validation, blockstorage: Separate code paths for reindex and saving new blocks

By calling SaveBlockToDisk only when we actually want to save a new
block to disk. In the reindex case, we now call UpdateBlockInfo
directly from validation.

This commit doesn't change behavior.

src/bench/readblock.cpp

@@ -18,7 +18,7 @@ static FlatFilePos WriteBlockToDisk(ChainstateManager& chainman)
     CBlock block;
     stream >> TX_WITH_WITNESS(block);
 
-    return chainman.m_blockman.SaveBlockToDisk(block, 0, nullptr);
+    return chainman.m_blockman.SaveBlockToDisk(block, 0);
 }
 
 static void ReadBlockFromDiskTest(benchmark::Bench& bench)

src/node/blockstorage.cpp

@@ -1147,28 +1147,20 @@ bool BlockManager::ReadRawBlockFromDisk(std::vector<uint8_t>& block, const FlatF
     return true;
 }
 
-FlatFilePos BlockManager::SaveBlockToDisk(const CBlock& block, int nHeight, const FlatFilePos* dbp)
+FlatFilePos BlockManager::SaveBlockToDisk(const CBlock& block, int nHeight)
 {
     unsigned int nBlockSize = ::GetSerializeSize(TX_WITH_WITNESS(block));
     FlatFilePos blockPos;
-    const auto position_known {dbp != nullptr};
-    if (position_known) {
-        blockPos = *dbp;
-        // position_known is set iff performing a reindex. In this case, no blocks need to be written, only the blockfile info database needs to be rebuilt.
-        UpdateBlockInfo(block, nHeight, *dbp);
-    } else {
-        // when known, blockPos.nPos points at the offset of the block data in the blk file. that already accounts for
-        // the serialization header present in the file (the 4 magic message start bytes + the 4 length bytes = 8 bytes = BLOCK_SERIALIZATION_HEADER_SIZE).
-        // we add BLOCK_SERIALIZATION_HEADER_SIZE only for new blocks since they will have the serialization header added when written to disk.
-        nBlockSize += static_cast<unsigned int>(BLOCK_SERIALIZATION_HEADER_SIZE);
-        if (!FindBlockPos(blockPos, nBlockSize, nHeight, block.GetBlockTime())) {
-            LogError("%s: FindBlockPos failed\n", __func__);
-            return FlatFilePos();
-        }
-        if (!WriteBlockToDisk(block, blockPos)) {
-            m_opts.notifications.fatalError(_("Failed to write block."));
-            return FlatFilePos();
-        }
+    // 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);
+    if (!FindBlockPos(blockPos, nBlockSize, nHeight, block.GetBlockTime())) {
+        LogError("%s: FindBlockPos failed\n", __func__);
+        return FlatFilePos();
+    }
+    if (!WriteBlockToDisk(block, blockPos)) {
+        m_opts.notifications.fatalError(_("Failed to write block."));
+        return FlatFilePos();
     }
     return blockPos;
 }

src/node/blockstorage.h

@@ -328,8 +328,6 @@ public:
         EXCLUSIVE_LOCKS_REQUIRED(::cs_main);
 
     /** Store block on disk and update block file statistics.
-     *  If dbp is non-null, it means the block data is already stored, and dbp contains the file position.
-     *  In this case, the block data will not be written, only the block file statistics will be updated. This case should only happen during reindexing.
      *
      * @param[in]  block        the block to be stored
      * @param[in]  nHeight      the height of the block
@@ -337,7 +335,7 @@ public:
      * @returns in case of success, the position to which the block was written to
      *          in case of an error, an empty FlatFilePos
      */
-    FlatFilePos SaveBlockToDisk(const CBlock& block, int nHeight, const FlatFilePos* dbp);
+    FlatFilePos SaveBlockToDisk(const CBlock& block, int nHeight);
 
     /** Update blockfile info while processing a block during reindex. The block must be available on disk.
      *

src/test/blockmanager_tests.cpp

@@ -35,20 +35,20 @@ BOOST_AUTO_TEST_CASE(blockmanager_find_block_pos)
     };
     BlockManager blockman{*Assert(m_node.shutdown), blockman_opts};
     // simulate adding a genesis block normally
-    BOOST_CHECK_EQUAL(blockman.SaveBlockToDisk(params->GenesisBlock(), 0, nullptr).nPos, BLOCK_SERIALIZATION_HEADER_SIZE);
+    BOOST_CHECK_EQUAL(blockman.SaveBlockToDisk(params->GenesisBlock(), 0).nPos, BLOCK_SERIALIZATION_HEADER_SIZE);
     // simulate what happens during reindex
     // simulate a well-formed genesis block being found at offset 8 in the blk00000.dat file
     // the block is found at offset 8 because there is an 8 byte serialization header
     // consisting of 4 magic bytes + 4 length bytes before each block in a well-formed blk file.
-    FlatFilePos pos{0, BLOCK_SERIALIZATION_HEADER_SIZE};
-    BOOST_CHECK_EQUAL(blockman.SaveBlockToDisk(params->GenesisBlock(), 0, &pos).nPos, BLOCK_SERIALIZATION_HEADER_SIZE);
+    const FlatFilePos pos{0, BLOCK_SERIALIZATION_HEADER_SIZE};
+    blockman.UpdateBlockInfo(params->GenesisBlock(), 0, pos);
     // now simulate what happens after reindex for the first new block processed
     // the actual block contents don't matter, just that it's a block.
     // verify that the write position is at offset 0x12d.
     // this is a check to make sure that https://github.com/bitcoin/bitcoin/issues/21379 does not recur
     // 8 bytes (for serialization header) + 285 (for serialized genesis block) = 293
     // add another 8 bytes for the second block's serialization header and we get 293 + 8 = 301
-    FlatFilePos actual{blockman.SaveBlockToDisk(params->GenesisBlock(), 1, nullptr)};
+    FlatFilePos actual{blockman.SaveBlockToDisk(params->GenesisBlock(), 1)};
     BOOST_CHECK_EQUAL(actual.nPos, BLOCK_SERIALIZATION_HEADER_SIZE + ::GetSerializeSize(TX_WITH_WITNESS(params->GenesisBlock())) + BLOCK_SERIALIZATION_HEADER_SIZE);
 }
 
