1 files changed, 19 insertions, 4 deletions
diff --git a/src/node/blockstorage.cpp b/src/node/blockstorage.cpp
index 57f81e6bb6..04d46f4361 100644
--- a/src/node/blockstorage.cpp
+++ b/src/node/blockstorage.cpp
@@ -524,6 +524,16 @@ void BlockManager::FlushUndoFile(int block_file, bool finalize)
 void BlockManager::FlushBlockFile(bool fFinalize, bool finalize_undo)
 {
     LOCK(cs_LastBlockFile);
+
+    if (m_blockfile_info.size() < 1) {
+        // Return if we haven't loaded any blockfiles yet. This happens during
+        // chainstate init, when we call ChainstateManager::MaybeRebalanceCaches() (which
+        // then calls FlushStateToDisk()), resulting in a call to this function before we
+        // have populated `m_blockfile_info` via LoadBlockIndexDB().
+        return;
+    }
+    assert(static_cast<int>(m_blockfile_info.size()) > m_last_blockfile);
+
     FlatFilePos block_pos_old(m_last_blockfile, m_blockfile_info[m_last_blockfile].nSize);
     if (!BlockFileSeq().Flush(block_pos_old, fFinalize)) {
         AbortNode("Flushing block file to disk failed. This is likely the result of an I/O error.");
@@ -789,19 +799,24 @@ bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const FlatFilePos& pos, c
     return true;
 }
 
-/** Store block on disk. If dbp is non-nullptr, the file is known to already reside on disk */
 FlatFilePos BlockManager::SaveBlockToDisk(const CBlock& block, int nHeight, CChain& active_chain, const CChainParams& chainparams, const FlatFilePos* dbp)
 {
     unsigned int nBlockSize = ::GetSerializeSize(block, CLIENT_VERSION);
     FlatFilePos blockPos;
-    if (dbp != nullptr) {
+    const auto position_known {dbp != nullptr};
+    if (position_known) {
         blockPos = *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 + 8, nHeight, active_chain, block.GetBlockTime(), dbp != nullptr)) {
+    if (!FindBlockPos(blockPos, nBlockSize, nHeight, active_chain, block.GetBlockTime(), position_known)) {
         error("%s: FindBlockPos failed", __func__);
         return FlatFilePos();
     }
-    if (dbp == nullptr) {
+    if (!position_known) {
         if (!WriteBlockToDisk(block, blockPos, chainparams.MessageStart())) {
             AbortNode("Failed to write block");
             return FlatFilePos();