Merge #15118: Refactor block file logic

04cca330944f859b4ed68cb8da8a79f5206fd630 Style cleanup. (Jim Posen)
4c01e4e159db82ce4b2acce75f709cac996367d7 flatfile: Unit tests for FlatFileSeq methods. (Jim Posen)
65a489e93d181d3c0f7a9cf79f7c11ff8cf2b0f0 scripted-diff: Rename CBlockDiskPos to FlatFilePos. (Jim Posen)
d6d8a78f26f52fdfe43293686135e2fc6919926c Move CDiskBlockPos from chain to flatfile. (Jim Posen)
e0380933e3745214331d358bda8c5e79299c84d2 validation: Refactor file flush logic into FlatFileSeq. (Jim Posen)
992404b31ed2f8cabeed59d074552f0ae10fda94 validation: Refactor block file pre-allocation into FlatFileSeq. (Jim Posen)
e2d2abb99fe353ffc2ff3bc1ff578fad31065335 validation: Refactor OpenDiskFile into method on FlatFileSeq. (Jim Posen)
9183d6ef656c8f3ed406821b99827f9b5f047665 validation: Extract basic block file logic into FlatFileSeq class. (Jim Posen)
62e7addb632cad77cbd5fbccbaee51c7b32505d0 util: Move CheckDiskSpace to util. (Jim Posen)

Pull request description:

  This cleans up and refactors block file helpers so that they may be used by the block filter indexer. Per [design discussion](https://github.com/bitcoin/bitcoin/pull/14121#issuecomment-451252591) about storing BIP 157 block filters, it has been suggested that they are stored in the same way as block and undo data. This refactor is sufficient to simplify file operations for this use case, though in the future perhaps more pruning-related logic ought to be moved into the new classes.

  The basic abstraction is a `FlatFileSeq` which manages access to a sequence of numbered files into which raw data is written.

Tree-SHA512: b2108756777f2dad8964a1a2ef2764486e708a4a4a8cfac47b5de8bcb0625388964438eb096b10cfd9ea39212c299b5cb32fa943e768db2333cf49ea7def157e

12 files changed, 417 insertions, 181 deletions

diff --git a/src/Makefile.am b/src/Makefile.am
index d491530ca1..2e297d3dac 100644
--- a/src/Makefile.am
+++ b/src/Makefile.am
@@ -128,6 +128,7 @@ BITCOIN_CORE_H = \
   core_io.h \
   core_memusage.h \
   cuckoocache.h \
+  flatfile.h \
   fs.h \
   httprpc.h \
   httpserver.h \
@@ -247,6 +248,7 @@ libbitcoin_server_a_SOURCES = \
   chain.cpp \
   checkpoints.cpp \
   consensus/tx_verify.cpp \
+  flatfile.cpp \
   httprpc.cpp \
   httpserver.cpp \
   index/base.cpp \
diff --git a/src/Makefile.test.include b/src/Makefile.test.include
index 84bc326cfe..09fef5a1dd 100644
--- a/src/Makefile.test.include
+++ b/src/Makefile.test.include
@@ -83,6 +83,7 @@ BITCOIN_TESTS =\
   test/cuckoocache_tests.cpp \
   test/denialofservice_tests.cpp \
   test/descriptor_tests.cpp \
+  test/flatfile_tests.cpp \
   test/fs_tests.cpp \
   test/getarg_tests.cpp \
   test/hash_tests.cpp \
diff --git a/src/chain.h b/src/chain.h
index 5a6f10b84f..2b6d2d082c 100644
--- a/src/chain.h
+++ b/src/chain.h
@@ -8,6 +8,7 @@
 
 #include <arith_uint256.h>
 #include <consensus/params.h>
+#include <flatfile.h>
 #include <primitives/block.h>
 #include <tinyformat.h>
 #include <uint256.h>
@@ -90,46 +91,6 @@ public:
      }
 };
 
-struct CDiskBlockPos
-{
-    int nFile;
-    unsigned int nPos;
-
-    ADD_SERIALIZE_METHODS;
-
-    template <typename Stream, typename Operation>
-    inline void SerializationOp(Stream& s, Operation ser_action) {
-        READWRITE(VARINT(nFile, VarIntMode::NONNEGATIVE_SIGNED));
-        READWRITE(VARINT(nPos));
-    }
-
-    CDiskBlockPos() {
-        SetNull();
-    }
-
-    CDiskBlockPos(int nFileIn, unsigned int nPosIn) {
-        nFile = nFileIn;
-        nPos = nPosIn;
-    }
-
-    friend bool operator==(const CDiskBlockPos &a, const CDiskBlockPos &b) {
-        return (a.nFile == b.nFile && a.nPos == b.nPos);
-    }
-
-    friend bool operator!=(const CDiskBlockPos &a, const CDiskBlockPos &b) {
-        return !(a == b);
-    }
-
-    void SetNull() { nFile = -1; nPos = 0; }
-    bool IsNull() const { return (nFile == -1); }
-
-    std::string ToString() const
-    {
-        return strprintf("CDiskBlockPos(nFile=%i, nPos=%i)", nFile, nPos);
-    }
-
-};
-
 enum BlockStatus: uint32_t {
     //! Unused.
     BLOCK_VALID_UNKNOWN      =    0,
@@ -266,8 +227,8 @@ public:
         nNonce         = block.nNonce;
     }
 
-    CDiskBlockPos GetBlockPos() const {
-        CDiskBlockPos ret;
+    FlatFilePos GetBlockPos() const {
+        FlatFilePos ret;
         if (nStatus & BLOCK_HAVE_DATA) {
             ret.nFile = nFile;
             ret.nPos  = nDataPos;
@@ -275,8 +236,8 @@ public:
         return ret;
     }
 