@@ -156,12 +156,11 @@ BOOST_AUTO_TEST_CASE(blockmanager_flush_block_file)
     // Blockstore is empty
     BOOST_CHECK_EQUAL(blockman.CalculateCurrentUsage(), 0);
 
-    // Write the first block; dbp=nullptr means this block doesn't already have a disk
-    // location, so allocate a free location and write it there.
-    FlatFilePos pos1{blockman.SaveBlockToDisk(block1, /*nHeight=*/1, /*dbp=*/nullptr)};
+    // Write the first block to a new location.
+    FlatFilePos pos1{blockman.SaveBlockToDisk(block1, /*nHeight=*/1)};
 
     // Write second block
-    FlatFilePos pos2{blockman.SaveBlockToDisk(block2, /*nHeight=*/2, /*dbp=*/nullptr)};
+    FlatFilePos pos2{blockman.SaveBlockToDisk(block2, /*nHeight=*/2)};
 
     // Two blocks in the file
     BOOST_CHECK_EQUAL(blockman.CalculateCurrentUsage(), (TEST_BLOCK_SIZE + BLOCK_SERIALIZATION_HEADER_SIZE) * 2);
@@ -181,22 +180,19 @@ BOOST_AUTO_TEST_CASE(blockmanager_flush_block_file)
         BOOST_CHECK_EQUAL(read_block.nVersion, 2);
     }
 
-    // When FlatFilePos* dbp is given, SaveBlockToDisk() will not write or
-    // overwrite anything to the flat file block storage. It will, however,
-    // update the blockfile metadata. This is to facilitate reindexing
-    // when the user has the blocks on disk but the metadata is being rebuilt.
+    // During reindex, the flat file block storage will not be written to.
+    // UpdateBlockInfo will, however, update the blockfile metadata.
     // Verify this behavior by attempting (and failing) to write block 3 data
     // to block 2 location.
     CBlockFileInfo* block_data = blockman.GetBlockFileInfo(0);
     BOOST_CHECK_EQUAL(block_data->nBlocks, 2);
-    BOOST_CHECK(blockman.SaveBlockToDisk(block3, /*nHeight=*/3, /*dbp=*/&pos2) == pos2);
+    blockman.UpdateBlockInfo(block3, /*nHeight=*/3, /*pos=*/pos2);
     // Metadata is updated...
     BOOST_CHECK_EQUAL(block_data->nBlocks, 3);
     // ...but there are still only two blocks in the file
     BOOST_CHECK_EQUAL(blockman.CalculateCurrentUsage(), (TEST_BLOCK_SIZE + BLOCK_SERIALIZATION_HEADER_SIZE) * 2);
 
     // Block 2 was not overwritten:
-    //   SaveBlockToDisk() did not call WriteBlockToDisk() because `FlatFilePos* dbp` was non-null
     blockman.ReadBlockFromDisk(read_block, pos2);
     BOOST_CHECK_EQUAL(read_block.nVersion, 2);
 }

src/validation.cpp

@@ -4342,10 +4342,16 @@ bool ChainstateManager::AcceptBlock(const std::shared_ptr<const CBlock>& pblock,
     // Write block to history file
     if (fNewBlock) *fNewBlock = true;
     try {
-        FlatFilePos blockPos{m_blockman.SaveBlockToDisk(block, pindex->nHeight, dbp)};
-        if (blockPos.IsNull()) {
-            state.Error(strprintf("%s: Failed to find position to write new block to disk", __func__));
-            return false;
+        FlatFilePos blockPos{};
+        if (dbp) {
+            blockPos = *dbp;
+            m_blockman.UpdateBlockInfo(block, pindex->nHeight, blockPos);
+        } else {
+            blockPos = m_blockman.SaveBlockToDisk(block, pindex->nHeight);
+            if (blockPos.IsNull()) {
+                state.Error(strprintf("%s: Failed to find position to write new block to disk", __func__));
+                return false;
+            }
         }
         ReceivedBlockTransactions(block, pindex, blockPos);
     } catch (const std::runtime_error& e) {
@@ -4845,7 +4851,7 @@ bool Chainstate::LoadGenesisBlock()
 
     try {
         const CBlock& block = params.GenesisBlock();
-        FlatFilePos blockPos{m_blockman.SaveBlockToDisk(block, 0, nullptr)};
+        FlatFilePos blockPos{m_blockman.SaveBlockToDisk(block, 0)};
         if (blockPos.IsNull()) {
             LogError("%s: writing genesis block to disk failed\n", __func__);
             return false;