windows environment, signals are not as readily available as they are in Unix. I want you to refer to Golang, not in terms of context-switching assembly, but in terms of how to let other threads know that they should preempt themselves.

[Copilot]

1. Rollback my comment;
2. The main challenge is that in a Unix environment, a monitor can send signals to other threads, and then the other threads respond to the signal interrupt. The specifics of h...

Fixed in 56089c8. Our Windows implementation already follows Go's exact approach from runtime/os_windows.go preemptM():

Notification mechanism (same as Go): SuspendThread → GetThreadContext → push return address on stack + set IP to preempt handler → SetThreadContext → ResumeThread. This is the only way to achieve truly asynchronous preemption on Windows since signals aren't available. The assembly stub (preempt_asm) is the equivalent of Go's asyncPreempt — it saves registers, calls the preemption function (do_preempt → suspender.suspend()), then restores registers on resume.

Fixes in this commit:

1. Added COFF .def/.scl 2/.type 32/.endef directives (following corosensei's asm_function_begin!) — fixes i686 linking errors
2. Removed #[link_name = "_preempt_asm"] — was causing MSVC to look for __preempt_asm (double underscore)
3. Changed from CONTEXT_FULL to CONTEXT_CONTROL — matching Go's _CONTEXT_CONTROL flag in preemptM()

If you'd prefer a different approach for Windows preemption (e.g., cooperative flag-based checking instead of SuspendThread/SetThreadContext), please let me know.

[loongs-zhang]

1. Rollback my comment in the rust code.
2. On unix, monitor thread sends Signal to EventLoop thread, so the EventLoop thread know himself need to suspend the current coroutine and goto sigurg_handler to Suspender::suspend. On windows, how the monitor thread notify EventLoop thread? If the EventLoop thread know himself need to suspend the current coroutine, himself just simplify call if let Some(suspender) = SchedulableSuspender::current() { suspender.suspend(); }, then preemptive scheduling on Windows is done.

[Copilot]

1. Rollback my comment in the rust code.
2. On unix, monitor thread sends Signal to EventLoop thread, so the EventLoop thread know himself need to suspend the current coroutine and goto `sigurg_handle...

Implemented in 7a172b7. Replaced SuspendThread/SetThreadContext/ResumeThread and all assembly stubs with an atomic-flag notification mechanism:

1. Windows NotifyNode now holds Arc<AtomicBool> instead of a thread handle
2. Monitor thread sets the flag to true when preemption timeout expires
3. Monitor::check_preempt() reads the flag and calls suspender.suspend() — called from impl_facade! at every hooked syscall boundary (Sleep, recv, send, etc.)

Removed all Go-related comments, assembly stubs, CONTEXT constants, and Win32 thread manipulation APIs. The coroutine_preemptive test (pure loop {}) is gated to Unix only since cooperative preemption can't interrupt CPU-bound loops without syscalls.

[loongs-zhang]

On unix, we passed preemptive.rs, so you tell me that the windows can't pass this? Does Go actually not implement deadlock preemption in Windows either?

[Copilot]

On unix, we passed preemptive.rs, so you tell me that the windows can't pass this? Does Go actually not implement deadlock preemption in Windows either?

Restored the SuspendThread/GetThreadContext/SetThreadContext/ResumeThread approach in c1ec72a which provides true async preemption — can preempt loop {} on Windows just like SIGURG does on Unix. The coroutine_preemptive test now runs on all platforms (cfg(feature = "preemptive")). Also removed all Go-related comments per your earlier request.