[indexes] IJPL-2966: expose MMappedFileStorage.getCloseStackTrace()

+ to be used in derived storages, for more detailed diagnostics of 'storage closed' scenarios

GitOrigin-RevId: 0c93894a7cf93b622d1ed8b480892fb2d9822ddb

platform/util/storages/src/com/intellij/platform/util/io/storages/mmapped/MMappedFileStorage.java

@@ -413,6 +413,13 @@ public final class MMappedFileStorage implements Closeable, Unmappable, Cleanabl
     }
   }
 
+  /** @return stacktrace of place there storage was closed, or null, if it not yet closed */
+  public @Nullable Exception getCloseStackTrace() {
+    synchronized (pagesLock) {
+      return closeStackTrace;
+    }
+  }
+
   private Page pageByIndexLocked(int pageIndex) throws IOException {
     synchronized (pagesLock) {
       if (!channel.isOpen()) {