threads: export should_yield / yield into thread_impl

Usually we can access static methods of a thread, such as yield, easily by just
calling seastar::thread::yield. Since the earliest commit that introduces
threads, however (cd0ae46), we have a separate "thread_impl" with some
of the methods alternatively exported. Although that is never explicitly
explained in that commit, my understanding is that using the thread methods
directly requires us to import thread.hh, which brings in the reactor, which
simply has no place in the future core machinery.

In preparation to yield from inside the future object, this patch exports
should_yield and yield into thread_impl.

v2: should_yield can now be inline and will not generate an explicit out of line
call.

Signed-off-by: Glauber Costa <[email protected]>

core/thread.cc

@@ -190,6 +190,10 @@ thread_context::main() {
 
 namespace thread_impl {
 
+void yield() {
+    g_current_context->thread->yield();
+}
+
 void switch_in(thread_context* to) {
     to->switch_in();
 }

core/thread_impl.hh

@@ -21,6 +21,7 @@
  */
 
 #pragma once
+#include "preempt.hh"
 #include <setjmp.h>
 #include <chrono>
 #include <experimental/optional>
@@ -51,6 +52,17 @@ inline thread_context* get() {
     return g_current_context->thread;
 }
 
+inline bool should_yield() {
+    if (need_preempt()) {
+        return true;
+    } else if (g_current_context->yield_at) {
+        return std::chrono::steady_clock::now() >= *(g_current_context->yield_at);
+    } else {
+        return false;
+    }
+}
+
+void yield();
 void switch_in(thread_context* to);
 void switch_out(thread_context* from);
 void init();