1 files changed, 27 insertions, 9 deletions

diff --git a/src/validation.cpp b/src/validation.cpp
index 866c26a31f..81b22a7e1f 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -1769,19 +1769,24 @@ DisconnectResult CChainState::DisconnectBlock(const CBlock& block, const CBlockI
     return fClean ? DISCONNECT_OK : DISCONNECT_UNCLEAN;
 }
 
-void static FlushBlockFile(bool fFinalize = false)
+static void FlushUndoFile(int block_file, bool finalize = false)
 {
-    LOCK(cs_LastBlockFile);
+    FlatFilePos undo_pos_old(block_file, vinfoBlockFile[block_file].nUndoSize);
+    if (!UndoFileSeq().Flush(undo_pos_old, finalize)) {
+        AbortNode("Flushing undo file to disk failed. This is likely the result of an I/O error.");
+    }
+}
 
+static void FlushBlockFile(bool fFinalize = false, bool finalize_undo = false)
+{
+    LOCK(cs_LastBlockFile);
     FlatFilePos block_pos_old(nLastBlockFile, vinfoBlockFile[nLastBlockFile].nSize);
-    FlatFilePos undo_pos_old(nLastBlockFile, vinfoBlockFile[nLastBlockFile].nUndoSize);
-
-    bool status = true;
-    status &= BlockFileSeq().Flush(block_pos_old, fFinalize);
-    status &= UndoFileSeq().Flush(undo_pos_old, fFinalize);
-    if (!status) {
+    if (!BlockFileSeq().Flush(block_pos_old, fFinalize)) {
         AbortNode("Flushing block file to disk failed. This is likely the result of an I/O error.");
     }
+    // we do not always flush the undo file, as the chain tip may be lagging behind the incoming blocks,
+    // e.g. during IBD or a sync after a node going offline
+    if (!fFinalize || finalize_undo) FlushUndoFile(nLastBlockFile, finalize_undo);
 }
 
 static bool FindUndoPos(BlockValidationState &state, int nFile, FlatFilePos &pos, unsigned int nAddSize);
@@ -1795,6 +1800,14 @@ static bool WriteUndoDataForBlock(const CBlockUndo& blockundo, BlockValidationSt
             return error("ConnectBlock(): FindUndoPos failed");
         if (!UndoWriteToDisk(blockundo, _pos, pindex->pprev->GetBlockHash(), chainparams.MessageStart()))
             return AbortNode(state, "Failed to write undo data");
+        // rev files are written in block height order, whereas blk files are written as blocks come in (often out of order)
+        // we want to flush the rev (undo) file once we've written the last block, which is indicated by the last height
+        // in the block file info as below; note that this does not catch the case where the undo writes are keeping up
+        // with the block writes (usually when a synced up node is getting newly mined blocks) -- this case is caught in
+        // the FindBlockPos function
+        if (_pos.nFile < nLastBlockFile && static_cast<uint32_t>(pindex->nHeight) == vinfoBlockFile[_pos.nFile].nHeightLast) {
+            FlushUndoFile(_pos.nFile, true);
+        }
 
         // update nUndoPos in block index
         pindex->nUndoPos = _pos.nPos;
@@ -3244,8 +3257,13 @@ static bool FindBlockPos(FlatFilePos &pos, unsigned int nAddSize, unsigned int n
         vinfoBlockFile.resize(nFile + 1);
     }
 
+    bool finalize_undo = false;
     if (!fKnown) {
         while (vinfoBlockFile[nFile].nSize + nAddSize >= MAX_BLOCKFILE_SIZE) {
+            // when the undo file is keeping up with the block file, we want to flush it explicitly
+            // when it is lagging behind (more blocks arrive than are being connected), we let the
+            // undo block write case handle it
+            finalize_undo = (vinfoBlockFile[nFile].nHeightLast == (unsigned int)ChainActive().Tip()->nHeight);
             nFile++;
             if (vinfoBlockFile.size() <= nFile) {
                 vinfoBlockFile.resize(nFile + 1);
@@ -3259,7 +3277,7 @@ static bool FindBlockPos(FlatFilePos &pos, unsigned int nAddSize, unsigned int n
         if (!fKnown) {
             LogPrintf("Leaving block file %i: %s\n", nLastBlockFile, vinfoBlockFile[nLastBlockFile].ToString());
         }
-        FlushBlockFile(!fKnown);
+        FlushBlockFile(!fKnown, finalize_undo);
         nLastBlockFile = nFile;
     }