-    CDiskBlockPos GetUndoPos() const {
-        CDiskBlockPos ret;
+    FlatFilePos GetUndoPos() const {
+        FlatFilePos ret;
         if (nStatus & BLOCK_HAVE_UNDO) {
             ret.nFile = nFile;
             ret.nPos  = nUndoPos;
diff --git a/src/flatfile.cpp b/src/flatfile.cpp
new file mode 100644
index 0000000000..8a8f7b681c
--- /dev/null
+++ b/src/flatfile.cpp
@@ -0,0 +1,98 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <stdexcept>
+
+#include <flatfile.h>
+#include <logging.h>
+#include <tinyformat.h>
+#include <util/system.h>
+
+FlatFileSeq::FlatFileSeq(fs::path dir, const char* prefix, size_t chunk_size) :
+    m_dir(std::move(dir)),
+    m_prefix(prefix),
+    m_chunk_size(chunk_size)
+{
+    if (chunk_size == 0) {
+        throw std::invalid_argument("chunk_size must be positive");
+    }
+}
+
+std::string FlatFilePos::ToString() const
+{
+    return strprintf("FlatFilePos(nFile=%i, nPos=%i)", nFile, nPos);
+}
+
+fs::path FlatFileSeq::FileName(const FlatFilePos& pos) const
+{
+    return m_dir / strprintf("%s%05u.dat", m_prefix, pos.nFile);
+}
+
+FILE* FlatFileSeq::Open(const FlatFilePos& pos, bool read_only)
+{
+    if (pos.IsNull()) {
+        return nullptr;
+    }
+    fs::path path = FileName(pos);
+    fs::create_directories(path.parent_path());
+    FILE* file = fsbridge::fopen(path, read_only ? "rb": "rb+");
+    if (!file && !read_only)
+        file = fsbridge::fopen(path, "wb+");
+    if (!file) {
+        LogPrintf("Unable to open file %s\n", path.string());
+        return nullptr;
+    }
+    if (pos.nPos && fseek(file, pos.nPos, SEEK_SET)) {
+        LogPrintf("Unable to seek to position %u of %s\n", pos.nPos, path.string());
+        fclose(file);
+        return nullptr;
+    }
+    return file;
+}
+
+size_t FlatFileSeq::Allocate(const FlatFilePos& pos, size_t add_size, bool& out_of_space)
+{
+    out_of_space = false;
+
+    unsigned int n_old_chunks = (pos.nPos + m_chunk_size - 1) / m_chunk_size;
+    unsigned int n_new_chunks = (pos.nPos + add_size + m_chunk_size - 1) / m_chunk_size;
+    if (n_new_chunks > n_old_chunks) {
+        size_t old_size = pos.nPos;
+        size_t new_size = n_new_chunks * m_chunk_size;
+        size_t inc_size = new_size - old_size;
+
+        if (CheckDiskSpace(m_dir, inc_size)) {
+            FILE *file = Open(pos);
+            if (file) {
+                LogPrintf("Pre-allocating up to position 0x%x in %s%05u.dat\n", new_size, m_prefix, pos.nFile);
+                AllocateFileRange(file, pos.nPos, inc_size);
+                fclose(file);
+                return inc_size;
+            }
+        } else {
+            out_of_space = true;
+        }
+    }
+    return 0;
+}
+
+bool FlatFileSeq::Flush(const FlatFilePos& pos, bool finalize)
+{
+    FILE* file = Open(FlatFilePos(pos.nFile, 0)); // Avoid fseek to nPos
+    if (!file) {
+        return error("%s: failed to open file %d", __func__, pos.nFile);
+    }
+    if (finalize && !TruncateFile(file, pos.nPos)) {
+        fclose(file);
+        return error("%s: failed to truncate file %d", __func__, pos.nFile);
+    }
+    if (!FileCommit(file)) {
+        fclose(file);
+        return error("%s: failed to commit file %d", __func__, pos.nFile);
+    }
+
+    fclose(file);
+    return true;
+}
diff --git a/src/flatfile.h b/src/flatfile.h
new file mode 100644
index 0000000000..374ceff411
--- /dev/null
+++ b/src/flatfile.h
@@ -0,0 +1,96 @@
+// Copyright (c) 2009-2010 Satoshi Nakamoto
+// Copyright (c) 2009-2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#ifndef BITCOIN_FLATFILE_H
+#define BITCOIN_FLATFILE_H
+
+#include <string>
+
+#include <fs.h>
+#include <serialize.h>
+
+struct FlatFilePos
+{
+    int nFile;
+    unsigned int nPos;
+
+    ADD_SERIALIZE_METHODS;
+
+    template <typename Stream, typename Operation>
+    inline void SerializationOp(Stream& s, Operation ser_action) {
+        READWRITE(VARINT(nFile, VarIntMode::NONNEGATIVE_SIGNED));
+        READWRITE(VARINT(nPos));
+    }
+
+    FlatFilePos() : nFile(-1), nPos(0) {}
+
+    FlatFilePos(int nFileIn, unsigned int nPosIn) :
+        nFile(nFileIn),
+        nPos(nPosIn)
+    {}
+
+    friend bool operator==(const FlatFilePos &a, const FlatFilePos &b) {
+        return (a.nFile == b.nFile && a.nPos == b.nPos);
+    }
+
+    friend bool operator!=(const FlatFilePos &a, const FlatFilePos &b) {
+        return !(a == b);
+    }
+
+    void SetNull() { nFile = -1; nPos = 0; }
+    bool IsNull() const { return (nFile == -1); }
+
+    std::string ToString() const;
+};
+
+/**
+ * FlatFileSeq represents a sequence of numbered files storing raw data. This class facilitates
+ * access to and efficient management of these files.
+ */
+class FlatFileSeq
+{
+private:
+    const fs::path m_dir;
+    const char* const m_prefix;
+    const size_t m_chunk_size;
+
+public:
+    /**
+     * Constructor
+     *
+     * @param dir The base directory that all files live in.
+     * @param prefix A short prefix given to all file names.
+     * @param chunk_size Disk space is pre-allocated in multiples of this amount.
+     */
+    FlatFileSeq(fs::path dir, const char* prefix, size_t chunk_size);
+
+    /** Get the name of the file at the given position. */
+    fs::path FileName(const FlatFilePos& pos) const;
+
+    /** Open a handle to the file at the given position. */
+    FILE* Open(const FlatFilePos& pos, bool read_only = false);
+
+    /**
+     * Allocate additional space in a file after the given starting position. The amount allocated
+     * will be the minimum multiple of the sequence chunk size greater than add_size.
+     *
+     * @param[in] pos The starting position that bytes will be allocated after.
+     * @param[in] add_size The minimum number of bytes to be allocated.
+     * @param[out] out_of_space Whether the allocation failed due to insufficient disk space.
+     * @return The number of bytes successfully allocated.
+     */
+    size_t Allocate(const FlatFilePos& pos, size_t add_size, bool& out_of_space);
+
+    /**
+     * Commit a file to disk, and optionally truncate off extra pre-allocated bytes if final.
+     *
+     * @param[in] pos The first unwritten position in the file to be flushed.
+     * @param[in] finalize True if no more data will be written to this file.
+     * @return true on success, false on failure.
+     */
+    bool Flush(const FlatFilePos& pos, bool finalize = false);
+};
+
+#endif // BITCOIN_FLATFILE_H
diff --git a/src/index/txindex.cpp b/src/index/txindex.cpp
index 10bc8419dd..7367ec7cb6 100644
--- a/src/index/txindex.cpp
+++ b/src/index/txindex.cpp
@@ -16,7 +16,7 @@ constexpr char DB_TXINDEX_BLOCK = 'T';
 
