Fix use after free problem

Author: JonasKunz

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/data/BlockUtils.java

@@ -13,6 +13,7 @@
 import org.elasticsearch.core.Releasable;
 import org.elasticsearch.core.Releasables;
 import org.elasticsearch.exponentialhistogram.ExponentialHistogram;
+import org.elasticsearch.exponentialhistogram.ExponentialHistogramCircuitBreaker;
 
 import java.util.ArrayList;
 import java.util.Arrays;
@@ -302,7 +303,11 @@ yield new AggregateMetricDoubleLiteral(
                     aggBlock.countBlock().getInt(offset)
                 );
             }
-            case EXPONENTIAL_HISTOGRAM -> ((ExponentialHistogramArrayBlock) block).getExponentialHistogram(offset);
+            case EXPONENTIAL_HISTOGRAM -> ExponentialHistogram.merge( //TODO: do a cleaner copy with builder here
+                100,
+                ExponentialHistogramCircuitBreaker.noop(),
+                ((ExponentialHistogramArrayBlock) block).getExponentialHistogram(offset)
+            );
             case UNKNOWN -> throw new IllegalArgumentException("can't read values from [" + block + "]");
         };
     }

x-pack/plugin/esql/compute/src/main/java/org/elasticsearch/compute/data/ExponentialHistogramArrayBlock.java

@@ -25,6 +25,7 @@ public final class ExponentialHistogramArrayBlock extends AbstractNonThreadSafeR
 
     // TODO: Add AggregateMetricDoubleBlock for min/max/sum/count
     private final BytesRefBlock encodedHistograms;
+    private boolean isClosed = false;
 
     public ExponentialHistogramArrayBlock(BytesRefBlock encodedHistograms) {
         this.encodedHistograms = encodedHistograms;
@@ -42,6 +43,7 @@ static BytesRef encode(ExponentialHistogram histogram, BytesRef growableScratch)
 
     @Override
     protected void closeInternal() {
+        isClosed = true;
         Releasables.close(encodedHistograms);
     }
 
@@ -207,9 +209,16 @@ private static void resizeIfRequired(BytesRef growableBuffer, int totalSize) {
 
         @Override
         public int scale() {
+            checkForUseAfterClose();
             return data.get(data.position() + SCALE_OFFSET);
         }
 
+        private void checkForUseAfterClose() {
+            if (ExponentialHistogramArrayBlock.this.isClosed) {
+                throw new IllegalStateException("Backing block has already been closed!");
+            }
+        }
+
         @Override
         public ZeroBucket zeroBucket() {
             // TODO: implement