Add support for deterministic database shards (XRPLF#2688):

This commit, if merged, adds support to allow multiple indepedent nodes to
produce a binary identical shard for a given range of ledgers. The advantage
is that servers can use content-addressable storage, and can more efficiently
retrieve shards by downloading from multiple peers at once and then verifying
the integrity of a shard by cross-checking its checksum with the checksum
other servers report.

Builds/CMake/RippledCore.cmake

@@ -509,6 +509,7 @@ target_sources (rippled PRIVATE
   src/ripple/nodestore/impl/DatabaseNodeImp.cpp
   src/ripple/nodestore/impl/DatabaseRotatingImp.cpp
   src/ripple/nodestore/impl/DatabaseShardImp.cpp
+  src/ripple/nodestore/impl/DeterministicShard.cpp
   src/ripple/nodestore/impl/DecodedBlob.cpp
   src/ripple/nodestore/impl/DummyScheduler.cpp
   src/ripple/nodestore/impl/EncodedBlob.cpp

src/ripple/nodestore/Backend.h

@@ -58,6 +58,25 @@ class Backend
     virtual void
     open(bool createIfMissing = true) = 0;
 
+    /** Open the backend.
+        @param createIfMissing Create the database files if necessary.
+        @param appType Deterministic appType used to create a backend.
+        @param uid Deterministic uid used to create a backend.
+        @param salt Deterministic salt used to create a backend.
+        This allows the caller to catch exceptions.
+    */
+    virtual void
+    open(
+        bool createIfMissing,
+        boost::optional<uint64_t> appType,
+        boost::optional<uint64_t> uid,
+        boost::optional<uint64_t> salt)
+    {
+        Throw<std::runtime_error>(std::string(
+            "Deterministic appType/uid/salt not supported by backend " +
+            getName()));
+    }
+
     /** Close the backend.
         This allows the caller to catch exceptions.
     */

src/ripple/nodestore/DeterministicShard.md

@@ -0,0 +1,103 @@
+# Deterministic Database Shards
+
+This doc describes the standard way to assemble the database shard. A shard assembled using this approach becomes deterministic i.e. if two independent sides assemble the shard consists of the same ledgers, accounts and transactions, then they will obtain the same shard files `nudb.dat` and `nudb.key`. The approach deals with the `NuDB` database format only, refer to `https://github.com/vinniefalco/NuDB`.
+
+
+## Headers
+
+Due to NuDB database definition, the following headers are used for database files:
+
+nudb.key:
+```
+char[8]         Type            The characters "nudb.key"
+uint16          Version         Holds the version number
+uint64          UID             Unique ID generated on creation
+uint64          Appnum          Application defined constant
+uint16          KeySize         Key size in bytes
+uint64          Salt            A random seed
+uint64          Pepper          The salt hashed
+uint16          BlockSize       Size of a file block in bytes
+uint16          LoadFactor      Target fraction in 65536ths
+uint8[56]       Reserved        Zeroes
+uint8[]         Reserved        Zero-pad to block size
+```
+
+nudb.dat:
+```
+char[8]         Type            The characters "nudb.dat"
+uint16          Version         Holds the version number
+uint64          UID             Unique ID generated on creation
+uint64          Appnum          Application defined constant
+uint16          KeySize         Key size in bytes
+uint8[64]       (reserved)      Zeroes
+```
+there all fields are saved using network byte order (most significant byte first).
+
+To make the shard deterministic the following parameters are used as values of header field both for `nudb.key` and `nudb.dat` files.
+```
+Version         2
+UID             digest(0)
+Appnum          digest(2) | 0x5348524400000000 /* 'SHRD' */
+KeySize         32
+Salt            digest(1)
+Pepper          XXH64(Salt)
+BlockSize       0x1000 (4096 bytes)
+LoadFactor      0.5 (numeric 0x8000)
+```
+Note: XXH64() is well-known hash algorithm.
+
+The `digest(i)` mentioned above defined as the follows:
+
+First, RIPEMD160 hash `H` calculated of the following structure
+```
+uint256         lastHash        Hash of last ledger in shard
+uint32          index           Index of the shard
+uint32          firstSeq        Sequence number of first ledger in the shard
+uint32          lastSeq         Sequence number of last ledger in the shard
+uint32          version         Version of shard, 2 at the present
+```
+there all 32-bit integers are hashed in network byte order.
+
+Then, `digest(i)` is defined as the following portion of the above hash `H`:
+```
+digest(0) = H[0] << 56 | H[2] << 48 | ... | H[14] << 0,
+digest(1) = H[1] << 56 | H[3] << 48 | ... | H[15] << 0,
+digest(2) = H[19] << 24 | H[18] << 16 | ... | H[16] << 0,
+```
+where `H[i]` denotes `i`-th byte of hash `H`.
+
+
+## Contents
+
+After deterministic shard is created using the above mentioned headers, it filled with objects. First, all objects of the shard are collected and sorted in according to their hashes. Here the objects are: ledgers, SHAmap tree nodes including accounts and transactions, and final key object with hash 0. Objects are sorted by increasing of their hashes, precisely, by increasing of hex representations of hashes in lexicographic order. 
+
+For example, the following is an example of sorted hashes in their hex representation:
+```
+0000000000000000000000000000000000000000000000000000000000000000
+154F29A919B30F50443A241C466691B046677C923EE7905AB97A4DBE8A5C2423
+2231553FC01D37A66C61BBEEACBB8C460994493E5659D118E19A8DDBB1444273
+272DCBFD8E4D5D786CF11A5444B30FB35435933B5DE6C660AA46E68CF0F5C447
+3C062FD9F0BCDCA31ACEBCD8E530D0BDAD1F1D1257B89C435616506A3EE6CB9E
+58A0E5AE427CDDC1C7C06448E8C3E4BF718DE036D827881624B20465C3E1334F
+...
+```
+
+Finally, objects added to the shard one by one in the sorted order from low to high hashes.
+
+
+## Tests
+
+To perform test to deterministic shards implementation one can enter the following command:
+```
+rippled --unittest ripple.NodeStore.DatabaseShard
+```
+
+The following is the right output of deterministic shards test:
+```
+ripple.NodeStore.DatabaseShard DatabaseShard deterministic_shard with backend nudb
+Iteration 0: RIPEMD160[nudb.key] = 4CFA8985836B549EC99D2E9705707F488DC91E4E
+Iteration 0: RIPEMD160[nudb.dat] = 8CC61F503C36339803F8C2FC652C1102DDB889F1
+Iteration 1: RIPEMD160[nudb.key] = 4CFA8985836B549EC99D2E9705707F488DC91E4E
+Iteration 1: RIPEMD160[nudb.dat] = 8CC61F503C36339803F8C2FC652C1102DDB889F1
+```
+

