Make ListBuffer's iterator fail when the buffer is mutated

build.sbt

@@ -451,6 +451,11 @@ val mimaFilterSettings = Seq {
     // this is safe because the default cannot be used; instead the single-param overload in
     // `IterableOnceOps` is chosen (https://github.com/scala/scala/pull/9232#discussion_r501554458)
     ProblemFilters.exclude[DirectMissingMethodProblem]("scala.collection.immutable.ArraySeq.copyToArray$default$2"),
+
+    // Fix for scala/bug#12009
+    ProblemFilters.exclude[MissingClassProblem]("scala.collection.mutable.MutationTracker"),
+    ProblemFilters.exclude[MissingClassProblem]("scala.collection.mutable.MutationTracker$"),
+    ProblemFilters.exclude[MissingClassProblem]("scala.collection.mutable.MutationTracker$CheckedIterator"),
   ),
 }
 

src/compiler/scala/tools/nsc/transform/async/AnfTransform.scala

@@ -136,19 +136,15 @@ private[async] trait AnfTransform extends TransformUtils {
           }
 
         case ValDef(mods, name, tpt, rhs) => atOwner(tree.symbol) {
-          // Capture current cursor of a non-empty `stats` buffer so we can efficiently restrict the
+          // Capture size of `stats` buffer so we can efficiently restrict the
           // `changeOwner` to the newly added items...
-          var statsIterator = if (currentStats.isEmpty) null else currentStats.iterator
+          val oldItemsCount = currentStats.length
 
           val expr = atOwner(currentOwner.owner)(transform(rhs))
 
-          // But, ListBuffer.empty.iterator doesn't reflect later mutation. Luckily we can just start
-          // from the beginning of the buffer
-          if (statsIterator == null) statsIterator = currentStats.iterator
-
           // Definitions within stats lifted out of the `ValDef` rhs should no longer be owned by the
           // the ValDef.
-          statsIterator.foreach(_.changeOwner((currentOwner, currentOwner.owner)))
+          currentStats.iterator.drop(oldItemsCount).foreach(_.changeOwner((currentOwner, currentOwner.owner)))
           val expr1 = if (isUnitType(expr.tpe)) {
             currentStats += expr
             literalBoxedUnit

src/library/scala/collection/mutable/Growable.scala

@@ -14,7 +14,7 @@ package scala
 package collection
 package mutable
 
-import scala.collection.IterableOnce
+import scala.annotation.nowarn
 
 /** This trait forms part of collections that can be augmented
   * using a `+=` operator and that can be cleared of all elements using
@@ -56,10 +56,14 @@ trait Growable[-A] extends Clearable {
    *  @param xs   the IterableOnce producing the elements to $add.
    *  @return  the $coll itself.
    */
+  @nowarn("msg=will most likely never compare equal")
   def addAll(xs: IterableOnce[A]): this.type = {
-    val it = xs.iterator
-    while (it.hasNext) {
-      addOne(it.next())
+    if (xs.asInstanceOf[AnyRef] eq this) addAll(Buffer.from(xs)) // avoid mutating under our own iterator
+    else {
+      val it = xs.iterator
+      while (it.hasNext) {
+        addOne(it.next())
+      }
     }
     this
   }

src/library/scala/collection/mutable/ListBuffer.scala

@@ -35,13 +35,15 @@ import scala.runtime.Statics.releaseFence
   *  @define mayNotTerminateInf
   *  @define willNotTerminateInf
   */
+@SerialVersionUID(-8428291952499836345L)
 class ListBuffer[A]
   extends AbstractBuffer[A]
      with SeqOps[A, ListBuffer, ListBuffer[A]]
      with StrictOptimizedSeqOps[A, ListBuffer, ListBuffer[A]]
      with ReusableBuilder[A, immutable.List[A]]
      with IterableFactoryDefaults[A, ListBuffer]
      with DefaultSerializable {
+  @transient private[this] var mutationCount: Int = 0
 
   private var first: List[A] = Nil
   private var last0: ::[A] = null
@@ -50,7 +52,7 @@ class ListBuffer[A]
 
   private type Predecessor[A0] = ::[A0] /*| Null*/
 
-  def iterator = first.iterator
+  def iterator: Iterator[A] = new MutationTracker.CheckedIterator(first.iterator, mutationCount)
 
   override def iterableFactory: SeqFactory[ListBuffer] = ListBuffer
 
@@ -69,7 +71,12 @@ class ListBuffer[A]
     aliased = false
   }
 
-  private def ensureUnaliased() = if (aliased) copyElems()
+  // we only call this before mutating things, so it's
+  // a good place to track mutations for the iterator
+  private def ensureUnaliased(): Unit = {
+    mutationCount += 1
+    if (aliased) copyElems()
+  }
 
