add unsafe append to tablecell

ydb/core/scheme/scheme_tablecell.cpp

@@ -258,6 +258,43 @@ bool TSerializedCellVec::DoTryParse(const TString& data) {
     return TryDeserializeCellVec(data, Buf, Cells);
 }
 
+bool TSerializedCellVec::UnsafeAppendCells(TConstArrayRef<TCell> cells, TString& serializedCellVec) {
+    if (Y_UNLIKELY(cells.size() == 0)) {
+        return true;
+    }
+
+    if (!serializedCellVec) {
+        TSerializedCellVec::Serialize(serializedCellVec, cells);
+        return true;
+    }
+
+    const char* buf = serializedCellVec.data();
+    const char* bufEnd = serializedCellVec.data() + serializedCellVec.size();
+
+    if (Y_UNLIKELY(bufEnd - buf < static_cast<ptrdiff_t>(sizeof(ui16)))) {
+        return false;
+    }
+
+    ui16 cellCount = ReadUnaligned<ui16>(buf);
+    cellCount += cells.size();
+
+    size_t newSize = serializedCellVec.size();
+
+    for (auto& cell : cells)
+        newSize += sizeof(TCellHeader) + cell.Size();
+
+    serializedCellVec.ReserveAndResize(newSize);
+
+    char* mutableBuf = serializedCellVec.Detach();
+    char* oldBufEnd = mutableBuf + (bufEnd - buf);
+
+    WriteUnaligned<ui16>(mutableBuf, cellCount);
+
+    SerializeCellVecBody(cells, oldBufEnd, nullptr);
+
+    return true;
+}
+
 TSerializedCellMatrix::TSerializedCellMatrix(TConstArrayRef<TCell> cells, ui32 rowCount, ui16 colCount)
     : RowCount(rowCount), ColCount(colCount)
 {

ydb/core/scheme/scheme_tablecell.h

@@ -555,6 +555,9 @@ class TSerializedCellVec {
         return Cells;
     }
 
+    // read headers, assuming the buf is correct and append additional cells at the end
+    static bool UnsafeAppendCells(TConstArrayRef<TCell> cells, TString& serializedCellVec);
+
     static void Serialize(TString& res, TConstArrayRef<TCell> cells);
 
     static TString Serialize(TConstArrayRef<TCell> cells);

ydb/core/scheme/scheme_tablecell_ut.cpp

@@ -507,4 +507,37 @@ Y_UNIT_TEST_SUITE(Scheme) {
             }
         }
     }
+
+    Y_UNIT_TEST(UnsafeAppend) {
+        TString appended = TSerializedCellVec::Serialize({});
+
+        UNIT_ASSERT(TSerializedCellVec::UnsafeAppendCells({}, appended));
+
+        UNIT_ASSERT_EQUAL(appended.size(), 0);
+
+        ui64 intVal = 42;
+        char bigStrVal[] = "This is a large string value that shouldn't be inlined";
+
+        TVector<TCell> cells;
+        cells.emplace_back(TCell::Make(intVal));
+        cells.emplace_back(bigStrVal, sizeof(bigStrVal));
+
+        UNIT_ASSERT(TSerializedCellVec::UnsafeAppendCells(cells, appended));
+        TString serialized = TSerializedCellVec::Serialize(cells);
+
+        UNIT_ASSERT_VALUES_EQUAL(appended, serialized);
+
+        UNIT_ASSERT(TSerializedCellVec::UnsafeAppendCells(cells, appended));
+
+        cells.emplace_back(TCell::Make(intVal));
+        cells.emplace_back(bigStrVal, sizeof(bigStrVal));
+
+        serialized = TSerializedCellVec::Serialize(cells);
+
+        UNIT_ASSERT_VALUES_EQUAL(appended, serialized);
+
+        appended.resize(1);
+
+        UNIT_ASSERT(!TSerializedCellVec::UnsafeAppendCells(cells, appended));
+    }
 }