 std::unique_ptr<TxIndex> g_txindex;
 
-struct CDiskTxPos : public CDiskBlockPos
+struct CDiskTxPos : public FlatFilePos
 {
     unsigned int nTxOffset; // after header
 
@@ -24,11 +24,11 @@ struct CDiskTxPos : public CDiskBlockPos
 
     template <typename Stream, typename Operation>
     inline void SerializationOp(Stream& s, Operation ser_action) {
-        READWRITEAS(CDiskBlockPos, *this);
+        READWRITEAS(FlatFilePos, *this);
         READWRITE(VARINT(nTxOffset));
     }
 
-    CDiskTxPos(const CDiskBlockPos &blockIn, unsigned int nTxOffsetIn) : CDiskBlockPos(blockIn.nFile, blockIn.nPos), nTxOffset(nTxOffsetIn) {
+    CDiskTxPos(const FlatFilePos &blockIn, unsigned int nTxOffsetIn) : FlatFilePos(blockIn.nFile, blockIn.nPos), nTxOffset(nTxOffsetIn) {
     }
 
     CDiskTxPos() {
@@ -36,7 +36,7 @@ struct CDiskTxPos : public CDiskBlockPos
     }
 
     void SetNull() {
-        CDiskBlockPos::SetNull();
+        FlatFilePos::SetNull();
         nTxOffset = 0;
     }
 };
diff --git a/src/init.cpp b/src/init.cpp
index e0da962030..ee9a451d0b 100644
--- a/src/init.cpp
+++ b/src/init.cpp
@@ -668,8 +668,8 @@ static void ThreadImport(std::vector<fs::path> vImportFiles)
     if (fReindex) {
         int nFile = 0;
         while (true) {
-            CDiskBlockPos pos(nFile, 0);
-            if (!fs::exists(GetBlockPosFilename(pos, "blk")))
+            FlatFilePos pos(nFile, 0);
+            if (!fs::exists(GetBlockPosFilename(pos)))
                 break; // No block files left to reindex
             FILE *file = OpenBlockFile(pos, true);
             if (!file)
@@ -1669,11 +1669,11 @@ bool AppInitMain(InitInterfaces& interfaces)
 
     // ********************************************************* Step 11: import blocks
 
