Fix VectorBuilder comodification bug in set operation

Author: Abduqodiri Qurbonzoda

kotlinx-collections-immutable/src/main/kotlin/kotlinx/collections/immutable/implementations/immutableList/PersistentVectorBuilder.kt

@@ -380,6 +380,10 @@ class PersistentVectorBuilder<E>(private var vector: PersistentList<E>,
         checkElementIndex(index, size)
         if (rootSize() <= index) {
             val mutableTail = makeMutable(tail)
+
+            // Creating new tail implies structural change.
+            if (mutableTail !== tail) { modCount++ }
+
             val tailIndex = index and MAX_BUFFER_SIZE_MINUS_ONE
             val oldElement = mutableTail[tailIndex]
             mutableTail[tailIndex] = element
@@ -399,6 +403,12 @@ class PersistentVectorBuilder<E>(private var vector: PersistentList<E>,
         val mutableRoot = makeMutable(root)
 
         if (shift == 0) {
+            // Creating new leaf implies structural change.
+            // Actually, while descending to this leaf several nodes could be recreated.
+            // However, this builder is exclusive owner of this leaf iff it is exclusive owner of all leaf's ancestors.
+            // Hence, checking recreation of this leaf is enough to determine if a structural change occurred.
+            if (mutableRoot !== root) { modCount++ }
+
             oldElementCarry.value = mutableRoot[bufferIndex]
             mutableRoot[bufferIndex] = e
             return mutableRoot

kotlinx-collections-immutable/tests/src/test/kotlin/kotlinx.collections.immutable/contractTests/immutableList/ImmutableListTest.kt

@@ -146,7 +146,17 @@ class ImmutableListTest {
     @Test fun subListOfBuilder() {
         val list = "abcxaxyz12".toImmutableList().toPersistentList()
         val builder = list.builder()
-        val subList = builder.subList(2, 5)
+
+        // builder needs to recreate the inner trie to apply the modification.
+        // So, structural changes in builder causes CME on subList iteration.
+        var subList = builder.subList(2, 5)
+        builder[4] = 'x'
+        assertFailsWith<ConcurrentModificationException> { subList.joinToString("") }
+
+        // builder is the exclusive owner of the inner trie.
+        // So, `set(index, value)` doesn't lead to structural changes.
+        subList = builder.subList(2, 5)
+        assertEquals("cxx", subList.joinToString(""))
         builder[4] = 'b'
         assertEquals("cxb", subList.joinToString(""))
         subList.removeAt(0)