[kernel] Add userspace threading

Implement the syscalls necessary for threads to create other threads in
their same process. This involved rearranging a number of syscalls, as
well as implementing object_wait and a basic implementation of a
process' list of handles.

src/kernel/syscalls/thread.cpp

@@ -0,0 +1,80 @@
+#include "j6/errors.h"
+#include "j6/types.h"
+
+#include "log.h"
+#include "objects/process.h"
+#include "scheduler.h"
+
+namespace syscalls {
+
+j6_status_t
+thread_koid(j6_koid_t *koid)
+{
+	if (koid == nullptr) {
+		return j6_err_invalid_arg;
+	}
+
+	TCB *tcb = scheduler::get().current();
+	*koid =  thread::from_tcb(tcb)->koid();
+	return j6_status_ok;
+}
+
+j6_status_t
+thread_create(void *rip, j6_handle_t *handle)
+{
+	scheduler &s = scheduler::get();
+	TCB *tcb = s.current();
+	thread *parent = thread::from_tcb(tcb);
+	process &p = parent->parent();
+
+	thread *child = p.create_thread(scheduler::default_priority);
+	child->add_thunk_user(reinterpret_cast<uintptr_t>(rip));
+	*handle = p.add_handle(child);
+	s.add_thread(child->tcb());
+
+	log::debug(logs::syscall, "Thread %llx spawned new thread %llx, handle %d",
+		parent->koid(), child->koid(), *handle);
+
+	return j6_status_ok;
+}
+
+j6_status_t
+thread_exit(int64_t status)
+{
+	auto &s = scheduler::get();
+	TCB *tcb = s.current();
+	thread *th = thread::from_tcb(tcb);
+	log::debug(logs::syscall, "Thread %llx exiting with code %d", th->koid(), status);
+
+	th->exit(status);
+	s.schedule();
+
+	log::error(logs::syscall, "returned to exit syscall");
+	return j6_err_unexpected;
+}
+
+j6_status_t
+thread_pause()
+{
+	auto &s = scheduler::get();
+	TCB *tcb = s.current();
+	thread *th = thread::from_tcb(tcb);
+	th->wait_on_signals(th, -1ull);
+	s.schedule();
+	return j6_status_ok;
+}
+
+j6_status_t
+thread_sleep(uint64_t til)
+{
+	auto &s = scheduler::get();
+	TCB *tcb = s.current();
+	thread *th = thread::from_tcb(tcb);
+	log::debug(logs::syscall, "Thread %llx sleeping until %llu", th->koid(), til);
+
+	th->wait_on_time(til);
+	s.schedule();
+	return j6_status_ok;
+}
+
+} // namespace syscalls