Adding classes, functions, and utilities for common LMDB operations.

12 files changed, 1552 insertions, 0 deletions

diff --git a/src/lmdb/CMakeLists.txt b/src/lmdb/CMakeLists.txt
new file mode 100644
index 000000000..1f369f114
--- /dev/null
+++ b/src/lmdb/CMakeLists.txt
@@ -0,0 +1,33 @@
+# Copyright (c) 2014-2018, The Monero Project
+# 
+# All rights reserved.
+# 
+# Redistribution and use in source and binary forms, with or without modification, are
+# permitted provided that the following conditions are met:
+# 
+# 1. Redistributions of source code must retain the above copyright notice, this list of
+#    conditions and the following disclaimer.
+# 
+# 2. Redistributions in binary form must reproduce the above copyright notice, this list
+#    of conditions and the following disclaimer in the documentation and/or other
+#    materials provided with the distribution.
+# 
+# 3. Neither the name of the copyright holder nor the names of its contributors may be
+#    used to endorse or promote products derived from this software without specific
+#    prior written permission.
+# 
+# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+# EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+# MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+# THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+# PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+# INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+# STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+# THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+set(lmdb_sources database.cpp error.cpp table.cpp value_stream.cpp)
+set(lmdb_headers database.h error.h key_stream.h table.h transaction.h util.h value_stream.h)
+
+monero_add_library(lmdb_lib ${lmdb_sources} ${lmdb_headers})
+target_link_libraries(lmdb_lib common ${LMDB_LIBRARY})
diff --git a/src/lmdb/database.cpp b/src/lmdb/database.cpp
new file mode 100644
index 000000000..c6b244671
--- /dev/null
+++ b/src/lmdb/database.cpp
@@ -0,0 +1,187 @@
+// Copyright (c) 2014-2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#include "database.h"
+#include "lmdb/error.h"
+#include "lmdb/util.h"
+
+#ifdef _WIN32
+namespace
+{
+    constexpr const mdb_mode_t open_flags = 0;
+}
+#else
+#include <sys/stat.h>
+
+namespace
+{
+    constexpr const mdb_mode_t open_flags = (S_IRUSR | S_IWUSR);
+}
+#endif
+
+namespace lmdb
+{
+    namespace
+    {
+        constexpr const std::size_t max_resize = 1 * 1024 * 1024 * 1024; // 1 GB
+        void acquire_context(context& ctx) noexcept
+        {
+            while (ctx.lock.test_and_set());
+            ++(ctx.active);
+            ctx.lock.clear();
+        }
+
+        void release_context(context& ctx) noexcept
+        {
+            --(ctx.active);
+        }
+    }
+
+    void release_read_txn::operator()(MDB_txn* ptr) const noexcept
+    {
+        if (ptr)
+        {
+            MDB_env* const env = mdb_txn_env(ptr);
+            abort_txn{}(ptr);
+            if (env)
+            {
+                context* ctx = reinterpret_cast<context*>(mdb_env_get_userctx(env));
+                if (ctx)
+                    release_context(*ctx);
+            }
+        }
+    }
+
+    expect<environment> open_environment(const char* path, MDB_dbi max_dbs) noexcept
+    {
+        MONERO_PRECOND(path != nullptr);
+
+        MDB_env* obj = nullptr;
+        MONERO_LMDB_CHECK(mdb_env_create(std::addressof(obj)));
+        environment out{obj};
+
+        MONERO_LMDB_CHECK(mdb_env_set_maxdbs(out.get(), max_dbs));
+        MONERO_LMDB_CHECK(mdb_env_open(out.get(), path, 0, open_flags));
+        return {std::move(out)};
+    }
+
+    expect<write_txn> database::do_create_txn(unsigned int flags) noexcept
+    {
+        MONERO_PRECOND(handle() != nullptr);
+
+        for (unsigned attempts = 0; attempts < 3; ++attempts)
+        {
+            acquire_context(ctx);
+
+            MDB_txn* txn = nullptr;
+            const int err =
+                mdb_txn_begin(handle(), nullptr, flags, &txn);
+            if (!err && txn != nullptr)
+                return write_txn{txn};
+
+            release_context(ctx);
+            if (err != MDB_MAP_RESIZED)
+                return {lmdb::error(err)};
+            MONERO_CHECK(this->resize());
+        }
+        return {lmdb::error(MDB_MAP_RESIZED)};
+    }
+
+    database::database(environment env)
+      : env(std::move(env)), ctx{{}, ATOMIC_FLAG_INIT}
+    {
+        if (handle())
+        {
+            const int err = mdb_env_set_userctx(handle(), std::addressof(ctx));
+            if (err)
