Fix #2275: Fix deadlock in @volatile lazy vals

Apply patch suggested in #2276

Author: allanrenucci

docs/docs/reference/changed/lazy-vals-spec.md

@@ -55,7 +55,7 @@ class Foo {
           LazyVals.wait4Notification(this, bitmap_offset, flag, <field-id>)
         case <state-3> =>
           retry = false
-          result = $target
+          result = value_0
         }
       }
     result

library/src/dotty/runtime/LazyVals.scala

@@ -43,7 +43,7 @@ object LazyVals {
     var retry = true
     while (retry) {
       val cur = get(t, offset)
-      if (STATE(cur, ord) == 1) retry = CAS(t, offset, cur, v, ord)
+      if (STATE(cur, ord) == 1) retry = !CAS(t, offset, cur, v, ord)
       else {
         // cur == 2, somebody is waiting on monitor
         if (CAS(t, offset, cur, v, ord)) {
@@ -67,7 +67,8 @@ object LazyVals {
       else if (state == 2) {
         val monitor = getMonitor(t, ord)
         monitor.synchronized {
-          monitor.wait()
+          if (STATE(get(t, offset), ord) == 2) // make sure notification did not happen yet.
+            monitor.wait()
         }
       }
       else retry = false

tests/run/i2275.scala

@@ -0,0 +1,19 @@
+object Test {
+  var count = 0
+
+  @volatile lazy val x: Int = {
+    if (count < 100) {
+      count += 1
+      ???
+    }
+    1
+  }
+
+  def main(args: Array[String]): Unit = {
+    def fetchLazy(): Unit = try x catch { case _: Throwable => fetchLazy() }
+
+    for (_ <- 0 until 10) {
+      new Thread(() => fetchLazy()).start()
+    }
+  }
+}