rt: Properly block tasks while waiting for port detach

Author: brson

src/libcore/comm.rs

@@ -42,7 +42,9 @@ native mod rustrt {
 
     fn new_port(unit_sz: ctypes::size_t) -> *rust_port;
     fn del_port(po: *rust_port);
-    fn rust_port_detach(po: *rust_port);
+    fn rust_port_begin_detach(po: *rust_port,
+                              yield: *ctypes::uintptr_t);
+    fn rust_port_end_detach(po: *rust_port);
     fn get_port_id(po: *rust_port) -> port_id;
     fn rust_port_size(po: *rust_port) -> ctypes::size_t;
     fn port_recv(dptr: *uint, po: *rust_port,
@@ -82,7 +84,17 @@ enum chan<T: send> {
 resource port_ptr<T: send>(po: *rust_port) {
     // Once the port is detached it's guaranteed not to receive further
     // messages
-    rustrt::rust_port_detach(po);
+    let yield = 0u;
+    let yieldp = ptr::addr_of(yield);
+    rustrt::rust_port_begin_detach(po, yieldp);
+    if yield != 0u {
+        // Need to wait for the port to be detached
+        // FIXME: If this fails then we're going to leave our port
+        // in a bogus state.
+        task::yield();
+    }
+    rustrt::rust_port_end_detach(po);
+
     // Drain the port so that all the still-enqueued items get dropped
     while rustrt::rust_port_size(po) > 0u {
         // FIXME: For some reason if we don't assign to something here

src/rt/rust_builtin.cpp

@@ -482,18 +482,23 @@ new_port(size_t unit_sz) {
 }
 
 extern "C" CDECL void
-rust_port_detach(rust_port *port) {
+rust_port_begin_detach(rust_port *port, uintptr_t *yield) {
     rust_task *task = rust_task_thread::get_task();
     LOG(task, comm, "rust_port_detach(0x%" PRIxPTR ")", (uintptr_t) port);
-    port->detach();
+    port->begin_detach(yield);
+}
+
+extern "C" CDECL void
+rust_port_end_detach(rust_port *port) {
+    port->end_detach();
 }
 
 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() == 1, "Expected port ref_count == 1");
-    port->deref();
+    A(task->thread, port->get_ref_count() == 0, "Expected port ref_count == 0");
+    delete port;
 }
 
 extern "C" CDECL size_t

src/rt/rust_port.cpp

@@ -20,13 +20,40 @@ rust_port::~rust_port() {
     task->deref();
 }
 
-void rust_port::detach() {
+void rust_port::delete_this() {
+    scoped_lock with(detach_lock);
+
+    if (task->blocked_on(&detach_cond)) {
+        // The port owner is waiting for the port to be detached
+        task->wakeup(&detach_cond);
+    }
+}
+
+void rust_port::begin_detach(uintptr_t *yield) {
+    *yield = false;
+
     task->release_port(id);
-    // FIXME: Busy waiting until we're the only ref
+
+    deref();
+
+    scoped_lock with(detach_lock);
+    if (get_ref_count() != 0) {
+        task->block(&detach_cond, "waiting for port detach");
+        *yield = true;
+    }
+}
+
+void rust_port::end_detach() {
+    // FIXME: For some reason, on rare occasion we can get here without
+    // actually having the ref count go to 0. Possibly related to #1923
     bool done = false;
     while (!done) {
-        done = get_ref_count() == 1;
+        done = 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);
 }
 
 void rust_port::send(void *sptr) {

src/rt/rust_port.h

@@ -3,6 +3,8 @@
 
 #include "rust_internal.h"
 
+class port_detach_cond : public rust_cond { };
+
 class rust_port : public kernel_owned<rust_port>, public rust_cond {
 public:
     RUST_ATOMIC_REFCOUNT();
@@ -16,15 +18,23 @@ class rust_port : public kernel_owned<rust_port>, public rust_cond {
 
     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() { delete this; }
+    void delete_this();
 
     void log_state();
     void send(void *sptr);
     void receive(void *dptr, uintptr_t *yield);
     size_t size();
-    void detach();
+
+    void begin_detach(uintptr_t *yield);
+    void end_detach();
 };
 
 //

src/rt/rustrt.def.in

@@ -34,7 +34,8 @@ rust_str_push
 rust_list_files
 rust_log_console_on
 rust_log_console_off
-rust_port_detach
+rust_port_begin_detach
+rust_port_end_detach
 rust_port_size
 rust_process_wait
 rust_ptr_eq