-    if (!CheckDiskSpace(/* additional_bytes */ 0, /* blocks_dir */ false)) {
+    if (!CheckDiskSpace(GetDataDir())) {
         InitError(strprintf(_("Error: Disk space is low for %s"), GetDataDir()));
         return false;
     }
-    if (!CheckDiskSpace(/* additional_bytes */ 0, /* blocks_dir */ true)) {
+    if (!CheckDiskSpace(GetBlocksDir())) {
         InitError(strprintf(_("Error: Disk space is low for %s"), GetBlocksDir()));
         return false;
     }
diff --git a/src/test/flatfile_tests.cpp b/src/test/flatfile_tests.cpp
new file mode 100644
index 0000000000..079a09f8f9
--- /dev/null
+++ b/src/test/flatfile_tests.cpp
@@ -0,0 +1,123 @@
+// Copyright (c) 2019 The Bitcoin Core developers
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <flatfile.h>
+#include <test/test_bitcoin.h>
+
+#include <boost/test/unit_test.hpp>
+
+BOOST_FIXTURE_TEST_SUITE(flatfile_tests, BasicTestingSetup)
+
+BOOST_AUTO_TEST_CASE(flatfile_filename)
+{
+    auto data_dir = SetDataDir("flatfile_test");
+
+    FlatFilePos pos(456, 789);
+
+    FlatFileSeq seq1(data_dir, "a", 16 * 1024);
+    BOOST_CHECK_EQUAL(seq1.FileName(pos), data_dir / "a00456.dat");
+
+    FlatFileSeq seq2(data_dir / "a", "b", 16 * 1024);
+    BOOST_CHECK_EQUAL(seq2.FileName(pos), data_dir / "a" / "b00456.dat");
+}
+
+BOOST_AUTO_TEST_CASE(flatfile_open)
+{
+    auto data_dir = SetDataDir("flatfile_test");
+    FlatFileSeq seq(data_dir, "a", 16 * 1024);
+
+    std::string line1("A purely peer-to-peer version of electronic cash would allow online "
+                      "payments to be sent directly from one party to another without going "
+                      "through a financial institution.");
+    std::string line2("Digital signatures provide part of the solution, but the main benefits are "
+                      "lost if a trusted third party is still required to prevent double-spending.");
+
+    size_t pos1 = 0;
+    size_t pos2 = pos1 + GetSerializeSize(line1, CLIENT_VERSION);
+
+    // Write first line to file.
+    {
+        CAutoFile file(seq.Open(FlatFilePos(0, pos1)), SER_DISK, CLIENT_VERSION);
+        file << LIMITED_STRING(line1, 256);
+    }
+
+    // Attempt to append to file opened in read-only mode.
+    {
+        CAutoFile file(seq.Open(FlatFilePos(0, pos2), true), SER_DISK, CLIENT_VERSION);
+        BOOST_CHECK_THROW(file << LIMITED_STRING(line2, 256), std::ios_base::failure);
+    }
+
+    // Append second line to file.
+    {
+        CAutoFile file(seq.Open(FlatFilePos(0, pos2)), SER_DISK, CLIENT_VERSION);
+        file << LIMITED_STRING(line2, 256);
+    }
+
+    // Read text from file in read-only mode.
+    {
+        std::string text;
+        CAutoFile file(seq.Open(FlatFilePos(0, pos1), true), SER_DISK, CLIENT_VERSION);
+
+        file >> LIMITED_STRING(text, 256);
+        BOOST_CHECK_EQUAL(text, line1);
+
+        file >> LIMITED_STRING(text, 256);
+        BOOST_CHECK_EQUAL(text, line2);
+    }
+
+    // Read text from file with position offset.
+    {
+        std::string text;
+        CAutoFile file(seq.Open(FlatFilePos(0, pos2)), SER_DISK, CLIENT_VERSION);
+
+        file >> LIMITED_STRING(text, 256);
+        BOOST_CHECK_EQUAL(text, line2);
+    }
+
+    // Ensure another file in the sequence has no data.
+    {
+        std::string text;
+        CAutoFile file(seq.Open(FlatFilePos(1, pos2)), SER_DISK, CLIENT_VERSION);
+        BOOST_CHECK_THROW(file >> LIMITED_STRING(text, 256), std::ios_base::failure);
+    }
+}
+
+BOOST_AUTO_TEST_CASE(flatfile_allocate)
+{
+    auto data_dir = SetDataDir("flatfile_test");
+    FlatFileSeq seq(data_dir, "a", 100);
+
+    bool out_of_space;
+
+    BOOST_CHECK_EQUAL(seq.Allocate(FlatFilePos(0, 0), 1, out_of_space), 100);
+    BOOST_CHECK_EQUAL(fs::file_size(seq.FileName(FlatFilePos(0, 0))), 100);
+    BOOST_CHECK(!out_of_space);
+
+    BOOST_CHECK_EQUAL(seq.Allocate(FlatFilePos(0, 99), 1, out_of_space), 0);
+    BOOST_CHECK_EQUAL(fs::file_size(seq.FileName(FlatFilePos(0, 99))), 100);
+    BOOST_CHECK(!out_of_space);
+
+    BOOST_CHECK_EQUAL(seq.Allocate(FlatFilePos(0, 99), 2, out_of_space), 101);
+    BOOST_CHECK_EQUAL(fs::file_size(seq.FileName(FlatFilePos(0, 99))), 200);
+    BOOST_CHECK(!out_of_space);
+}
+
+BOOST_AUTO_TEST_CASE(flatfile_flush)
+{
+    auto data_dir = SetDataDir("flatfile_test");
+    FlatFileSeq seq(data_dir, "a", 100);
+
+    bool out_of_space;
+    seq.Allocate(FlatFilePos(0, 0), 1, out_of_space);
+
+    // Flush without finalize should not truncate file.
+    seq.Flush(FlatFilePos(0, 1));
+    BOOST_CHECK_EQUAL(fs::file_size(seq.FileName(FlatFilePos(0, 1))), 100);
+
+    // Flush with finalize should truncate file.
+    seq.Flush(FlatFilePos(0, 1), true);
+    BOOST_CHECK_EQUAL(fs::file_size(seq.FileName(FlatFilePos(0, 1))), 1);
+}
+
+BOOST_AUTO_TEST_SUITE_END()
diff --git a/src/util/system.cpp b/src/util/system.cpp
index 5ae0604894..9594dd81bf 100644
--- a/src/util/system.cpp
+++ b/src/util/system.cpp
@@ -135,6 +135,14 @@ bool DirIsWritable(const fs::path& directory)
     return true;
 }
 
+bool CheckDiskSpace(const fs::path& dir, uint64_t additional_bytes)
+{
+    constexpr uint64_t min_disk_space = 52428800; // 50 MiB
+
+    uint64_t free_bytes_available = fs::space(dir).available;
+    return free_bytes_available >= min_disk_space + additional_bytes;
+}
+
 /**
  * Interpret a string argument as a boolean.
  *
diff --git a/src/util/system.h b/src/util/system.h
index 6899e38c9e..54eb88e261 100644
--- a/src/util/system.h
+++ b/src/util/system.h
@@ -72,6 +72,7 @@ bool RenameOver(fs::path src, fs::path dest);
 bool LockDirectory(const fs::path& directory, const std::string lockfile_name, bool probe_only=false);
 void UnlockDirectory(const fs::path& directory, const std::string& lockfile_name);
 bool DirIsWritable(const fs::path& directory);
+bool CheckDiskSpace(const fs::path& dir, uint64_t additional_bytes = 0);
 
 /** Release all directory locks. This is used for unit testing only, at runtime
  * the global destructor will take care of the locks.
diff --git a/src/validation.cpp b/src/validation.cpp
index f3d34dca70..358992b74d 100644
--- a/src/validation.cpp
+++ b/src/validation.cpp
@@ -15,6 +15,7 @@
 #include <consensus/tx_verify.h>
 #include <consensus/validation.h>
 #include <cuckoocache.h>
+#include <flatfile.h>
 #include <hash.h>
 #include <index/txindex.h>
 #include <policy/fees.h>
@@ -165,7 +166,7 @@ public:
      * that it doesn't descend from an invalid block, and then add it to mapBlockIndex.
      */
     bool AcceptBlockHeader(const CBlockHeader& block, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
-    bool AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const CDiskBlockPos* dbp, bool* fNewBlock) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+    bool AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const FlatFilePos* dbp, bool* fNewBlock) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
     // Block (dis)connection on a given view:
     DisconnectResult DisconnectBlock(const CBlock& block, const CBlockIndex* pindex, CCoinsViewCache& view);
@@ -204,7 +205,7 @@ private:
 
     void InvalidBlockFound(CBlockIndex *pindex, const CValidationState &state) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
     CBlockIndex* FindMostWorkChain() EXCLUSIVE_LOCKS_REQUIRED(cs_main);
-    void ReceivedBlockTransactions(const CBlock& block, CBlockIndex* pindexNew, const CDiskBlockPos& pos, const Consensus::Params& consensusParams) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
+    void ReceivedBlockTransactions(const CBlock& block, CBlockIndex* pindexNew, const FlatFilePos& pos, const Consensus::Params& consensusParams) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
 