   // Avoids copying where possible.
   override def toList: List[A] = {
@@ -97,6 +104,7 @@ class ListBuffer[A]
   }
 
   def clear(): Unit = {
+    mutationCount += 1
     first = Nil
     len = 0
     last0 = null
@@ -114,18 +122,28 @@ class ListBuffer[A]
 
   // Overridden for performance
   override final def addAll(xs: IterableOnce[A]): this.type = {
-    val it = xs.iterator
-    if (it.hasNext) {
-      ensureUnaliased()
-      val last1 = new ::[A](it.next(), Nil)
-      if (len == 0) first = last1 else last0.next = last1
-      last0 = last1
-      len += 1
-      while (it.hasNext) {
+    if (xs.asInstanceOf[AnyRef] eq this) { // avoid mutating under our own iterator
+      if (len > 0) {
+        ensureUnaliased()
+        val copy = ListBuffer.from(this)
+        last0.next = copy.first
+        last0 = copy.last0
+        len *= 2
+      }
+    } else {
+      val it = xs.iterator
+      if (it.hasNext) {
+        ensureUnaliased()
         val last1 = new ::[A](it.next(), Nil)
-        last0.next = last1
+        if (len == 0) first = last1 else last0.next = last1
         last0 = last1
         len += 1
+        while (it.hasNext) {
+          val last1 = new ::[A](it.next(), Nil)
+          last0.next = last1
+          last0 = last1
+          len += 1
+        }
       }
     }
     this
@@ -230,13 +248,29 @@ class ListBuffer[A]
   }
 
   def insertAll(idx: Int, elems: IterableOnce[A]): Unit = {
-    ensureUnaliased()
-    val it = elems.iterator
-    if (it.hasNext) {
-      ensureUnaliased()
-      if (idx < 0 || idx > len) throw new IndexOutOfBoundsException(s"$idx is out of bounds (min 0, max ${len-1})")
-      if (idx == len) ++=(elems)
-      else insertAfter(locate(idx), it)
+    if (idx < 0 || idx > len) throw new IndexOutOfBoundsException(s"$idx is out of bounds (min 0, max ${len-1})")
+    elems match {
+      case elems: AnyRef if elems eq this => // avoid mutating under our own iterator
+        if (len > 0) {
+          val copy = ListBuffer.from(this)
+          if (idx == 0 || idx == len) { // prepend/append
+            last0.next = copy.first
+            last0 = copy.last0
+          } else {
+            val prev = locate(idx) // cannot be `null` because other condition catches that
+            val follow = prev.next
+            prev.next = copy.first
+            copy.last0.next = follow
+          }
+          len *= 2
+        }
+      case elems =>
+        val it = elems.iterator
+        if (it.hasNext) {
+          ensureUnaliased()
+          if (idx == len) ++=(elems)
+          else insertAfter(locate(idx), it)
+        }
     }
   }
 
@@ -275,15 +309,17 @@ class ListBuffer[A]
   }
 
   def mapInPlace(f: A => A): this.type = {
-    ensureUnaliased()
+    mutationCount += 1
     val buf = new ListBuffer[A]
     for (elem <- this) buf += f(elem)
     first = buf.first
     last0 = buf.last0
+    aliased = false // we just assigned from a new instance
     this
   }
 
   def flatMapInPlace(f: A => IterableOnce[A]): this.type = {
+    mutationCount += 1
     var src = first
     var dst: List[A] = null
     last0 = null
@@ -299,6 +335,7 @@ class ListBuffer[A]
       src = src.tail
     }
     first = if(dst eq null) Nil else dst
+    aliased = false // we just rebuilt a fresh, unaliased instance
     this
   }
 
@@ -322,12 +359,24 @@ class ListBuffer[A]
   }
 
   def patchInPlace(from: Int, patch: collection.IterableOnce[A], replaced: Int): this.type = {
-    val i = math.min(math.max(from, 0), length)
-    val n = math.min(math.max(replaced, 0), length)
-    ensureUnaliased()
-    val p = locate(i)
-    removeAfter(p, math.min(n, len - i))
-    insertAfter(p, patch.iterator)
+    val _len = len
+    val _from = math.max(from, 0)         // normalized
+    val _replaced = math.max(replaced, 0) // normalized
+    val it = patch.iterator
+
+    val nonEmptyPatch = it.hasNext
+    val nonEmptyReplace = (_from < _len) && (_replaced > 0)
+
+    // don't want to add a mutation or check aliasing (potentially expensive)
+    // if there's no patching to do
+    if (nonEmptyPatch || nonEmptyReplace) {
+      ensureUnaliased()
+      val i = math.min(_from, _len)
+      val n = math.min(_replaced, _len)
+      val p = locate(i)
+      removeAfter(p, math.min(n, _len - i))
+      insertAfter(p, it)
+    }
     this
   }
 

