Fix bbq_hnsw merge file cleanup on random IO exceptions (#119691)

Since we open two temporary files during quantized vector merging, its
possible that the second file fails to be created. In that case, we
should ensure the previously created temporary files are removed.

closes #119392

Author: benwtrent

docs/changelog/119691.yaml

@@ -0,0 +1,6 @@
+pr: 119691
+summary: Fix `bbq_hnsw` merge file cleanup on random IO exceptions
+area: Vector Search
+type: bug
+issues:
+ - 119392

server/src/main/java/org/elasticsearch/index/codec/vectors/es818/ES818BinaryQuantizedVectorsWriter.java

@@ -443,21 +443,27 @@ private CloseableRandomVectorScorerSupplier mergeOneFieldToIndex(
         float cDotC
     ) throws IOException {
         long vectorDataOffset = binarizedVectorData.alignFilePointer(Float.BYTES);
-        final IndexOutput tempQuantizedVectorData = segmentWriteState.directory.createTempOutput(
-            binarizedVectorData.getName(),
-            "temp",
-            segmentWriteState.context
-        );
-        final IndexOutput tempScoreQuantizedVectorData = segmentWriteState.directory.createTempOutput(
-            binarizedVectorData.getName(),
-            "score_temp",
-            segmentWriteState.context
-        );
         IndexInput binarizedDataInput = null;
         IndexInput binarizedScoreDataInput = null;
+        IndexOutput tempQuantizedVectorData = null;
+        IndexOutput tempScoreQuantizedVectorData = null;
         boolean success = false;
         OptimizedScalarQuantizer quantizer = new OptimizedScalarQuantizer(fieldInfo.getVectorSimilarityFunction());
         try {
+            // Since we are opening two files, it's possible that one or the other fails to open
+            // we open them within the try to ensure they are cleaned
+            tempQuantizedVectorData = segmentWriteState.directory.createTempOutput(
+                binarizedVectorData.getName(),
+                "temp",
+                segmentWriteState.context
+            );
+            tempScoreQuantizedVectorData = segmentWriteState.directory.createTempOutput(
+                binarizedVectorData.getName(),
+                "score_temp",
+                segmentWriteState.context
+            );
+            final String tempQuantizedVectorDataName = tempQuantizedVectorData.getName();
+            final String tempScoreQuantizedVectorDataName = tempScoreQuantizedVectorData.getName();
             FloatVectorValues floatVectorValues = KnnVectorsWriter.MergedVectorValues.mergeFloatVectorValues(fieldInfo, mergeState);
             if (fieldInfo.getVectorSimilarityFunction() == COSINE) {
                 floatVectorValues = new NormalizedFloatVectorValues(floatVectorValues);
@@ -516,8 +522,8 @@ private CloseableRandomVectorScorerSupplier mergeOneFieldToIndex(
                 IOUtils.close(finalBinarizedDataInput, finalBinarizedScoreDataInput);
                 IOUtils.deleteFilesIgnoringExceptions(
                     segmentWriteState.directory,
-                    tempQuantizedVectorData.getName(),
-                    tempScoreQuantizedVectorData.getName()
+                    tempQuantizedVectorDataName,
+                    tempScoreQuantizedVectorDataName
                 );
             });
         } finally {
@@ -528,11 +534,12 @@ private CloseableRandomVectorScorerSupplier mergeOneFieldToIndex(
                     binarizedDataInput,
                     binarizedScoreDataInput
                 );
-                IOUtils.deleteFilesIgnoringExceptions(
-                    segmentWriteState.directory,
-                    tempQuantizedVectorData.getName(),
-                    tempScoreQuantizedVectorData.getName()
-                );
+                if (tempQuantizedVectorData != null) {
+                    IOUtils.deleteFilesIgnoringExceptions(segmentWriteState.directory, tempQuantizedVectorData.getName());
+                }
+                if (tempScoreQuantizedVectorData != null) {
+                    IOUtils.deleteFilesIgnoringExceptions(segmentWriteState.directory, tempScoreQuantizedVectorData.getName());
+                }
             }
         }
     }