thread_sync.c (rb_mutex_synchronize): only unlock if we own the mutex

If an exception is raised inside Mutex#sleep (via ConditionVariable#wait),
we cannot guarantee we can own the mutex in the ensure callback.

However, who owns the mutex at that point does not matter.  What
matters is the Mutex is usable after an exception occurs.

* thread_sync.c (rb_mutex_synchronize): only unlock if we own the mutex

* spec/ruby/library/conditionvariable/wait_spec.rb: only test lock
  usability after thread kill.  Who owns the lock at any
  particular moment is an implementation detail which we cannot
  easily guarantee.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64441 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

spec/ruby/library/conditionvariable/wait_spec.rb

@@ -23,21 +23,15 @@ describe "ConditionVariable#wait" do
     th.join
   end
 
-  it "reacquires the lock even if the thread is killed" do
+  it "the lock remains usable even if the thread is killed" do
     m = Mutex.new
     cv = ConditionVariable.new
     in_synchronize = false
-    owned = nil
 
     th = Thread.new do
       m.synchronize do
         in_synchronize = true
-        begin
         cv.wait(m)
-        ensure
-          owned = m.owned?
-          $stderr.puts "\nThe Thread doesn't own the Mutex!" unless owned
-        end
       end
     end
 
@@ -49,24 +43,19 @@ describe "ConditionVariable#wait" do
     th.kill
     th.join
 
-    owned.should == true
+    m.try_lock.should == true
+    m.unlock
   end
 
-  it "reacquires the lock even if the thread is killed after being signaled" do
+  it "lock remains usable even if the thread is killed after being signaled" do
     m = Mutex.new
     cv = ConditionVariable.new
     in_synchronize = false
-    owned = nil
 
     th = Thread.new do
       m.synchronize do
         in_synchronize = true
-        begin
         cv.wait(m)
-        ensure
-          owned = m.owned?
-          $stderr.puts "\nThe Thread doesn't own the Mutex!" unless owned
-        end
       end
     end
 
@@ -84,7 +73,8 @@ describe "ConditionVariable#wait" do
     }
 
     th.join
-    owned.should == true
+    m.try_lock.should == true
+    m.unlock
   end
 
   it "supports multiple Threads waiting on the same ConditionVariable and Mutex" do

thread_sync.c

@@ -321,21 +321,14 @@ rb_mutex_owned_p(VALUE self)
     return owned;
 }
 
-static const char *
+static void
-rb_mutex_unlock_th(rb_mutex_t *mutex, rb_thread_t *th)
+mutex_do_unlock(rb_mutex_t *mutex, rb_thread_t *th)
 {
-    const char *err = NULL;
-
-    if (mutex->th == 0) {
-	err = "Attempt to unlock a mutex which is not locked";
-    }
-    else if (mutex->th != th) {
-	err = "Attempt to unlock a mutex which is locked by another thread";
-    }
-    else {
     struct sync_waiter *cur = 0, *next = 0;
     rb_mutex_t **th_mutex = &th->keeping_mutexes;
 
+    VM_ASSERT(mutex->th == th);
+
     mutex->th = 0;
     list_for_each_safe(&mutex->waitq, cur, next, node) {
         list_del_init(&cur->node);
@@ -358,6 +351,21 @@ rb_mutex_unlock_th(rb_mutex_t *mutex, rb_thread_t *th)
     }
     *th_mutex = mutex->next_mutex;
     mutex->next_mutex = NULL;
+}
+
+static const char *
+rb_mutex_unlock_th(rb_mutex_t *mutex, rb_thread_t *th)
+{
+    const char *err = NULL;
+
+    if (mutex->th == 0) {
+	err = "Attempt to unlock a mutex which is not locked";
+    }
+    else if (mutex->th != th) {
+	err = "Attempt to unlock a mutex which is locked by another thread";
+    }
+    else {
+        mutex_do_unlock(mutex, th);
     }
 
     return err;
@@ -497,6 +505,20 @@ mutex_sleep(int argc, VALUE *argv, VALUE self)
     return rb_mutex_sleep(self, timeout);
 }
 
+static VALUE
+mutex_unlock_if_owned(VALUE self)
+{
+    rb_thread_t *th = GET_THREAD();
+    rb_mutex_t *mutex;
+    GetMutexPtr(self, mutex);
+
+    /* we may not own the mutex if an exception occured */
+    if (mutex->th == th) {
+        mutex_do_unlock(mutex, th);
+    }
+    return Qfalse;
+}
+
 /*
  * call-seq:
  *    mutex.synchronize { ... }    -> result of the block
@@ -509,7 +531,7 @@ VALUE
 rb_mutex_synchronize(VALUE mutex, VALUE (*func)(VALUE arg), VALUE arg)
 {
     rb_mutex_lock(mutex);
-    return rb_ensure(func, arg, rb_mutex_unlock, mutex);
+    return rb_ensure(func, arg, mutex_unlock_if_owned, mutex);
 }
 
 /*