+                MONERO_THROW(lmdb::error(err), "Failed to set user context");
+        }
+    }
+
+    database::~database() noexcept
+    {
+        while (ctx.active);
+    }
+
+    expect<void> database::resize() noexcept
+    {
+        MONERO_PRECOND(handle() != nullptr);
+
+        while (ctx.lock.test_and_set());
+        while (ctx.active);
+
+        MDB_envinfo info{};
+        MONERO_LMDB_CHECK(mdb_env_info(handle(), &info));
+
+        const std::size_t resize = std::min(info.me_mapsize, max_resize);
+        const int err = mdb_env_set_mapsize(handle(), info.me_mapsize + resize);
+        ctx.lock.clear();
+        if (err)
+            return {lmdb::error(err)};
+        return success();
+    }
+
+    expect<read_txn> database::create_read_txn(suspended_txn txn) noexcept
+    {
+        if (txn)
+        {
+            acquire_context(ctx);
+            const int err = mdb_txn_renew(txn.get());
+            if (err)
+            {
+                release_context(ctx);
+                return {lmdb::error(err)};
+            }
+            return read_txn{txn.release()};
+        }
+        auto new_txn = do_create_txn(MDB_RDONLY);
+        if (new_txn)
+            return read_txn{new_txn->release()};
+        return new_txn.error();
+    }
+
+    expect<suspended_txn> database::reset_txn(read_txn txn) noexcept
+    {
+        MONERO_PRECOND(txn != nullptr);
+        mdb_txn_reset(txn.get());
+        release_context(ctx);
+        return suspended_txn{txn.release()};
+    }
+
+    expect<write_txn> database::create_write_txn() noexcept
+    {
+        return do_create_txn(0);
+    }
+
+    expect<void> database::commit(write_txn txn) noexcept
+    {
+        MONERO_PRECOND(txn != nullptr);
+        MONERO_LMDB_CHECK(mdb_txn_commit(txn.get()));
+        txn.release();
+        release_context(ctx);
+        return success();
+    }
+} // lmdb
diff --git a/src/lmdb/database.h b/src/lmdb/database.h
new file mode 100644
index 000000000..269f8c8a1
--- /dev/null
+++ b/src/lmdb/database.h
@@ -0,0 +1,138 @@
+// Copyright (c) 2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#pragma once
+
+#include <atomic>
+#include <cstddef>
+#include <lmdb.h>
+#include <memory>
+#include <type_traits>
+
+#include "common/expect.h"
+#include "lmdb/error.h"
+#include "lmdb/transaction.h"
+
+namespace lmdb
+{
+    //! Closes LMDB environment handle.
+    struct close_env
+    {
+        void operator()(MDB_env* ptr) const noexcept
+        {
+            if (ptr)
+                mdb_env_close(ptr);
+        }
+    };
+
+    using environment = std::unique_ptr<MDB_env, close_env>;
+
+    //! \return LMDB environment at `path` with a max of `max_dbs` tables.
+    expect<environment> open_environment(const char* path, MDB_dbi max_dbs) noexcept;
+
+    //! Context given to LMDB.
+    struct context
+    {
+        std::atomic<std::size_t> active;
+        std::atomic_flag lock;
+    };
+
+    //! Manages a LMDB environment for safe memory-map resizing. Thread-safe.
+    class database
+    {
+        environment env;
+        context ctx;
+
+        //! \return The LMDB environment associated with the object.
+        MDB_env* handle() const noexcept { return env.get(); }
+
+        expect<write_txn> do_create_txn(unsigned int flags) noexcept;
+
+    public: 
+        database(environment env);
+
+        database(database&&) = delete;
+        database(database const&) = delete;
+
+        virtual ~database() noexcept;
+
+        database& operator=(database&&) = delete;
+        database& operator=(database const&) = delete;
+
+        /*!
+            Resize the memory map for the LMDB environment. Will block until
+            all reads/writes on the environment complete.
+        */
+        expect<void> resize() noexcept;
+
+        //! \return A read only LMDB transaction, reusing `txn` if provided.
+        expect<read_txn> create_read_txn(suspended_txn txn = nullptr) noexcept;
+
+        //! \return `txn` after releasing context.
+        expect<suspended_txn> reset_txn(read_txn txn) noexcept;
+
+        //! \return A read-write LMDB transaction.
+        expect<write_txn> create_write_txn() noexcept;
+
+        //! Commit the read-write transaction.
+        expect<void> commit(write_txn txn) noexcept;
+
+        /*!
+            Create a write transaction, pass it to `f`, then try to commit
+            the write if `f` succeeds.
+
+            \tparam F must be callable with signature `expect<T>(MDB_txn&)`.
+            \param f must be re-startable if `lmdb::error(MDB_MAP_FULL)`.
+
+            \return The result of calling `f`.
