fix potential concurrent modification exception in PendingPool (#7813)

Author: tbenr

ethereum/statetransition/src/main/java/tech/pegasys/teku/statetransition/util/PendingPool.java

@@ -97,6 +97,7 @@ public synchronized void add(T item) {
 
     final Bytes32 itemRoot = hashTreeRootFunction.apply(item);
     final Collection<Bytes32> requiredRoots = requiredBlockRootsFunction.apply(item);
+    final ArrayList<Bytes32> newRequiredRoots = new ArrayList<>();
 
     requiredRoots.forEach(
         requiredRoot ->
@@ -106,12 +107,15 @@ public synchronized void add(T item) {
                     requiredRoot,
                     (key) -> {
                       final Set<Bytes32> dependants = new HashSet<>();
-                      requiredBlockRootSubscribers.forEach(
-                          c -> c.onRequiredBlockRoot(requiredRoot));
+                      newRequiredRoots.add(requiredRoot);
                       return dependants;
                     })
                 .add(itemRoot));
 
+    newRequiredRoots.forEach(
+        requiredRoot ->
+            requiredBlockRootSubscribers.forEach(s -> s.onRequiredBlockRoot(requiredRoot)));
+
     // Index item by root
     if (pendingItems.putIfAbsent(itemRoot, item) == null) {
       LOG.trace(

ethereum/statetransition/src/test/java/tech/pegasys/teku/statetransition/util/PendingPoolTest.java

@@ -59,6 +59,25 @@ private void setSlot(final UInt64 slot) {
     pendingPool.onSlot(slot);
   }
 
+  @Test
+  public void add_shouldDeferSubscribersCallToAvoidConcurrentModificationException() {
+    final SignedBeaconBlock block =
+        dataStructureUtil.randomSignedBeaconBlock(currentSlot.longValue());
+
+    final SignedBeaconBlock block2 =
+        dataStructureUtil.randomSignedBeaconBlock(currentSlot.longValue());
+
+    pendingPool.subscribeRequiredBlockRoot(
+        blockRoot -> {
+          if (blockRoot.equals(block.getParentRoot())) {
+            pendingPool.add(block2);
+          }
+        });
+    pendingPool.add(block);
+
+    assertThat(pendingPool.contains(block2)).isTrue();
+  }
+
   @Test
   public void add_blockForCurrentSlot() {
     final SignedBeaconBlock block =