sched: implement thread::join()

While generally useful, it helps now to avoid wakeups to threads that are
no longer there.

arch/x64/arch-switch.hh

@@ -62,8 +62,7 @@ void thread::init_stack()
 void thread::on_thread_stack(thread* t)
 {
     t->_func();
-    t->_waiting = true;
-    schedule();
+    t->complete();
 }
 
 void thread::setup_tcb()
@@ -80,8 +79,7 @@ void thread_main_c(thread* t)
 {
     s_current = t;
     t->main();
-    t->_waiting = true;
-    schedule();
+    t->complete();
 }
 
 }

core/sched.cc

@@ -83,6 +83,8 @@ thread::thread(std::function<void ()> func, stack_info stack, bool main)
     , _on_runqueue(!main)
     , _waiting(false)
     , _stack(stack)
+    , _terminated(false)
+    , _joiner()
 {
     with_lock(thread_list_mutex, [this] {
         thread_list.push_back(*this);
@@ -151,6 +153,24 @@ void thread::stop_wait()
     _waiting = false;
 }
 
+void thread::complete()
+{
+    _waiting = true;
+    _terminated = true;
+    if (_joiner) {
+        _joiner->wake();
+    }
+    while (true) {
+        schedule();
+    }
+}
+
+void thread::join()
+{
+    _joiner = current();
+    wait_until([this] { return _terminated; });
+}
+
 thread::stack_info thread::get_stack_info()
 {
     return _stack;

include/sched.hh

@@ -49,6 +49,7 @@ public:
     static thread* current();
     stack_info get_stack_info();
     cpu* tcpu();
+    void join();
 private:
     void main();
     void switch_to();
@@ -58,6 +59,7 @@ private:
     void stop_wait();
     void init_stack();
     void setup_tcb();
+    void complete();
     static void on_thread_stack(thread* t);
 private:
     std::function<void ()> _func;
@@ -67,6 +69,7 @@ private:
     std::atomic_bool _waiting;
     stack_info _stack;
     cpu* _cpu;
+    bool _terminated;
     friend void thread_main_c(thread* t);
     friend class wait_guard;
     friend class cpu;
@@ -74,6 +77,7 @@ private:
     friend void ::smp_launch();
     friend void init(elf::tls_data tls, std::function<void ()> cont);
 public:
+    thread* _joiner;
     bi::list_member_hook<> _runqueue_link;
     // for the debugger
     bi::list_member_hook<> _thread_list_link;