 
     bool RollforwardBlock(const CBlockIndex* pindex, CCoinsViewCache& inputs, const CChainParams& params) EXCLUSIVE_LOCKS_REQUIRED(cs_main);
@@ -317,7 +318,9 @@ static bool FlushStateToDisk(const CChainParams& chainParams, CValidationState &
 static void FindFilesToPruneManual(std::set<int>& setFilesToPrune, int nManualPruneHeight);
 static void FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPruneAfterHeight);
 bool CheckInputs(const CTransaction& tx, CValidationState &state, const CCoinsViewCache &inputs, bool fScriptChecks, unsigned int flags, bool cacheSigStore, bool cacheFullScriptStore, PrecomputedTransactionData& txdata, std::vector<CScriptCheck> *pvChecks = nullptr);
-static FILE* OpenUndoFile(const CDiskBlockPos &pos, bool fReadOnly = false);
+static FILE* OpenUndoFile(const FlatFilePos &pos, bool fReadOnly = false);
+static FlatFileSeq BlockFileSeq();
+static FlatFileSeq UndoFileSeq();
 
 bool CheckFinalTx(const CTransaction &tx, int flags)
 {
@@ -1042,7 +1045,7 @@ bool GetTransaction(const uint256& hash, CTransactionRef& txOut, const Consensus
 // CBlock and CBlockIndex
 //
 
-static bool WriteBlockToDisk(const CBlock& block, CDiskBlockPos& pos, const CMessageHeader::MessageStartChars& messageStart)
+static bool WriteBlockToDisk(const CBlock& block, FlatFilePos& pos, const CMessageHeader::MessageStartChars& messageStart)
 {
     // Open history file to append
     CAutoFile fileout(OpenBlockFile(pos), SER_DISK, CLIENT_VERSION);
@@ -1063,7 +1066,7 @@ static bool WriteBlockToDisk(const CBlock& block, CDiskBlockPos& pos, const CMes
     return true;
 }
 
-bool ReadBlockFromDisk(CBlock& block, const CDiskBlockPos& pos, const Consensus::Params& consensusParams)
+bool ReadBlockFromDisk(CBlock& block, const FlatFilePos& pos, const Consensus::Params& consensusParams)
 {
     block.SetNull();
 
@@ -1089,7 +1092,7 @@ bool ReadBlockFromDisk(CBlock& block, const CDiskBlockPos& pos, const Consensus:
 
 bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex, const Consensus::Params& consensusParams)
 {
-    CDiskBlockPos blockPos;
+    FlatFilePos blockPos;
     {
         LOCK(cs_main);
         blockPos = pindex->GetBlockPos();
@@ -1103,9 +1106,9 @@ bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex, const Consensus
     return true;
 }
 
-bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const CDiskBlockPos& pos, const CMessageHeader::MessageStartChars& message_start)
+bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const FlatFilePos& pos, const CMessageHeader::MessageStartChars& message_start)
 {
-    CDiskBlockPos hpos = pos;
+    FlatFilePos hpos = pos;
     hpos.nPos -= 8; // Seek back 8 bytes for meta header
     CAutoFile filein(OpenBlockFile(hpos, true), SER_DISK, CLIENT_VERSION);
     if (filein.IsNull()) {
@@ -1140,7 +1143,7 @@ bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const CDiskBlockPos& pos,
 
 bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const CBlockIndex* pindex, const CMessageHeader::MessageStartChars& message_start)
 {
-    CDiskBlockPos block_pos;
+    FlatFilePos block_pos;
     {
         LOCK(cs_main);
         block_pos = pindex->GetBlockPos();
@@ -1452,7 +1455,7 @@ bool CheckInputs(const CTransaction& tx, CValidationState &state, const CCoinsVi
 
 namespace {
 
-bool UndoWriteToDisk(const CBlockUndo& blockundo, CDiskBlockPos& pos, const uint256& hashBlock, const CMessageHeader::MessageStartChars& messageStart)
+bool UndoWriteToDisk(const CBlockUndo& blockundo, FlatFilePos& pos, const uint256& hashBlock, const CMessageHeader::MessageStartChars& messageStart)
 {
     // Open history file to append
     CAutoFile fileout(OpenUndoFile(pos), SER_DISK, CLIENT_VERSION);
@@ -1481,7 +1484,7 @@ bool UndoWriteToDisk(const CBlockUndo& blockundo, CDiskBlockPos& pos, const uint
 
 static bool UndoReadFromDisk(CBlockUndo& blockundo, const CBlockIndex *pindex)
 {
-    CDiskBlockPos pos = pindex->GetUndoPos();
+    FlatFilePos pos = pindex->GetUndoPos();
     if (pos.IsNull()) {
         return error("%s: no undo data available", __func__);
     }
@@ -1627,37 +1630,24 @@ void static FlushBlockFile(bool fFinalize = false)
 {
     LOCK(cs_LastBlockFile);
 
-    CDiskBlockPos posOld(nLastBlockFile, 0);
-    bool status = true;
-
-    FILE *fileOld = OpenBlockFile(posOld);
-    if (fileOld) {
-        if (fFinalize)
-            status &= TruncateFile(fileOld, vinfoBlockFile[nLastBlockFile].nSize);
-        status &= FileCommit(fileOld);
-        fclose(fileOld);
-    }
-
-    fileOld = OpenUndoFile(posOld);
-    if (fileOld) {
-        if (fFinalize)
-            status &= TruncateFile(fileOld, vinfoBlockFile[nLastBlockFile].nUndoSize);
-        status &= FileCommit(fileOld);
-        fclose(fileOld);
-    }
+    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) {
         AbortNode("Flushing block file to disk failed. This is likely the result of an I/O error.");
     }
 }
 