src/library/scala/collection/mutable/MutationTracker.scala

@@ -0,0 +1,78 @@
+/*
+ * Scala (https://www.scala-lang.org)
+ *
+ * Copyright EPFL and Lightbend, Inc.
+ *
+ * Licensed under Apache License 2.0
+ * (http://www.apache.org/licenses/LICENSE-2.0).
+ *
+ * See the NOTICE file distributed with this work for
+ * additional information regarding copyright ownership.
+ */
+
+package scala
+package collection
+package mutable
+
+import java.util.ConcurrentModificationException
+
+/**
+ * Utilities to check that mutations to a client that tracks
+ * its mutations have not occurred since a given point.
+ * [[Iterator `Iterator`]]s that perform this check automatically
+ * during iteration can be created by wrapping an `Iterator`
+ * in a [[MutationTracker.CheckedIterator `CheckedIterator`]],
+ * or by manually using the [[MutationTracker.checkMutations() `checkMutations`]]
+ * and [[MutationTracker.checkMutationsForIteration() `checkMutationsForIteration`]]
+ * methods.
+ */
+private object MutationTracker {
+
+  /**
+   * Checks whether or not the actual mutation count differs from
+   * the expected one, throwing an exception, if it does.
+   *
+   * @param expectedCount the expected mutation count
+   * @param actualCount   the actual mutation count
+   * @param message the exception message in case of mutations
+   * @throws ConcurrentModificationException if the expected and actual
+   *                                         mutation counts differ
+   */
+  @throws[ConcurrentModificationException]
+  def checkMutations(expectedCount: Int, actualCount: Int, message: String): Unit = {
+    if (actualCount != expectedCount) throw new ConcurrentModificationException(message)
+  }
+
+  /**
+   * Checks whether or not the actual mutation count differs from
+   * the expected one, throwing an exception, if it does. This method
+   * produces an exception message saying that it was called because a
+   * backing collection was mutated during iteration.
+   *
+   * @param expectedCount the expected mutation count
+   * @param actualCount   the actual mutation count
+   * @throws ConcurrentModificationException if the expected and actual
+   *                                         mutation counts differ
+   */
+  @throws[ConcurrentModificationException]
+  @inline def checkMutationsForIteration(expectedCount: Int, actualCount: Int): Unit =
+    checkMutations(expectedCount, actualCount, "mutation occurred during iteration")
+
+  /**
+   * An iterator wrapper that checks if the underlying collection has
+   * been mutated.
+   *
+   * @param underlying    the underlying iterator
+   * @param mutationCount a by-name provider of the current mutation count
+   * @tparam A the type of the iterator's elements
+   */
+  final class CheckedIterator[A](underlying: Iterator[A], mutationCount: => Int) extends AbstractIterator[A] {
+    private[this] val expectedCount = mutationCount
+
+    def hasNext: Boolean = {
+      checkMutationsForIteration(expectedCount, mutationCount)
+      underlying.hasNext
+    }
+    def next(): A = underlying.next()
+  }
+}

src/library/scala/collection/mutable/Shrinkable.scala

@@ -13,7 +13,7 @@
 package scala
 package collection.mutable
 
-import scala.annotation.tailrec
+import scala.annotation.{nowarn, tailrec}
 
 /** This trait forms part of collections that can be reduced
   *  using a `-=` operator.
@@ -52,16 +52,24 @@ trait Shrinkable[-A] {
     *  @param xs   the iterator producing the elements to remove.
     *  @return the $coll itself
     */
+  @nowarn("msg=will most likely never compare equal")
   def subtractAll(xs: collection.IterableOnce[A]): this.type = {
     @tailrec def loop(xs: collection.LinearSeq[A]): Unit = {
       if (xs.nonEmpty) {
         subtractOne(xs.head)
         loop(xs.tail)
       }
     }
-    xs match {
-      case xs: collection.LinearSeq[A] => loop(xs)
-      case xs => xs.iterator.foreach(subtractOne)
+    if (xs.asInstanceOf[AnyRef] eq this) { // avoid mutating under our own iterator
+      xs match {
+        case xs: Clearable => xs.clear()
+        case xs            => subtractAll(Buffer.from(xs))
+      }
+    } else {
+      xs match {
+        case xs: collection.LinearSeq[A] => loop(xs)
+        case xs => xs.iterator.foreach(subtractOne)
+      }
     }
     this
   }