Add a documentation for JobSupport.addLastAtomic

* Document internal invariants
* Add memory footprint test
* This explanation can be used to reasonably review follow-up PR that gets rid of addLastIf

Author: qwwdfsad

kotlinx-coroutines-core/common/src/JobSupport.kt

@@ -464,8 +464,9 @@ public open class JobSupport constructor(active: Boolean) : Job, ChildJob, Paren
                     if (state.isActive) {
                         // try move to SINGLE state
                         if (_state.compareAndSet(state, node)) return node
-                    } else
+                    } else {
                         promoteEmptyToNodeList(state) // that way we can add listener for non-active coroutine
+                    }
                 }
                 is Incomplete -> {
                     val list = state.list
@@ -522,7 +523,32 @@ public open class JobSupport constructor(active: Boolean) : Job, ChildJob, Paren
         return node
     }
 
-    private fun addLastAtomic(expect: Any, list: NodeList, node: JobNode) =
+    /**
+     * Adds the [node] to the given [list] if the state of this job is equal to [expect].
+     *
+     * This method is invoked only from [invokeOnCompletion] (two places) and [expect] is always a snapshot of the
+     * `state` that [invokeOnCompletion] works on.
+     *
+     * It is required to preserve the following invariants:
+     *
+     * 1) Exactly once handler invocation.
+     *    Without atomic addIf, the following scenario is possible:
+     *    * T1 reads `state`, attempts to add a handler to the list, gets preempted
+     *    * T2 finalizes the state, CASes it and invokes all the handlers from the list
+     *    * T1 adds its handler to the list to be never invoked
+     * If T1 starts to re-check the state after the addition, then the handler might be invoked twice.
+     *
+     * 2) Parent finalization.
+     *
+     * NB: it's totally unobvious from the signature, but [tryWaitForChildren] if's over `invokeOnCompletion` result:
+     * if it's `NonDisposableHandle` then the addition failed, otherwise it's successfully added.
+     * If handler fails to be added, then child is cancelled/completed and there is no need to
+     * either wait for it **or** invoke its handler.
+     *
+     * Additional implementation note: all internal handlers implement [LockFreeLinkedListNode] and
+     * LFLL **does not** support additions and removals of the same node (with the same identity) more than once.
+     */
+    private fun addLastAtomic(expect: Any, list: NodeList, node: JobNode): Boolean =
         list.addLastIf(node) { this.state === expect }
 
     private fun promoteEmptyToNodeList(state: Empty) {
@@ -904,7 +930,7 @@ public open class JobSupport constructor(active: Boolean) : Job, ChildJob, Paren
         notifyRootCause?.let { notifyCancelling(list, it) }
         // now wait for children
         val child = firstChild(state)
-        if (child != null && tryWaitForChild(finishing, child, proposedUpdate))
+        if (child != null && tryWaitForChildren(finishing, child, proposedUpdate))
             return COMPLETING_WAITING_CHILDREN
         // otherwise -- we have not children left (all were already cancelled?)
         return finalizeFinishingState(finishing, proposedUpdate)
@@ -916,16 +942,26 @@ public open class JobSupport constructor(active: Boolean) : Job, ChildJob, Paren
     private fun firstChild(state: Incomplete) =
         state as? ChildHandleNode ?: state.list?.nextChild()
 
-    // return false when there is no more incomplete children to wait
-    // ## IMPORTANT INVARIANT: Only one thread can be concurrently invoking this method.
-    private tailrec fun tryWaitForChild(state: Finishing, child: ChildHandleNode, proposedUpdate: Any?): Boolean {
+    /**
+     * This method is invoked by the JobSupport lifecycle when it transitions into "completing" state (also includes "cancelling"),
+     * and in order to be transitioned into its final state it has to wait for all the children.
+     * If this method returns `true` then the current job is waiting for its children.
+     * It's done via an addition of a special completion handler to the child that, when invoked, invokes
+     * [continueCompleting] that almost immediately fallbacks to [tryWaitForChildren].
+     *
+     * If it returns `false` then there is no children to wait for and it's safe to finalize the final job's state.
+     * Note that at this point, new children can appear:
+     * * For `cancelling` state they are immediately cancelled.
+     * * For `completing` state they are not cancelled and left hanging out to dry.
+     */
+    private tailrec fun tryWaitForChildren(state: Finishing, child: ChildHandleNode, proposedUpdate: Any?): Boolean {
         val handle = child.childJob.invokeOnCompletion(
             invokeImmediately = false,
             handler = ChildCompletion(this, state, child, proposedUpdate).asHandler
         )
         if (handle !== NonDisposableHandle) return true // child is not complete and we've started waiting for it
         val nextChild = child.nextChild() ?: return false
-        return tryWaitForChild(state, nextChild, proposedUpdate)
+        return tryWaitForChildren(state, nextChild, proposedUpdate)
     }
 
     // ## IMPORTANT INVARIANT: Only one thread can be concurrently invoking this method.
@@ -934,7 +970,7 @@ public open class JobSupport constructor(active: Boolean) : Job, ChildJob, Paren
         // figure out if we need to wait for next child
         val waitChild = lastChild.nextChild()
         // try wait for next child
-        if (waitChild != null && tryWaitForChild(state, waitChild, proposedUpdate)) return // waiting for next child
+        if (waitChild != null && tryWaitForChildren(state, waitChild, proposedUpdate)) return // waiting for next child
         // no more children to wait -- try update state
         val finalState = finalizeFinishingState(state, proposedUpdate)
         afterCompletion(finalState)

kotlinx-coroutines-core/jvm/test/MemoryFootprintTest.kt

@@ -5,14 +5,32 @@
 package kotlinx.coroutines
 
 import org.junit.Test
-import org.openjdk.jol.info.ClassLayout
+import org.openjdk.jol.info.*
 import kotlin.test.*
 
 
 class MemoryFootprintTest : TestBase(true) {
 
     @Test
-    fun testJobLayout() = assertLayout(Job().javaClass, 24)
+    fun testJobLayout() {
+        assertLayout(jobWithChildren(0).javaClass, 24)
+    }
+
+    @Test
+    fun testJobSize() {
+        assertTotalSize(jobWithChildren(1), 112)
+        assertTotalSize(jobWithChildren(2), 192) // + 80
+        assertTotalSize(jobWithChildren(3), 248) // + 56
+        assertTotalSize(jobWithChildren(4), 304) // + 56
+    }
+
+    private fun jobWithChildren(numberOfChildren: Int): Job {
+        val result = Job()
+        repeat(numberOfChildren) {
+            Job(result)
+        }
+        return result
+    }
 
     @Test
     fun testCancellableContinuationFootprint() = assertLayout(CancellableContinuationImpl::class.java, 48)
@@ -22,4 +40,9 @@ class MemoryFootprintTest : TestBase(true) {
 //        println(ClassLayout.parseClass(clz).toPrintable())
         assertEquals(expectedSize.toLong(), size)
     }
+
+    private fun assertTotalSize(instance: Job, expectedSize: Int) {
+        val size = GraphLayout.parseInstance(instance).totalSize()
+        assertEquals(expectedSize.toLong(), size)
+    }
 }