rt: Change the rust_port refcounting scheme to avoid races

Hopefully...

src/rt/rust_builtin.cpp

@@ -497,7 +497,6 @@ extern "C" CDECL void
 del_port(rust_port *port) {
     rust_task *task = rust_task_thread::get_task();
     LOG(task, comm, "del_port(0x%" PRIxPTR ")", (uintptr_t) port);
-    A(task->thread, port->get_ref_count() == 0, "Expected port ref_count == 0");
     delete port;
 }
 

src/rt/rust_port.cpp

@@ -20,12 +20,19 @@ rust_port::~rust_port() {
     task->deref();
 }
 
-void rust_port::delete_this() {
-    scoped_lock with(detach_lock);
+void rust_port::ref() {
+    scoped_lock with(ref_lock);
+    ref_count++;
+}
 
-    if (task->blocked_on(&detach_cond)) {
-        // The port owner is waiting for the port to be detached
-        task->wakeup(&detach_cond);
+void rust_port::deref() {
+    scoped_lock with(ref_lock);
+    ref_count--;
+    if (!ref_count) {
+        if (task->blocked_on(&detach_cond)) {
+            // The port owner is waiting for the port to be detached
+            task->wakeup(&detach_cond);
+        }
     }
 }
 
@@ -34,23 +41,20 @@ void rust_port::begin_detach(uintptr_t *yield) {
 
     task->release_port(id);
 
-    deref();
+    scoped_lock with(ref_lock);
+    ref_count--;
 
-    scoped_lock with(detach_lock);
-    if (get_ref_count() != 0) {
+    if (ref_count != 0) {
         task->block(&detach_cond, "waiting for port detach");
         *yield = true;
     }
 }
 
 void rust_port::end_detach() {
-    // FIXME: For some reason we can sometimes get here without the
-    // refcount decreasing to 0. This is definitely a bug
-    while (get_ref_count() != 0) { }
-
     // Just take the lock to make sure that the thread that signaled
     // the detach_cond isn't still holding it
-    scoped_lock with(detach_lock);
+    scoped_lock with(ref_lock);
+    I(task->thread, ref_count == 0);
 }
 
 void rust_port::send(void *sptr) {

src/rt/rust_port.h

@@ -6,9 +6,17 @@
 class port_detach_cond : public rust_cond { };
 
 class rust_port : public kernel_owned<rust_port>, public rust_cond {
-public:
-    RUST_ATOMIC_REFCOUNT();
+private:
+    // Protects ref_count and detach_cond
+    lock_and_signal ref_lock;
+    intptr_t ref_count;
+    port_detach_cond detach_cond;
 
+public:
+    void ref();
+    void deref();
+
+public:
     rust_port_id id;
 
     rust_kernel *kernel;
@@ -18,15 +26,9 @@ public:
 
     lock_and_signal lock;
 
-private:
-    // Protects blocking and signaling on detach_cond
-    lock_and_signal detach_lock;
-    port_detach_cond detach_cond;
-
 public:
     rust_port(rust_task *task, size_t unit_sz);
     ~rust_port();
-    void delete_this();
 
     void log_state();
     void send(void *sptr);