src/ripple/nodestore/backend/NuDBFactory.cpp

@@ -38,7 +38,10 @@ namespace NodeStore {
 class NuDBBackend : public Backend
 {
 public:
-    static constexpr std::size_t currentType = 1;
+    static constexpr std::uint64_t currentType = 1;
+    static constexpr std::uint64_t deterministicType = 0x5348524400000000ull;
+    /* "SHRD" in ASCII */
+    static constexpr std::uint64_t deterministicMask = 0xFFFFFFFF00000000ull;
 
     beast::Journal const j_;
     size_t const keyBytes_;
@@ -93,7 +96,11 @@ class NuDBBackend : public Backend
     }
 
     void
-    open(bool createIfMissing) override
+    open(
+        bool createIfMissing,
+        boost::optional<uint64_t> appType,
+        boost::optional<uint64_t> uid,
+        boost::optional<uint64_t> salt) override
     {
         using namespace boost::filesystem;
         if (db_.is_open())
@@ -114,8 +121,9 @@ class NuDBBackend : public Backend
                 dp,
                 kp,
                 lp,
-                currentType,
-                nudb::make_salt(),
+                appType.value_or(currentType),
+                uid.value_or(nudb::make_uid()),
+                salt.value_or(nudb::make_salt()),
                 keyBytes_,
                 nudb::block_size(kp),
                 0.50,
@@ -128,10 +136,27 @@ class NuDBBackend : public Backend
         db_.open(dp, kp, lp, ec);
         if (ec)
             Throw<nudb::system_error>(ec);
-        if (db_.appnum() != currentType)
+
+        /** Old value currentType is accepted for appnum in traditional
+         *  databases, new value is used for deterministic shard databases.
+         *  New 64-bit value is constructed from fixed and random parts.
+         *  Fixed part is bounded by bitmask deterministicMask,
+         *  and the value of fixed part is deterministicType.
+         *  Random part depends on the contents of the shard and may be any.
+         *  The contents of appnum field should match either old or new rule.
+         */
+        if (db_.appnum() != appType.value_or(currentType) &&
+            (appType ||
+             (db_.appnum() & deterministicMask) != deterministicType))
             Throw<std::runtime_error>("nodestore: unknown appnum");
     }
 
+    void
+    open(bool createIfMissing) override
+    {
+        open(createIfMissing, boost::none, boost::none, boost::none);
+    }
+
     void
     close() override
     {