+        */
+        template<typename F>
+        typename std::result_of<F(MDB_txn&)>::type try_write(F f, unsigned attempts = 3)
+        {
+            for (unsigned i = 0; i < attempts; ++i)
+            {
+                expect<write_txn> txn = create_write_txn();
+                if (!txn)
+                    return txn.error();
+
+                MONERO_PRECOND(*txn != nullptr);
+                const auto wrote = f(*(*txn));
+                if (wrote)
+                {
+                    MONERO_CHECK(commit(std::move(*txn)));
+                    return wrote;
+                }
+                if (wrote != lmdb::error(MDB_MAP_FULL))
+                    return wrote;
+
+                txn->reset();
+                MONERO_CHECK(this->resize());
+            }
+            return {lmdb::error(MDB_MAP_FULL)};
+        }
+    };
+} // lmdb
+
diff --git a/src/lmdb/error.cpp b/src/lmdb/error.cpp
new file mode 100644
index 000000000..359677064
--- /dev/null
+++ b/src/lmdb/error.cpp
@@ -0,0 +1,98 @@
+// Copyright (c) 2014-2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+#include "error.h"
+
+#include <lmdb.h>
+#include <string>
+
+namespace {
+    struct category final : std::error_category
+    {
+        virtual const char* name() const noexcept override final
+        {
+            return "lmdb::error_category()";
+        }
+
+        virtual std::string message(int value) const override final
+        {
+            char const* const msg = mdb_strerror(value);
+            if (msg)
+                return msg;
+            return "Unknown lmdb::error_category() value";
+        }
+
+        virtual std::error_condition default_error_condition(int value) const noexcept override final
+        {
+            switch (value)
+            {
+                case MDB_KEYEXIST:
+                case MDB_NOTFOUND:
+                    break; // map to nothing generic
+                case MDB_PAGE_NOTFOUND:
+                case MDB_CORRUPTED:
+                    return std::errc::state_not_recoverable;
+                case MDB_PANIC:
+                case MDB_VERSION_MISMATCH:
+                case MDB_INVALID:
+                    break; // map to nothing generic
+                case MDB_MAP_FULL:
+                    return std::errc::no_buffer_space;
+                case MDB_DBS_FULL:
+                    break; // map to nothing generic
+                case MDB_READERS_FULL:
+                case MDB_TLS_FULL:
+                    return std::errc::no_lock_available;
+                case MDB_TXN_FULL:
+                case MDB_CURSOR_FULL:
+                case MDB_PAGE_FULL:
+                case MDB_MAP_RESIZED:
+                    break; // map to nothing generic
+                case MDB_INCOMPATIBLE:
+                    return std::errc::invalid_argument;
+                case MDB_BAD_RSLOT:
+                case MDB_BAD_TXN:
+                case MDB_BAD_VALSIZE:
+                case MDB_BAD_DBI:
+                    return std::errc::invalid_argument;
+                default:
+                    return std::error_condition{value, std::generic_category()};
+            }
+            return std::error_condition{value, *this};
+        }
+    };
+}
+
+namespace lmdb
+{
+    std::error_category const& error_category() noexcept
+    {
+        static const category instance{};
+        return instance;
+    }
+}
+
diff --git a/src/lmdb/error.h b/src/lmdb/error.h
new file mode 100644
index 000000000..2944adf78
--- /dev/null
+++ b/src/lmdb/error.h
@@ -0,0 +1,64 @@
+// Copyright (c) 2014-2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#pragma once
+
+#include <system_error>
+#include <type_traits>
+
+//! Executes a LMDB command, and returns errors via `lmdb::error` enum.
+#define MONERO_LMDB_CHECK(...)         \
+    do                                 \
+    {                                  \
+        const int err = __VA_ARGS__ ;  \
+        if (err)                       \
+            return {lmdb::error(err)}; \
+    } while (0)
+
+namespace lmdb
+{
+    //! Tracks LMDB error codes.
+    enum class error : int
+    {
+        // 0 is reserved for no error, as per expect<T>
+        // All other errors are the values reported by LMDB
+    };
+
+    std::error_category const& error_category() noexcept;
+
+    inline std::error_code make_error_code(error value) noexcept
+    {
+        return std::error_code{int(value), error_category()};
+    }
+}
+
+namespace std
+{
+    template<>
+    struct is_error_code_enum<::lmdb::error>
+      : true_type
+    {};
+}
diff --git a/src/lmdb/key_stream.h b/src/lmdb/key_stream.h
new file mode 100644
index 000000000..40434d3a1
--- /dev/null
+++ b/src/lmdb/key_stream.h
@@ -0,0 +1,264 @@
+// Copyright (c) 2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#pragma once
+
+#include <boost/range/iterator_range.hpp>
+#include <cstdint>
+#include <cstring>
+#include <iterator>
+#include <lmdb.h>
+#include <utility>
+
+#include "lmdb/value_stream.h"
+#include "span.h"
+
+namespace lmdb
+{
+
+    /*!
+        An InputIterator for a fixed-sized LMDB key and value. `operator++`
+        iterates over keys.
+
+        \tparam K Key type in database records.
+        \tparam V Value type in database records.
+
+        \note This meets requirements for an InputIterator only. The iterator
+            can only be incremented and dereferenced. All copies of an iterator
+            share the same LMDB cursor, and therefore incrementing any copy will
+            change the cursor state for all (incrementing an iterator will
+            invalidate all prior copies of the iterator). Usage is identical
+            to `std::istream_iterator`.
