SERVER-43052 Sanitize writeConflictRetry loops for duplicate fields in result

jstests/noPassthroughWithMongod/dropindexes_duplicate_fields.js

@@ -0,0 +1,48 @@
+/*
+ * Test that dropIndexes does not return a message containing multiple "nIndexesWas" fields.
+ *
+ * @tags: [requires_wiredtiger]
+ */
+
+(function() {
+"use strict";
+
+let oldRes, newRes;
+let coll = db.dropindexes_duplicate_fields;
+
+try {
+    // Repeat 100 times for the sake of probabilities
+    for (let i = 0; i < 100; i++) {
+        coll.drop();
+        assert.commandWorked(coll.insert({x: 1}));
+        assert.commandWorked(coll.createIndex({"x": 1}));
+
+        assert.commandWorked(db.adminCommand(
+            {configureFailPoint: 'WTWriteConflictException', mode: {activationProbability: 0.1}}));
+
+        // Will blow up if writeConflictRetry causes duplicate fields to be appended to result
+        let res = db.runCommand({dropIndexes: coll.getName(), index: {"x": 1}});
+        if (!oldRes && !newRes) {
+            oldRes = res;
+        } else if (oldRes && !newRes) {
+            newRes = res;
+        } else {
+            oldRes = newRes;
+            newRes = res;
+        }
+
+        assert.commandWorked(newRes ? newRes : oldRes);
+
+        // Responses between runs should be the same independent of a WriteConflict
+        if (oldRes && newRes) {
+            assert.eq(oldRes, newRes);
+        }
+
+        assert.commandWorked(
+            db.adminCommand({configureFailPoint: 'WTWriteConflictException', mode: "off"}));
+    }
+} finally {
+    assert.commandWorked(
+        db.adminCommand({configureFailPoint: 'WTWriteConflictException', mode: "off"}));
+}
+})();

src/mongo/db/catalog/drop_indexes.cpp

@@ -226,12 +226,16 @@ Status dropIndexes(OperationContext* opCtx,
         WriteUnitOfWork wunit(opCtx);
         OldClientContext ctx(opCtx, nss.ns());
 
-        Status status = wrappedRun(opCtx, collection, cmdObj, result);
+        // Use an empty BSONObjBuilder to avoid duplicate appends to result on retry loops.
+        BSONObjBuilder tempObjBuilder;
+        Status status = wrappedRun(opCtx, collection, cmdObj, &tempObjBuilder);
         if (!status.isOK()) {
             return status;
         }
 
         wunit.commit();
+
+        result->appendElementsUnique(tempObjBuilder.done());  // This append will only happen once.
         return Status::OK();
     });
 }