Don't wrap errors during merging

Author: albertzaharovits

server/src/main/java/org/elasticsearch/index/engine/ThreadPoolMergeScheduler.java

@@ -322,7 +322,7 @@ public void doRun() throws Exception {
                     // OK to ignore. This is what Lucene's ConcurrentMergeScheduler does
                 } else if (t instanceof Exception == false) {
                     // onFailure and onAfter should better be called for Errors too
-                    throw new RuntimeException(t);
+                    throw new ExceptionWrappingError(t);
                 } else {
                     throw t;
                 }
@@ -331,8 +331,10 @@ public void doRun() throws Exception {
 
         @Override
         public void onAfter() {
-            assert isAutoThrottle == false || activeThrottledMergeTasksAcrossSchedulersSet.contains(this)
-                : "onAfter should always be invoked on active (and run) merges";
+            assert onGoingMerge.getMerge().isAborted()
+                || isAutoThrottle == false
+                || activeThrottledMergeTasksAcrossSchedulersSet.contains(this)
+                : "onAfter should always be invoked on aborted or active (and run) merges";
             assert this.mergeStartTimeNS.get() != null : "onAfter should always be invoked after doRun";
             try {
                 if (verbose()) {
@@ -376,7 +378,7 @@ public void onFailure(Exception e) {
             // but keep this in case something believes calling `MergeTask#onFailure` is a sane way to abort a merge
             abortOnGoingMerge();
             mergeDone(this);
-            handleMergeException(e);
+            handleMergeException(ExceptionWrappingError.maybeUnwrapCause(e));
         }
 
         @Override
@@ -446,4 +448,17 @@ private static String rateToString(double mbPerSec) {
             return String.format(Locale.ROOT, "%.1f MB/sec", mbPerSec);
         }
     }
+
+    private static class ExceptionWrappingError extends RuntimeException {
+        private static Throwable maybeUnwrapCause(Exception e) {
+            if (e instanceof ExceptionWrappingError exceptionWrappingError) {
+                return exceptionWrappingError.getCause();
+            }
+            return e;
+        }
+
+        private ExceptionWrappingError(Throwable errorCause) {
+            super(errorCause);
+        }
+    }
 }