Fix reading strings from the hash index overflow file

When there is multi-threaded contention, data was being appended to the
result multiple times because of how optimisticRead behaves.
Replacing the relevant portion of the string instead of appending makes
sure that the result is correct and overwrites the null data from
previous runs when the optimisticRead finally manages a successful read.

Fixes #2916

src/storage/storage_structure/disk_overflow_file.cpp

@@ -32,10 +32,13 @@ std::string DiskOverflowFile::readString(TransactionType trxType, const ku_strin
             auto numBytesToReadInPage = std::min(
                 static_cast<uint32_t>(remainingLength), END_OF_PAGE - cursor.elemPosInPage);
             page_idx_t nextPage;
+            auto startPosInSrc = retVal.size();
             bufferManager->optimisticRead(*fileHandleToPin, pageIdxToPin, [&](uint8_t* frame) {
-                retVal +=
+                // Replace rather than append, since optimistic read may call the function multiple
+                // times
+                retVal.replace(startPosInSrc, numBytesToReadInPage,
                     std::string_view(reinterpret_cast<const char*>(frame) + cursor.elemPosInPage,
-                        numBytesToReadInPage);
+                        numBytesToReadInPage));
                 nextPage = *(page_idx_t*)(frame + END_OF_PAGE);
             });
             remainingLength -= numBytesToReadInPage;