+    */
+    template<typename K, typename V>
+    class key_iterator
+    {
+        MDB_cursor* cur;
+        epee::span<const std::uint8_t> key;
+        
+        void increment()
+        {
+            // MDB_NEXT_MULTIPLE doesn't work if only one value is stored :/
+            if (cur)
+                key = lmdb::stream::get(*cur, MDB_NEXT_NODUP, sizeof(K), sizeof(V)).first;
+        }
+
+    public:
+        using value_type = std::pair<K, boost::iterator_range<value_iterator<V>>>;
+        using reference = value_type;
+        using pointer = void;
+        using difference_type = std::size_t;
+        using iterator_category = std::input_iterator_tag;
+
+        //! Construct an "end" iterator.
+        key_iterator() noexcept
+          : cur(nullptr), key()
+        {}
+
+        /*!
+            \param cur Iterate over keys starting at this cursor position.
+            \throw std::system_error if unexpected LMDB error. This can happen
+                if `cur` is invalid.
+        */
+        key_iterator(MDB_cursor* cur)
+          : cur(cur), key()
+        {
+            if (cur)
+                key = lmdb::stream::get(*cur, MDB_GET_CURRENT, sizeof(K), sizeof(V)).first;
+        }
+
+        //! \return True if `this` is one-past the last key.
+        bool is_end() const noexcept { return key.empty(); }
+
+        //! \return True iff `rhs` is referencing `this` key.
+        bool equal(key_iterator const& rhs) const noexcept
+        {
+            return
+                (key.empty() && rhs.key.empty()) ||
+                key.data() == rhs.key.data();
+        }
+
+        /*!
+            Moves iterator to next key or end. Invalidates all prior copies of
+            the iterator.
+        */
+        key_iterator& operator++()
+        {
+            increment();
+            return *this;
+        }
+
+        /*!
+            Moves iterator to next key or end.
+
+            \return A copy that is already invalidated, ignore
+        */
+        key_iterator operator++(int)
+        {
+            key_iterator out{*this};
+            increment();
+            return out;
+        }
+
+        //! \pre `!is_end()` \return {current key, current value range}
+        value_type operator*() const
+        {
+            return {get_key(), make_value_range()};
+        }
+
+        //! \pre `!is_end()` \return Current key
+        K get_key() const noexcept
+        {
+            assert(!is_end());
+            K out;
+            std::memcpy(std::addressof(out), key.data(), sizeof(out));
+            return out;
+        }
+
+        /*!
+            Return a C++ iterator over database values from current cursor
+            position that will reach `.is_end()` after the last duplicate key
+            record. Calling `make_iterator()` will return an iterator whose
+            `operator*` will return an entire value (`V`).
+            `make_iterator<MONERO_FIELD(account, id)>()` will return an
+            iterator whose `operator*` will return a `decltype(account.id)`
+            object - the other fields in the struct `account` are never copied
+            from the database.
+
+            \throw std::system_error if LMDB has unexpected errors.
+            \return C++ iterator starting at current cursor position.
+        */
+        template<typename T = V, typename F = T, std::size_t offset = 0>
+        value_iterator<T, F, offset> make_value_iterator() const
+        {
+            static_assert(std::is_same<T, V>(), "bad MONERO_FIELD usage?");
+            return {cur};
+        }
+
+        /*!
+            Return a range from current cursor position until last duplicate
+            key record. Useful in for-each range loops or in templated code
+            expecting a range of elements. Calling `make_range()` will return
+            a range of `T` objects. `make_range<MONERO_FIELD(account, id)>()`
+            will return a range of `decltype(account.id)` objects - the other
+            fields in the struct `account` are never copied from the database.
+
+            \throw std::system_error if LMDB has unexpected errors.
+            \return An InputIterator range over values at cursor position.
+        */
+        template<typename T = V, typename F = T, std::size_t offset = 0>
+        boost::iterator_range<value_iterator<T, F, offset>> make_value_range() const
+        {
+            return {make_value_iterator<T, F, offset>(), value_iterator<T, F, offset>{}};
+        }
+    };
+
+    /*!
+        C++ wrapper for a LMDB read-only cursor on a fixed-sized key `K` and
+        value `V`.
+
+        \tparam K key type being stored by each record.
+        \tparam V value type being stored by each record.
+        \tparam D cleanup functor for the cursor; usually unique per db/table.
+    */
+    template<typename K, typename V, typename D>
+    class key_stream
+    {
+        std::unique_ptr<MDB_cursor, D> cur;
+    public:
+
+        //! Take ownership of `cur` without changing position. `nullptr` valid.
+        explicit key_stream(std::unique_ptr<MDB_cursor, D> cur)
+          : cur(std::move(cur))
+        {}
+
+        key_stream(key_stream&&) = default;
+        key_stream(key_stream const&) = delete;
+        ~key_stream() = default;
+        key_stream& operator=(key_stream&&) = default;
+        key_stream& operator=(key_stream const&) = delete;
+
+        /*!
+            Give up ownership of the cursor. `make_iterator()` and
+            `make_range()` can still be invoked, but return the empty set.
+
+            \return Currently owned LMDB cursor.
+        */
+        std::unique_ptr<MDB_cursor, D> give_cursor() noexcept
+        {
+            return {std::move(cur)};
+        }
+
+        /*!