-static bool FindUndoPos(CValidationState &state, int nFile, CDiskBlockPos &pos, unsigned int nAddSize);
+static bool FindUndoPos(CValidationState &state, int nFile, FlatFilePos &pos, unsigned int nAddSize);
 
 static bool WriteUndoDataForBlock(const CBlockUndo& blockundo, CValidationState& state, CBlockIndex* pindex, const CChainParams& chainparams)
 {
     // Write undo information to disk
     if (pindex->GetUndoPos().IsNull()) {
-        CDiskBlockPos _pos;
+        FlatFilePos _pos;
         if (!FindUndoPos(state, pindex->nFile, _pos, ::GetSerializeSize(blockundo, CLIENT_VERSION) + 40))
             return error("ConnectBlock(): FindUndoPos failed");
         if (!UndoWriteToDisk(blockundo, _pos, pindex->pprev->GetBlockHash(), chainparams.MessageStart()))
@@ -2134,8 +2124,9 @@ bool static FlushStateToDisk(const CChainParams& chainparams, CValidationState &
         // Write blocks and block index to disk.
         if (fDoFullFlush || fPeriodicWrite) {
             // Depend on nMinDiskSpace to ensure we can write block index
-            if (!CheckDiskSpace(0, true))
-                return state.Error("out of disk space");
+            if (!CheckDiskSpace(GetBlocksDir())) {
+                return AbortNode(state, "Disk space is low!", _("Error: Disk space is low!"));
+            }
             // First make sure all block and undo data is flushed to disk.
             FlushBlockFile();
             // Then update all block file information (which may refer to block and undo files).
@@ -2168,8 +2159,9 @@ bool static FlushStateToDisk(const CChainParams& chainparams, CValidationState &
             // twice (once in the log, and once in the tables). This is already
             // an overestimation, as most will delete an existing entry or
             // overwrite one. Still, use a conservative safety factor of 2.
-            if (!CheckDiskSpace(48 * 2 * 2 * pcoinsTip->GetCacheSize()))
-                return state.Error("out of disk space");
+            if (!CheckDiskSpace(GetDataDir(), 48 * 2 * 2 * pcoinsTip->GetCacheSize())) {
+                return AbortNode(state, "Disk space is low!", _("Error: Disk space is low!"));
+            }
             // Flush the chainstate (which may refer to block index entries).
             if (!pcoinsTip->Flush())
                 return AbortNode(state, "Failed to write to coin database");
@@ -2922,7 +2914,7 @@ CBlockIndex* CChainState::AddToBlockIndex(const CBlockHeader& block)
 }
 
 /** Mark a block as having its data received and checked (up to BLOCK_VALID_TRANSACTIONS). */
-void CChainState::ReceivedBlockTransactions(const CBlock& block, CBlockIndex* pindexNew, const CDiskBlockPos& pos, const Consensus::Params& consensusParams)
+void CChainState::ReceivedBlockTransactions(const CBlock& block, CBlockIndex* pindexNew, const FlatFilePos& pos, const Consensus::Params& consensusParams)
 {
     pindexNew->nTx = block.vtx.size();
     pindexNew->nChainTx = 0;
@@ -2968,7 +2960,7 @@ void CChainState::ReceivedBlockTransactions(const CBlock& block, CBlockIndex* pi
     }
 }
 
-static bool FindBlockPos(CDiskBlockPos &pos, unsigned int nAddSize, unsigned int nHeight, uint64_t nTime, bool fKnown = false)
+static bool FindBlockPos(FlatFilePos &pos, unsigned int nAddSize, unsigned int nHeight, uint64_t nTime, bool fKnown = false)
 {
     LOCK(cs_LastBlockFile);
 
@@ -3003,21 +2995,13 @@ static bool FindBlockPos(CDiskBlockPos &pos, unsigned int nAddSize, unsigned int
         vinfoBlockFile[nFile].nSize += nAddSize;
 
     if (!fKnown) {
-        unsigned int nOldChunks = (pos.nPos + BLOCKFILE_CHUNK_SIZE - 1) / BLOCKFILE_CHUNK_SIZE;
-        unsigned int nNewChunks = (vinfoBlockFile[nFile].nSize + BLOCKFILE_CHUNK_SIZE - 1) / BLOCKFILE_CHUNK_SIZE;
-        if (nNewChunks > nOldChunks) {
-            if (fPruneMode)
-                fCheckForPruning = true;
-            if (CheckDiskSpace(nNewChunks * BLOCKFILE_CHUNK_SIZE - pos.nPos, true)) {
-                FILE *file = OpenBlockFile(pos);
-                if (file) {
-                    LogPrintf("Pre-allocating up to position 0x%x in blk%05u.dat\n", nNewChunks * BLOCKFILE_CHUNK_SIZE, pos.nFile);
-                    AllocateFileRange(file, pos.nPos, nNewChunks * BLOCKFILE_CHUNK_SIZE - pos.nPos);
-                    fclose(file);
-                }
-            }
-            else
-                return error("out of disk space");
+        bool out_of_space;
+        size_t bytes_allocated = BlockFileSeq().Allocate(pos, nAddSize, out_of_space);
+        if (out_of_space) {
+            return AbortNode("Disk space is low!", _("Error: Disk space is low!"));
+        }
+        if (bytes_allocated != 0 && fPruneMode) {
+            fCheckForPruning = true;
         }
     }
 
@@ -3025,32 +3009,23 @@ static bool FindBlockPos(CDiskBlockPos &pos, unsigned int nAddSize, unsigned int
     return true;
 }
 
-static bool FindUndoPos(CValidationState &state, int nFile, CDiskBlockPos &pos, unsigned int nAddSize)
+static bool FindUndoPos(CValidationState &state, int nFile, FlatFilePos &pos, unsigned int nAddSize)
 {
     pos.nFile = nFile;
 
     LOCK(cs_LastBlockFile);
 
-    unsigned int nNewSize;
     pos.nPos = vinfoBlockFile[nFile].nUndoSize;
-    nNewSize = vinfoBlockFile[nFile].nUndoSize += nAddSize;
+    vinfoBlockFile[nFile].nUndoSize += nAddSize;
     setDirtyFileInfo.insert(nFile);
 
