Fix error that fail to dispose handler when job is in SINGLE/SINGLE+ state

Author: uchuhimo

kotlinx-coroutines-core/src/main/kotlin/kotlinx/coroutines/experimental/Job.kt

@@ -642,7 +642,7 @@ public open class JobSupport(active: Boolean) : AbstractCoroutineContextElement(
             when (state) {
                 // SINGE/SINGLE+ state -- one completion handler
                 is JobNode<*> -> {
-                    if (state !== this) return // a different job node --> we were already removed
+                    if (state !== node) return // a different job node --> we were already removed
                     // try remove and revert back to empty state
                     if (STATE.compareAndSet(this, state, EmptyActive)) return
                 }

kotlinx-coroutines-core/src/test/kotlin/kotlinx/coroutines/experimental/JobTest.kt

@@ -127,7 +127,7 @@ class JobTest : TestBase() {
         var fireCount = 0
         for (i in 0 until n) job.invokeOnCompletion { fireCount++ }.dispose()
     }
-    
+
     @Test
     fun testCancelledParent() {
         val parent = Job()
@@ -136,4 +136,25 @@ class JobTest : TestBase() {
         val child = Job(parent)
         check(!child.isActive)
     }
+
+    @Test
+    fun testDisposeSingleHandler() {
+        val job = Job()
+        var fireCount = 0
+        val handler = job.invokeOnCompletion { fireCount++ }
+        handler.dispose()
+        job.cancel()
+        assertEquals(0, fireCount)
+    }
+
+    @Test
+    fun testDisposeMultipleHandler() {
+        val job = Job()
+        val handlerCount = 10
+        var fireCount = 0
+        val handlers = Array(handlerCount) { job.invokeOnCompletion { fireCount++ } }
+        handlers.forEach { it.dispose() }
+        job.cancel()
+        assertEquals(0, fireCount)
+    }
 }