+            Place the stream back at the first key/value. Newly created
+            iterators will start at the first value again.
+
+            \note Invalidates all current iterators, including those created
+                with `make_iterator` or `make_range`. Also invalidates all
+                `value_iterator`s created with `key_iterator`.
+        */
+        void reset()
+        {
+            if (cur)
+                lmdb::stream::get(*cur, MDB_FIRST, 0, 0);
+        }
+
+        /*!
+            \throw std::system_error if LMDB has unexpected errors.
+            \return C++ iterator over database keys from current cursor
+                position that will reach `.is_end()` after the last key.
+        */
+        key_iterator<K, V> make_iterator() const
+        {
+            return {cur.get()};
+        }
+
+        /*!
+            \throw std::system_error if LMDB has unexpected errors.
+            \return Range from current cursor position until last key record.
+                Useful in for-each range loops or in templated code
+        */
+        boost::iterator_range<key_iterator<K, V>> make_range() const
+        {
+            return {make_iterator(), key_iterator<K, V>{}};
+        }
+    };
+
+    template<typename K, typename V>
+    inline
+    bool operator==(key_iterator<K, V> const& lhs, key_iterator<K, V> const& rhs) noexcept
+    {
+        return lhs.equal(rhs);
+    }
+
+    template<typename K, typename V>
+    inline
+    bool operator!=(key_iterator<K, V> const& lhs, key_iterator<K, V> const& rhs) noexcept
+    {
+        return !lhs.equal(rhs);
+    }
+} // lmdb
+
diff --git a/src/lmdb/table.cpp b/src/lmdb/table.cpp
new file mode 100644
index 000000000..0818b74e6
--- /dev/null
+++ b/src/lmdb/table.cpp
@@ -0,0 +1,43 @@
+// Copyright (c) 2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#include "table.h"
+
+namespace lmdb
+{
+    expect<MDB_dbi> table::open(MDB_txn& write_txn) const noexcept
+    {
+        MONERO_PRECOND(name != nullptr);
+
+        MDB_dbi out;
+        MONERO_LMDB_CHECK(mdb_dbi_open(&write_txn, name, flags, &out));
+        if (key_cmp && !(flags & MDB_INTEGERKEY))
+            MONERO_LMDB_CHECK(mdb_set_compare(&write_txn, out, key_cmp));
+        if (value_cmp && !(flags & MDB_INTEGERDUP))
+            MONERO_LMDB_CHECK(mdb_set_dupsort(&write_txn, out, value_cmp));
+        return out;
+    }
+}
diff --git a/src/lmdb/table.h b/src/lmdb/table.h
new file mode 100644
index 000000000..41a3de296
--- /dev/null
+++ b/src/lmdb/table.h
@@ -0,0 +1,120 @@
+#pragma once
+
+#include <utility>
+
+#include "common/expect.h"
+#include "lmdb/error.h"
+#include "lmdb/key_stream.h"
+#include "lmdb/util.h"
+#include "lmdb/value_stream.h"
+
+namespace lmdb
+{
+    //! Helper for grouping typical LMDB DBI options.
+    struct table
+    {
+        char const* const name;
+        const unsigned flags;
+        MDB_cmp_func const* const key_cmp;
+        MDB_cmp_func const* const value_cmp;
+
+        //! \pre `name != nullptr` \return Open table.
+        expect<MDB_dbi> open(MDB_txn& write_txn) const noexcept;
+    };
+
+    //! Helper for grouping typical LMDB DBI options when key and value are fixed types.
+    template<typename K, typename V>
+    struct basic_table : table
+    {
+        using key_type = K;
+        using value_type = V;
+
+        //! \return Additional LMDB flags based on `flags` value.
+        static constexpr unsigned compute_flags(const unsigned flags) noexcept
+        {
+            return flags | ((flags & MDB_DUPSORT) ? MDB_DUPFIXED : 0);
+        }
+
+        constexpr explicit basic_table(const char* name, unsigned flags = 0, MDB_cmp_func value_cmp = nullptr) noexcept
+          : table{name, compute_flags(flags), &lmdb::less<lmdb::native_type<K>>, value_cmp}
+        {}
+
+        /*!
+            \tparam U must be same as `V`; used for sanity checking.
+            \tparam F is the type within `U` that is being extracted.
+            \tparam offset to `F` within `U`.
+
+            \note If using `F` and `offset` to retrieve a specific field, use
+                `MONERO_FIELD` macro in `src/lmdb/util.h` which calculates the
+                offset automatically.
+
+            \return Value of type `F` at `offset` within `value` which has
+                type `U`.
+        */
+        template<typename U, typename F = U, std::size_t offset = 0>
+        static expect<F> get_value(MDB_val value) noexcept
+        {
+            static_assert(std::is_same<U, V>(), "bad MONERO_FIELD?");
+            static_assert(std::is_pod<F>(), "F must be POD");
+            static_assert(sizeof(F) + offset <= sizeof(U), "bad field type and/or offset");
+
+            if (value.mv_size != sizeof(U))
+                return {lmdb::error(MDB_BAD_VALSIZE)};
+
+            F out;
+            std::memcpy(std::addressof(out), static_cast<char*>(value.mv_data) + offset, sizeof(out));
+            return out;
+        }
+
+        /*!