-    unsigned int nOldChunks = (pos.nPos + UNDOFILE_CHUNK_SIZE - 1) / UNDOFILE_CHUNK_SIZE;
-    unsigned int nNewChunks = (nNewSize + UNDOFILE_CHUNK_SIZE - 1) / UNDOFILE_CHUNK_SIZE;
-    if (nNewChunks > nOldChunks) {
-        if (fPruneMode)
-            fCheckForPruning = true;
-        if (CheckDiskSpace(nNewChunks * UNDOFILE_CHUNK_SIZE - pos.nPos, true)) {
-            FILE *file = OpenUndoFile(pos);
-            if (file) {
-                LogPrintf("Pre-allocating up to position 0x%x in rev%05u.dat\n", nNewChunks * UNDOFILE_CHUNK_SIZE, pos.nFile);
-                AllocateFileRange(file, pos.nPos, nNewChunks * UNDOFILE_CHUNK_SIZE - pos.nPos);
-                fclose(file);
-            }
-        }
-        else
-            return state.Error("out of disk space");
+    bool out_of_space;
+    size_t bytes_allocated = UndoFileSeq().Allocate(pos, nAddSize, out_of_space);
+    if (out_of_space) {
+        return AbortNode(state, "Disk space is low!", _("Error: Disk space is low!"));
+    }
+    if (bytes_allocated != 0 && fPruneMode) {
+        fCheckForPruning = true;
     }
 
     return true;
@@ -3435,26 +3410,26 @@ bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& headers, CValidatio
 }
 
 /** Store block on disk. If dbp is non-nullptr, the file is known to already reside on disk */
-static CDiskBlockPos SaveBlockToDisk(const CBlock& block, int nHeight, const CChainParams& chainparams, const CDiskBlockPos* dbp) {
+static FlatFilePos SaveBlockToDisk(const CBlock& block, int nHeight, const CChainParams& chainparams, const FlatFilePos* dbp) {
     unsigned int nBlockSize = ::GetSerializeSize(block, CLIENT_VERSION);
-    CDiskBlockPos blockPos;
+    FlatFilePos blockPos;
     if (dbp != nullptr)
         blockPos = *dbp;
     if (!FindBlockPos(blockPos, nBlockSize+8, nHeight, block.GetBlockTime(), dbp != nullptr)) {
         error("%s: FindBlockPos failed", __func__);
-        return CDiskBlockPos();
+        return FlatFilePos();
     }
     if (dbp == nullptr) {
         if (!WriteBlockToDisk(block, blockPos, chainparams.MessageStart())) {
             AbortNode("Failed to write block");
-            return CDiskBlockPos();
+            return FlatFilePos();
         }
     }
     return blockPos;
 }
 
 /** Store block on disk. If dbp is non-nullptr, the file is known to already reside on disk */
-bool CChainState::AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const CDiskBlockPos* dbp, bool* fNewBlock)
+bool CChainState::AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CValidationState& state, const CChainParams& chainparams, CBlockIndex** ppindex, bool fRequested, const FlatFilePos* dbp, bool* fNewBlock)
 {
     const CBlock& block = *pblock;
 
@@ -3516,7 +3491,7 @@ bool CChainState::AcceptBlock(const std::shared_ptr<const CBlock>& pblock, CVali
     // Write block to history file
     if (fNewBlock) *fNewBlock = true;
     try {
-        CDiskBlockPos blockPos = SaveBlockToDisk(block, pindex->nHeight, chainparams, dbp);
+        FlatFilePos blockPos = SaveBlockToDisk(block, pindex->nHeight, chainparams, dbp);
         if (blockPos.IsNull()) {
             state.Error(strprintf("%s: Failed to find position to write new block to disk", __func__));
             return false;
@@ -3647,9 +3622,9 @@ void PruneOneBlockFile(const int fileNumber)
 void UnlinkPrunedFiles(const std::set<int>& setFilesToPrune)
 {
     for (std::set<int>::iterator it = setFilesToPrune.begin(); it != setFilesToPrune.end(); ++it) {
-        CDiskBlockPos pos(*it, 0);
-        fs::remove(GetBlockPosFilename(pos, "blk"));
-        fs::remove(GetBlockPosFilename(pos, "rev"));
+        FlatFilePos pos(*it, 0);
+        fs::remove(BlockFileSeq().FileName(pos));
+        fs::remove(UndoFileSeq().FileName(pos));
         LogPrintf("Prune: %s deleted blk/rev (%05u)\n", __func__, *it);
     }
 }
@@ -3757,52 +3732,28 @@ static void FindFilesToPrune(std::set<int>& setFilesToPrune, uint64_t nPruneAfte
            nLastBlockWeCanPrune, count);
 }
 
-bool CheckDiskSpace(uint64_t nAdditionalBytes, bool blocks_dir)
+static FlatFileSeq BlockFileSeq()
 {
-    uint64_t nFreeBytesAvailable = fs::space(blocks_dir ? GetBlocksDir() : GetDataDir()).available;
-
-    // Check for nMinDiskSpace bytes (currently 50MB)
-    if (nFreeBytesAvailable < nMinDiskSpace + nAdditionalBytes)
-        return AbortNode("Disk space is low!", _("Error: Disk space is low!"));
-
-    return true;
+    return FlatFileSeq(GetBlocksDir(), "blk", BLOCKFILE_CHUNK_SIZE);
 }
 
-static FILE* OpenDiskFile(const CDiskBlockPos &pos, const char *prefix, bool fReadOnly)
+static FlatFileSeq UndoFileSeq()
 {
-    if (pos.IsNull())
-        return nullptr;
-    fs::path path = GetBlockPosFilename(pos, prefix);
-    fs::create_directories(path.parent_path());
-    FILE* file = fsbridge::fopen(path, fReadOnly ? "rb": "rb+");
-    if (!file && !fReadOnly)
-        file = fsbridge::fopen(path, "wb+");
-    if (!file) {
-        LogPrintf("Unable to open file %s\n", path.string());
-        return nullptr;
-    }
-    if (pos.nPos) {
-        if (fseek(file, pos.nPos, SEEK_SET)) {
-            LogPrintf("Unable to seek to position %u of %s\n", pos.nPos, path.string());
-            fclose(file);
-            return nullptr;
-        }
-    }
-    return file;
+    return FlatFileSeq(GetBlocksDir(), "rev", UNDOFILE_CHUNK_SIZE);
 }
 
-FILE* OpenBlockFile(const CDiskBlockPos &pos, bool fReadOnly) {
-    return OpenDiskFile(pos, "blk", fReadOnly);
+FILE* OpenBlockFile(const FlatFilePos &pos, bool fReadOnly) {
+    return BlockFileSeq().Open(pos, fReadOnly);
 }
 
 /** Open an undo file (rev?????.dat) */
-static FILE* OpenUndoFile(const CDiskBlockPos &pos, bool fReadOnly) {
-    return OpenDiskFile(pos, "rev", fReadOnly);
+static FILE* OpenUndoFile(const FlatFilePos &pos, bool fReadOnly) {
+    return UndoFileSeq().Open(pos, fReadOnly);
 }
 
