refactor,blocks: deduplicate block's serialized size calculations

For consistency `UNDO_DATA_DISK_OVERHEAD` was also extracted to avoid the constant's ambiguity.
Asserts were added to help with the review - they are removed in the next commit.

Co-authored-by: Ryan Ofsky <ryan@ofsky.org>

Author: l0rinc

src/node/blockstorage.cpp

@@ -945,7 +945,9 @@ bool BlockManager::WriteUndoDataForBlock(const CBlockUndo& blockundo, BlockValid
     // Write undo information to disk
     if (block.GetUndoPos().IsNull()) {
         FlatFilePos pos;
-        if (!FindUndoPos(state, block.nFile, pos, ::GetSerializeSize(blockundo) + 40)) {
+        const unsigned int blockundo_size{static_cast<unsigned int>(GetSerializeSize(blockundo))};
+        static_assert(UNDO_DATA_DISK_OVERHEAD == 40); // TODO remove
+        if (!FindUndoPos(state, block.nFile, pos, blockundo_size + UNDO_DATA_DISK_OVERHEAD)) {
             LogError("%s: FindUndoPos failed\n", __func__);
             return false;
         }
@@ -957,12 +959,11 @@ bool BlockManager::WriteUndoDataForBlock(const CBlockUndo& blockundo, BlockValid
         }
 
         // Write index header
-        unsigned int nSize = GetSerializeSize(blockundo);
-        fileout << GetParams().MessageStart() << nSize;
-
+        assert(blockundo_size == GetSerializeSize(blockundo)); // TODO remove
+        fileout << GetParams().MessageStart() << blockundo_size;
         // Write undo data
-        long fileOutPos = fileout.tell();
-        pos.nPos = (unsigned int)fileOutPos;
+        pos.nPos += BLOCK_SERIALIZATION_HEADER_SIZE;
+        assert(pos.nPos == fileout.tell()); // TODO remove
         fileout << blockundo;
 
         // Calculate & write checksum
@@ -1092,33 +1093,26 @@ bool BlockManager::ReadRawBlockFromDisk(std::vector<uint8_t>& block, const FlatF
 
 FlatFilePos BlockManager::SaveBlockToDisk(const CBlock& block, int nHeight)
 {
-    unsigned int nBlockSize = ::GetSerializeSize(TX_WITH_WITNESS(block));
-    // Account for the 4 magic message start bytes + the 4 length bytes (8 bytes total,
-    // defined as BLOCK_SERIALIZATION_HEADER_SIZE)
-    nBlockSize += static_cast<unsigned int>(BLOCK_SERIALIZATION_HEADER_SIZE);
-    FlatFilePos pos{FindNextBlockPos(nBlockSize, nHeight, block.GetBlockTime())};
+    const unsigned int block_size{static_cast<unsigned int>(GetSerializeSize(TX_WITH_WITNESS(block)))};
+    FlatFilePos pos{FindNextBlockPos(block_size + BLOCK_SERIALIZATION_HEADER_SIZE, nHeight, block.GetBlockTime())};
     if (pos.IsNull()) {
         LogError("%s: FindNextBlockPos failed\n", __func__);
         return FlatFilePos();
     }
-
-    // Open history file to append
     AutoFile fileout{OpenBlockFile(pos)};
     if (fileout.IsNull()) {
         LogError("%s: OpenBlockFile failed\n", __func__);
         m_opts.notifications.fatalError(_("Failed to write block."));
         return FlatFilePos();
     }
 
+    assert(block_size == GetSerializeSize(TX_WITH_WITNESS(block))); // TODO remove
     // Write index header
-    unsigned int nSize = GetSerializeSize(TX_WITH_WITNESS(block));
-    fileout << GetParams().MessageStart() << nSize;
-
+    fileout << GetParams().MessageStart() << block_size;
     // Write block
-    long fileOutPos = fileout.tell();
-    pos.nPos = (unsigned int)fileOutPos;
+    pos.nPos += BLOCK_SERIALIZATION_HEADER_SIZE;
+    assert(pos.nPos == fileout.tell()); // TODO remove
     fileout << TX_WITH_WITNESS(block);
-
     return pos;
 }
 

src/node/blockstorage.h

@@ -74,8 +74,11 @@ static const unsigned int UNDOFILE_CHUNK_SIZE = 0x100000; // 1 MiB
 /** The maximum size of a blk?????.dat file (since 0.8) */
 static const unsigned int MAX_BLOCKFILE_SIZE = 0x8000000; // 128 MiB
 
-/** Size of header written by SaveBlockToDisk before a serialized CBlock */
-static constexpr size_t BLOCK_SERIALIZATION_HEADER_SIZE = std::tuple_size_v<MessageStartChars> + sizeof(unsigned int);
+/** Size of header written by SaveBlockToDisk before a serialized CBlock (8 bytes) */
+static constexpr size_t BLOCK_SERIALIZATION_HEADER_SIZE{std::tuple_size_v<MessageStartChars> + sizeof(unsigned int)};
+
+/** Total overhead when writing undo data: header (8 bytes) plus checksum (32 bytes) */
+static constexpr size_t UNDO_DATA_DISK_OVERHEAD{BLOCK_SERIALIZATION_HEADER_SIZE + uint256::size()};
 
 // Because validation code takes pointers to the map's CBlockIndex objects, if
 // we ever switch to another associative container, we need to either use a