+            \pre `cur != nullptr`.
+            \param cur Active cursor on table. Returned in object on success,
+                otherwise destroyed.
+            \return A handle to the first key/value in the table linked
+                to `cur` or an empty `key_stream`.
+        */
+        template<typename D>
+        expect<key_stream<K, V, D>>
+        static get_key_stream(std::unique_ptr<MDB_cursor, D> cur) noexcept
+        {
+            MONERO_PRECOND(cur != nullptr);
+
+            MDB_val key;
+            MDB_val value;
+            const int err = mdb_cursor_get(cur.get(), &key, &value, MDB_FIRST);
+            if (err)
+            {
+                if (err != MDB_NOTFOUND)
+                    return {lmdb::error(err)};
+                cur.reset(); // return empty set
+            }
+            return key_stream<K, V, D>{std::move(cur)};
+        }
+
+        /*!
+            \pre `cur != nullptr`.
+            \param cur Active cursor on table. Returned in object on success,
+                otherwise destroyed.
+            \return A handle to the first value at `key` in the table linked
+                to `cur` or an empty `value_stream`.
+        */
+        template<typename D>
+        expect<value_stream<V, D>>
+        static get_value_stream(K const& key, std::unique_ptr<MDB_cursor, D> cur) noexcept
+        {
+            MONERO_PRECOND(cur != nullptr);
+
+            MDB_val key_bytes = lmdb::to_val(key);
+            MDB_val value;
+            const int err = mdb_cursor_get(cur.get(), &key_bytes, &value, MDB_SET);
+            if (err)
+            {
+                if (err != MDB_NOTFOUND)
+                    return {lmdb::error(err)};
+                cur.reset(); // return empty set
+            }
+            return value_stream<V, D>{std::move(cur)};
+        }
+    };
+} // lmdb
+
diff --git a/src/lmdb/transaction.h b/src/lmdb/transaction.h
new file mode 100644
index 000000000..cdd80696c
--- /dev/null
+++ b/src/lmdb/transaction.h
@@ -0,0 +1,95 @@
+// Copyright (c) 2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#pragma once
+
+#include <lmdb.h>
+#include <memory>
+
+#include "lmdb/error.h"
+
+//! Uses C++ type system to differentiate between cursors
+#define MONERO_CURSOR(name)                                    \
+    struct close_ ## name : ::lmdb::close_cursor {};           \
+    using name = std::unique_ptr< MDB_cursor, close_ ## name >;
+
+namespace lmdb
+{
+    struct abort_txn
+    {
+        void operator()(MDB_txn* ptr) const noexcept
+        {
+            if (ptr)
+                mdb_txn_abort(ptr);
+        }
+    };
+
+    /*!
+        Only valid if used via `create_read_txn()`. Decrements active count in
+        associated `context`, and aborts a LMDB transaction (`mdb_txn_abort`).
+    */
+    struct release_read_txn
+    { 
+        void operator()(MDB_txn* ptr) const noexcept;
+        // implementation in database.cpp
+    };
+
+    /*!
+        Only valid if used via `create_write_txn()`. Decrements active count in
+        associated `context`, and aborts a LMDB transaction (`mdb_txn_abort`).
+    */
+    struct abort_write_txn
+    {
+        void operator()(MDB_txn* ptr) const noexcept
+        {
+            release_read_txn{}(ptr);
+        }
+    };
+
+    struct close_cursor
+    {
+        void operator()(MDB_cursor* ptr) const noexcept
+        {
+            if (ptr)
+                mdb_cursor_close(ptr);
+        }
+    };
+
+    template<typename D>
+    inline expect<std::unique_ptr<MDB_cursor, D>>
+    open_cursor(MDB_txn& txn, MDB_dbi tbl) noexcept
+    {
+        MDB_cursor* cur = nullptr;
+        MONERO_LMDB_CHECK(mdb_cursor_open(&txn, tbl, &cur));
+        return std::unique_ptr<MDB_cursor, D>{cur};
+    }
+
+    // The below use the C++ type system to designate `MDB_txn` status.
+
+    using suspended_txn = std::unique_ptr<MDB_txn, abort_txn>;
+    using read_txn = std::unique_ptr<MDB_txn, release_read_txn>;
+    using write_txn = std::unique_ptr<MDB_txn, abort_write_txn>;
+} // lmdb
diff --git a/src/lmdb/util.h b/src/lmdb/util.h
new file mode 100644
index 000000000..50162b7c8
--- /dev/null
+++ b/src/lmdb/util.h
@@ -0,0 +1,149 @@
+// Copyright (c) 2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#pragma once
+
+#include <cstddef>
+#include <cstring>
+#include <lmdb.h>
+#include <type_traits>
+#include <utility>
+
+#include "span.h"
+
+/*! Calculates types and offset of struct field. Use in template arguments for
+    `table::get_value`, `value_iterator::get_value`,
+    `value_stream::make_iterator`, or `value_stream::make_range`. */
+#define MONERO_FIELD(obj, field)                                     \
+    obj , decltype(std::declval<obj>().field) , offsetof(obj, field)
+
+//! Expands to `lmdb::less` for the value `field` within `obj`.