-fs::path GetBlockPosFilename(const CDiskBlockPos &pos, const char *prefix)
+fs::path GetBlockPosFilename(const FlatFilePos &pos)
 {
-    return GetBlocksDir() / strprintf("%s%05u.dat", prefix, pos.nFile);
+    return BlockFileSeq().FileName(pos);
 }
 
 CBlockIndex * CChainState::InsertBlockIndex(const uint256& hash)
@@ -3909,7 +3860,7 @@ bool static LoadBlockIndexDB(const CChainParams& chainparams) EXCLUSIVE_LOCKS_RE
     }
     for (std::set<int>::iterator it = setBlkDataFiles.begin(); it != setBlkDataFiles.end(); it++)
     {
-        CDiskBlockPos pos(*it, 0);
+        FlatFilePos pos(*it, 0);
         if (CAutoFile(OpenBlockFile(pos, true), SER_DISK, CLIENT_VERSION).IsNull()) {
             return false;
         }
@@ -4342,7 +4293,7 @@ bool CChainState::LoadGenesisBlock(const CChainParams& chainparams)
 
     try {
         const CBlock& block = chainparams.GenesisBlock();
-        CDiskBlockPos blockPos = SaveBlockToDisk(block, 0, chainparams, nullptr);
+        FlatFilePos blockPos = SaveBlockToDisk(block, 0, chainparams, nullptr);
         if (blockPos.IsNull())
             return error("%s: writing genesis block to disk failed", __func__);
         CBlockIndex *pindex = AddToBlockIndex(block);
@@ -4359,10 +4310,10 @@ bool LoadGenesisBlock(const CChainParams& chainparams)
     return g_chainstate.LoadGenesisBlock(chainparams);
 }
 
-bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskBlockPos *dbp)
+bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, FlatFilePos *dbp)
 {
     // Map of disk positions for blocks with unknown parent (only used for reindex)
-    static std::multimap<uint256, CDiskBlockPos> mapBlocksUnknownParent;
+    static std::multimap<uint256, FlatFilePos> mapBlocksUnknownParent;
     int64_t nStart = GetTimeMillis();
 
     int nLoaded = 0;
@@ -4448,9 +4399,9 @@ bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskB
                 while (!queue.empty()) {
                     uint256 head = queue.front();
                     queue.pop_front();
-                    std::pair<std::multimap<uint256, CDiskBlockPos>::iterator, std::multimap<uint256, CDiskBlockPos>::iterator> range = mapBlocksUnknownParent.equal_range(head);
+                    std::pair<std::multimap<uint256, FlatFilePos>::iterator, std::multimap<uint256, FlatFilePos>::iterator> range = mapBlocksUnknownParent.equal_range(head);
                     while (range.first != range.second) {
-                        std::multimap<uint256, CDiskBlockPos>::iterator it = range.first;
+                        std::multimap<uint256, FlatFilePos>::iterator it = range.first;
                         std::shared_ptr<CBlock> pblockrecursive = std::make_shared<CBlock>();
                         if (ReadBlockFromDisk(*pblockrecursive, it->second, chainparams.GetConsensus()))
                         {
diff --git a/src/validation.h b/src/validation.h
index 1975846b69..4ef82ee2c7 100644
--- a/src/validation.h
+++ b/src/validation.h
@@ -181,9 +181,6 @@ extern arith_uint256 nMinimumChainWork;
 /** Best header we've seen so far (used for getheaders queries' starting points). */
 extern CBlockIndex *pindexBestHeader;
 
-/** Minimum disk space required - used in CheckDiskSpace() */
-static const uint64_t nMinDiskSpace = 52428800;
-
 /** Pruning-related variables and constants */
 /** True if any block files have ever been pruned. */
 extern bool fHavePruned;
@@ -245,14 +242,12 @@ bool ProcessNewBlock(const CChainParams& chainparams, const std::shared_ptr<cons
  */
 bool ProcessNewBlockHeaders(const std::vector<CBlockHeader>& block, CValidationState& state, const CChainParams& chainparams, const CBlockIndex** ppindex = nullptr, CBlockHeader* first_invalid = nullptr) LOCKS_EXCLUDED(cs_main);
 
-/** Check whether enough disk space is available for an incoming block */
-bool CheckDiskSpace(uint64_t nAdditionalBytes = 0, bool blocks_dir = false);
 /** Open a block file (blk?????.dat) */
-FILE* OpenBlockFile(const CDiskBlockPos &pos, bool fReadOnly = false);
+FILE* OpenBlockFile(const FlatFilePos &pos, bool fReadOnly = false);
 /** Translation to a filesystem path */
-fs::path GetBlockPosFilename(const CDiskBlockPos &pos, const char *prefix);
+fs::path GetBlockPosFilename(const FlatFilePos &pos);
 /** Import blocks from an external file */
-bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, CDiskBlockPos *dbp = nullptr);
+bool LoadExternalBlockFile(const CChainParams& chainparams, FILE* fileIn, FlatFilePos *dbp = nullptr);
 /** Ensures we have a genesis block in the block tree, possibly writing one to disk. */
 bool LoadGenesisBlock(const CChainParams& chainparams);
 /** Load the block tree and coins database from disk,
@@ -391,9 +386,9 @@ void InitScriptExecutionCache();
 
 
 /** Functions for disk access for blocks */
-bool ReadBlockFromDisk(CBlock& block, const CDiskBlockPos& pos, const Consensus::Params& consensusParams);
+bool ReadBlockFromDisk(CBlock& block, const FlatFilePos& pos, const Consensus::Params& consensusParams);
 bool ReadBlockFromDisk(CBlock& block, const CBlockIndex* pindex, const Consensus::Params& consensusParams);
-bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const CDiskBlockPos& pos, const CMessageHeader::MessageStartChars& message_start);
+bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const FlatFilePos& pos, const CMessageHeader::MessageStartChars& message_start);
 bool ReadRawBlockFromDisk(std::vector<uint8_t>& block, const CBlockIndex* pindex, const CMessageHeader::MessageStartChars& message_start);
 
 /** Functions for validating blocks and updating the block tree */