Faster -reindex by initially deserializing only headers

When a block is initially read from a blk*.dat file during reindexing,
it can be added to the block index only if all of its ancestor blocks
have been added, which is rare. If the block's ancestors have not been
added, the block must be re-read from disk later when it can be added.

This commit: During the initial block read, deserialize only its header,
rather than the entire block, since this is sufficient to determine
if its parent (and thus all its ancestors) has been added. This is a
performance improvement.

1 files changed, 28 insertions, 18 deletions

diff --git a/src/validation.cpp b/src/validation.cpp
index 37e68cfe4a..b9105b9090 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -4393,6 +4393,8 @@ void Chainstate::LoadExternalBlockFile(
     try {
         // This takes over fileIn and calls fclose() on it in the CBufferedFile destructor
         CBufferedFile blkdat(fileIn, 2*MAX_BLOCK_SERIALIZED_SIZE, MAX_BLOCK_SERIALIZED_SIZE+8, SER_DISK, CLIENT_VERSION);
+        // nRewind indicates where to resume scanning in case something goes wrong,
+        // such as a block fails to deserialize.
         uint64_t nRewind = blkdat.GetPos();
         while (!blkdat.eof()) {
             if (ShutdownRequested()) return;
@@ -4416,28 +4418,30 @@ void Chainstate::LoadExternalBlockFile(
                     continue;
             } catch (const std::exception&) {
                 // no valid block header found; don't complain
+                // (this happens at the end of every blk.dat file)
                 break;
             }
             try {
-                // read block
-                uint64_t nBlockPos = blkdat.GetPos();
+                // read block header
+                const uint64_t nBlockPos{blkdat.GetPos()};
                 if (dbp)
                     dbp->nPos = nBlockPos;
                 blkdat.SetLimit(nBlockPos + nSize);
-                std::shared_ptr<CBlock> pblock = std::make_shared<CBlock>();
-                CBlock& block = *pblock;
-                blkdat >> block;
-                nRewind = blkdat.GetPos();
-
-                uint256 hash = block.GetHash();
+                CBlockHeader header;
+                blkdat >> header;
+                const uint256 hash{header.GetHash()};
+                // Skip the rest of this block (this may read from disk into memory); position to the marker before the
+                // next block, but it's still possible to rewind to the start of the current block (without a disk read).
+                nRewind = nBlockPos + nSize;
+                blkdat.SkipTo(nRewind);
                 {
                     LOCK(cs_main);
                     // detect out of order blocks, and store them for later
-                    if (hash != params.GetConsensus().hashGenesisBlock && !m_blockman.LookupBlockIndex(block.hashPrevBlock)) {
+                    if (hash != params.GetConsensus().hashGenesisBlock && !m_blockman.LookupBlockIndex(header.hashPrevBlock)) {
                         LogPrint(BCLog::REINDEX, "%s: Out of order block %s, parent %s not known\n", __func__, hash.ToString(),
-                                block.hashPrevBlock.ToString());
+                                 header.hashPrevBlock.ToString());
                         if (dbp && blocks_with_unknown_parent) {
-                            blocks_with_unknown_parent->emplace(block.hashPrevBlock, *dbp);
+                            blocks_with_unknown_parent->emplace(header.hashPrevBlock, *dbp);
                         }
                         continue;
                     }
@@ -4445,13 +4449,19 @@ void Chainstate::LoadExternalBlockFile(
                     // process in case the block isn't known yet
                     const CBlockIndex* pindex = m_blockman.LookupBlockIndex(hash);
                     if (!pindex || (pindex->nStatus & BLOCK_HAVE_DATA) == 0) {
-                      BlockValidationState state;
-                      if (AcceptBlock(pblock, state, nullptr, true, dbp, nullptr, true)) {
-                          nLoaded++;
-                      }
-                      if (state.IsError()) {
-                          break;
-                      }
+                        // This block can be processed immediately; rewind to its start, read and deserialize it.
+                        blkdat.SetPos(nBlockPos);
+                        std::shared_ptr<CBlock> pblock{std::make_shared<CBlock>()};
+                        blkdat >> *pblock;
+                        nRewind = blkdat.GetPos();
+
+                        BlockValidationState state;
+                        if (AcceptBlock(pblock, state, nullptr, true, dbp, nullptr, true)) {
+                            nLoaded++;
+                        }
+                        if (state.IsError()) {
+                            break;
+                        }
                     } else if (hash != params.GetConsensus().hashGenesisBlock && pindex->nHeight % 1000 == 0) {
                         LogPrint(BCLog::REINDEX, "Block Import: already had block %s at height %d\n", hash.ToString(), pindex->nHeight);
                     }