Merge pull request scala/scala#10924 from lrytz/t13048

Fix reentrancy issue in LongMap/AnyRefMap.getOrElseUpdate

Author: lrytz

library/src/scala/collection/mutable/AnyRefMap.scala

@@ -161,13 +161,17 @@ class AnyRefMap[K <: AnyRef, V] private[collection] (defaultEntry: K => V, initi
     val h = hashOf(key)
     var i = seekEntryOrOpen(h, key)
     if (i < 0) {
-      // It is possible that the default value computation was side-effecting
-      // Our hash table may have resized or even contain what we want now
-      // (but if it does, we'll replace it)
       val value = {
-        val oh = _hashes
+        val ohs = _hashes
+        val j = i & IndexMask
+        val oh = ohs(j)
         val ans = defaultValue
-        if (oh ne _hashes) {
+        // Evaluating `defaultValue` may change the map
+        //   - repack: the array is different
+        //   - element added at `j`: since `i < 0`, the key was missing and `oh` is either 0 or MinValue.
+        //     If `defaultValue` added an element at `j` then `_hashes(j)` must be different now.
+        //     (`hashOf` never returns 0 or MinValue.)
+        if (ohs.ne(_hashes) || oh != _hashes(j)) {
           i = seekEntryOrOpen(h, key)
           if (i >= 0) _size -= 1
         }

library/src/scala/collection/mutable/LongMap.scala

@@ -185,13 +185,17 @@ final class LongMap[V] private[collection] (defaultEntry: Long => V, initialBuff
     else {
       var i = seekEntryOrOpen(key)
       if (i < 0) {
-        // It is possible that the default value computation was side-effecting
-        // Our hash table may have resized or even contain what we want now
-        // (but if it does, we'll replace it)
         val value = {
-          val ok = _keys
+          val oks = _keys
+          val j = i & IndexMask
+          val ok = oks(j)
           val ans = defaultValue
-          if (ok ne _keys) {
+          // Evaluating `defaultValue` may change the map
+          //   - repack: the array is different
+          //   - element added at `j`: since `i < 0`, the key was missing and `ok` is either 0 or MinValue.
+          //     If `defaultValue` added an element at `j` then `_keys(j)` must be different now.
+          //     (`_keys` never contains 0 or MinValue.)
+          if (oks.ne(_keys) || ok != _keys(j)) {
             i = seekEntryOrOpen(key)
             if (i >= 0) _size -= 1
           }