+#define MONERO_SORT_BY(obj, field)                              \
+    &::lmdb::less<                                              \
+        lmdb::native_type<decltype(std::declval<obj>().field)>, \
+        offsetof(obj, field)                                    \
+    >
+
+//! Expands to `lmdb::compare` for the value `field` within `obj`.
+#define MONERO_COMPARE(obj, field)           \
+    &::lmdb::compare<                        \
+        decltype(std::declval<obj>().field), \
+        offsetof(obj, field)                 \
+    >
+
+namespace lmdb
+{
+    //! Prevent instantiation of `std::underlying_type<T>` when `T` is not enum.
+    template<typename T>
+    struct identity
+    {
+        using type = T;
+    };
+
+    /*!
+        Get the native type for enums, or return `T` unchanged. Useful for
+        merging generated machine code for templated functions that use enums
+        with identical size-widths without relying on aggressive identical
+        comdat folding (ICF) support in linker. So with enum defintion
+        `enum class enum_foo : unsigned long {};` will always yield
+        `assert(&func_foo<unsigned long> == &func_foo<native_type<enum_foo>>)`.
+    */
+    template<typename T>
+    using native_type = typename std::conditional<
+        std::is_enum<T>::value, std::underlying_type<T>, identity<T>
+    >::type::type;
+
+    //! \return `value` as its native type.
+    template<typename T, typename U = typename std::underlying_type<T>::type>
+    inline constexpr U to_native(T value) noexcept
+    {
+        return U(value);
+    }
+
+    //! \return `value` bytes in a LMDB `MDB_val` object.
+    template<typename T>
+    inline MDB_val to_val(T&& value) noexcept
+    {
+        // lmdb does not touch user data, so const_cast is acceptable
+        static_assert(!std::is_rvalue_reference<T&&>(), "cannot use temporary value");
+        void const* const temp = reinterpret_cast<void const*>(std::addressof(value));
+        return MDB_val{sizeof(value), const_cast<void*>(temp)};
+    }
+
+    //! \return A span over the same chunk of memory as `value`.
+    inline constexpr epee::span<const std::uint8_t> to_byte_span(MDB_val value) noexcept
+    {
+        return {static_cast<const std::uint8_t*>(value.mv_data), value.mv_size};
+    }
+
+    /*!
+        A LMDB comparison function that uses `operator<`.
+
+        \tparam T has a defined `operator<` .
+        \tparam offset to `T` within the value.
+
+        \return -1 if `left < right`, 1 if `right < left`, and 0 otherwise.
+    */
+    template<typename T, std::size_t offset = 0>
+    inline int less(MDB_val const* left, MDB_val const* right) noexcept
+    {
+        if (!left || !right || left->mv_size < sizeof(T) + offset || right->mv_size < sizeof(T) + offset)
+        {
+            assert("invalid use of custom comparison" == 0);
+            return -1;
+        }
+
+        T left_val;
+        T right_val;
+        std::memcpy(std::addressof(left_val), static_cast<char*>(left->mv_data) + offset, sizeof(T));
+        std::memcpy(std::addressof(right_val), static_cast<char*>(right->mv_data) + offset, sizeof(T));
+        return left_val < right_val ? -1 : bool(right_val < left_val);
+    }
+
+    /*!
+        A LMDB comparison function that uses `std::memcmp`.
+
+        \toaram T is `!epee::has_padding`
+        \tparam offset to `T` within the value.
+
+        \return The result of `std::memcmp` over the value.
