Merge #17994: validation: flush undo files after last block write

ac94141af0 validation: delay flushing undo files in syncing node case (Karl-Johan Alm)

Pull request description:

  Fixes #17890. Replaces #17892.

  Data files (`{blk|rev}<number>.dat`) pre-allocate space as they are written, and then trims down to the final size once they move on to the next sequence ("finalized flush"). The code currently assumes (incorrectly) that blk and rev files finish at the same time, but because blk files are written as blocks come in, and rev files are written in block height order, rev files end up being written to for awhile after moving on to the next block file, resulting in pre-allocation and waste of up to 1 MB of space per rev file.

  The exact point at which rev file writing finishes is the highest height block found inside the corresponding block file, which is already available in the CBlockFileInfo vector. This PR moves finalized flushing of undo files to to directly after the undo data for the previous block file has been written.

  There is a branch with annotation that demonstrates how this is handling flushing here: https://github.com/kallewoof/bitcoin/tree/200124-rev-files-annotated

ACKs for top commit:
  vasild:
    ACK ac94141af (no changes in the code since ed34e00da).
  fjahr:
    Code review re-ACK ac94141af0
  jonatack:
    Code review ACK ac94141af0

Tree-SHA512: 1d4e3b3d1d99bd7ebe7a2f632b1231146dd4f9f993c54db3a4090d9c086d95d2e4c327fd936066392b3afc6277b8f3a908d5c5993d4c8e49f72b92a417716dd2

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;
     }