http2: avoid corruption in priority write scheduler

When removing a stream containing children in the priority
tree, it was possible for some children to not be correctly
moved to the parent of the removed stream.

Fixes golang/go#66514

Change-Id: Ie8a8743a6213a6b1a2426e023111878afff78f9e
Reviewed-on: https://go-review.googlesource.com/c/net/+/589255
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Jonathan Amsterdam <jba@google.com>

http2/writesched_priority.go

@@ -443,8 +443,8 @@ func (ws *priorityWriteScheduler) addClosedOrIdleNode(list *[]*priorityNode, max
 }
 
 func (ws *priorityWriteScheduler) removeNode(n *priorityNode) {
-	for k := n.kids; k != nil; k = k.next {
-		k.setParent(n.parent)
+	for n.kids != nil {
+		n.kids.setParent(n.parent)
 	}
 	n.setParent(nil)
 	delete(ws.nodes, n.id)

http2/writesched_priority_test.go

@@ -562,3 +562,37 @@ func TestPriorityRstStreamOnNonOpenStreams(t *testing.T) {
 		t.Error(err)
 	}
 }
+
+// https://go.dev/issue/66514
+func TestPriorityIssue66514(t *testing.T) {
+	addDep := func(ws *priorityWriteScheduler, child uint32, parent uint32) {
+		ws.AdjustStream(child, PriorityParam{
+			StreamDep: parent,
+			Exclusive: false,
+			Weight:    16,
+		})
+	}
+
+	validateDepTree := func(ws *priorityWriteScheduler, id uint32, t *testing.T) {
+		for n := ws.nodes[id]; n != nil; n = n.parent {
+			if n.parent == nil {
+				if n.id != uint32(0) {
+					t.Errorf("detected nodes not parented to 0")
+				}
+			}
+		}
+	}
+
+	ws := NewPriorityWriteScheduler(nil).(*priorityWriteScheduler)
+
+	// Root entry
+	addDep(ws, uint32(1), uint32(0))
+	addDep(ws, uint32(3), uint32(1))
+	addDep(ws, uint32(5), uint32(1))
+
+	for id := uint32(7); id < uint32(100); id += uint32(4) {
+		addDep(ws, id, id-uint32(4))
+		addDep(ws, id+uint32(2), id-uint32(4))
+		validateDepTree(ws, id, t)
+	}
+}