+    */
+    template<typename T, std::size_t offset = 0>
+    inline int compare(MDB_val const* left, MDB_val const* right) noexcept
+    {
+        static_assert(!epee::has_padding<T>(), "memcmp will not work");
+        if (!left || !right || left->mv_size < sizeof(T) + offset || right->mv_size < sizeof(T) + offset)
+        {
+            assert("invalid use of custom comparison" == 0);
+            return -1;
+        }
+        return std::memcmp(
+            static_cast<char*>(left->mv_data) + offset,
+            static_cast<char*>(right->mv_data) + offset,
+            sizeof(T)
+        );
+    }
+} // lmdb
diff --git a/src/lmdb/value_stream.cpp b/src/lmdb/value_stream.cpp
new file mode 100644
index 000000000..1024deb06
--- /dev/null
+++ b/src/lmdb/value_stream.cpp
@@ -0,0 +1,74 @@
+// Copyright (c) 2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#include "value_stream.h"
+
+#include <stdexcept>
+
+#include "common/expect.h"
+#include "lmdb/error.h"
+#include "lmdb/util.h"
+
+namespace lmdb
+{
+    namespace stream
+    {
+        std::size_t count(MDB_cursor* cur)
+        {
+            std::size_t out = 0;
+            if (cur)
+            {
+                const int rc = mdb_cursor_count(cur, &out);
+                if (rc)
+                    MONERO_THROW(lmdb::error(rc), "mdb_cursor_count");
+            }
+            return out;
+        }
+
+        std::pair<epee::span<const std::uint8_t>, epee::span<const std::uint8_t>>
+        get(MDB_cursor& cur, MDB_cursor_op op, std::size_t key, std::size_t value)
+        {
+            MDB_val key_bytes{};
+            MDB_val value_bytes{};
+            const int rc = mdb_cursor_get(&cur, &key_bytes, &value_bytes, op);
+            if (rc)
+            {
+                if (rc == MDB_NOTFOUND)
+                    return {};
+                MONERO_THROW(lmdb::error(rc), "mdb_cursor_get");
+            }
+
+            if (key && key != key_bytes.mv_size)
+                MONERO_THROW(lmdb::error(MDB_BAD_VALSIZE), "mdb_cursor_get key");
+
+            if (value && (value_bytes.mv_size % value != 0 || value_bytes.mv_size == 0))
+                MONERO_THROW(lmdb::error(MDB_BAD_VALSIZE), "mdb_cursor_get value");
+
+            return {lmdb::to_byte_span(key_bytes), lmdb::to_byte_span(value_bytes)};
+        }
+    }
+}
+
diff --git a/src/lmdb/value_stream.h b/src/lmdb/value_stream.h
new file mode 100644
index 000000000..c9977221f
--- /dev/null
+++ b/src/lmdb/value_stream.h
@@ -0,0 +1,287 @@
+// Copyright (c) 2018, The Monero Project
+// All rights reserved.
+//
+// Redistribution and use in source and binary forms, with or without modification, are
+// permitted provided that the following conditions are met:
+//
+// 1. Redistributions of source code must retain the above copyright notice, this list of
+//    conditions and the following disclaimer.
+//
+// 2. Redistributions in binary form must reproduce the above copyright notice, this list
+//    of conditions and the following disclaimer in the documentation and/or other
+//    materials provided with the distribution.
+//
+// 3. Neither the name of the copyright holder nor the names of its contributors may be
+//    used to endorse or promote products derived from this software without specific
+//    prior written permission.
+//
+// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
+// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
+// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
+// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
+// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
+// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+#pragma once
+
+#include <boost/range/iterator_range.hpp>
+#include <cstdint>
+#include <cstring>
+#include <iterator>
+#include <lmdb.h>
+#include <utility>
+
+#include "span.h"
+
+namespace lmdb
+{
+    namespace stream
+    {
+        /*
+            \throw std::system_error if unexpected LMDB error.
+            \return 0 if `cur == nullptr`, otherwise count of values at current key.
+        */
+        std::size_t count(MDB_cursor* cur);
+
+        /*!
+            Calls `mdb_cursor_get` and does some error checking.
+
+            \param cur is given to `mdb_cursor_get` without modification.
+            \param op is passed to `mdb_cursor_get` without modification.
+            \param key expected key size or 0 to skip key size check.
+            \param value expected value size or 0 to skip value size check.
+
+            \throw std::system_error if `key != 0` and `key_.mv_size != key`.
+            \throw std::system_error if `value != 0` and `value_.mv_size != value`.
+            \throw std::system_error if `mdb_cursor_get` returns any error
+                other than `MDB_NOTFOUND`.
+
+            \return {key bytes, value bytes} or two empty spans if `MDB_NOTFOUND`.
+        */
+        std::pair<epee::span<const std::uint8_t>, epee::span<const std::uint8_t>>
+            get(MDB_cursor& cur, MDB_cursor_op op, std::size_t key, std::size_t value);
+    }
+
+    /*!
+        An InputIterator for a fixed-sized LMDB value at a specific key.
+
+        \tparam T The value type at the specific key.
+        \tparam F The value type being returned when dereferenced.
+        \tparam offset to `F` within `T`.
+
+        \note This meets requirements for an InputIterator only. The iterator
+            can only be incremented and dereferenced. All copies of an iterator
+            share the same LMDB cursor, and therefore incrementing any copy will
+            change the cursor state for all (incrementing an iterator will
+            invalidate all prior copies of the iterator). Usage is identical
+            to `std::istream_iterator`.
+    */
+    template<typename T, typename F = T, std::size_t offset = 0>
+    class value_iterator
+    {
+        MDB_cursor* cur;
+        epee::span<const std::uint8_t> values;
+        
+        void increment()
+        {
+            values.remove_prefix(sizeof(T));
+            if (values.empty() && cur)
+                values = lmdb::stream::get(*cur, MDB_NEXT_DUP, 0, sizeof(T)).second;
+        }
+
+    public:
+        using value_type = F;
+        using reference = value_type;
+        using pointer = void;
+        using difference_type = std::size_t;
+        using iterator_category = std::input_iterator_tag;
+
+        //! Construct an "end" iterator.
+        value_iterator() noexcept
+          : cur(nullptr), values()
+        {}
+
+        /*!
+            \param cur Iterate over values starting at this cursor position.
+            \throw std::system_error if unexpected LMDB error. This can happen
+                if `cur` is invalid.
+        */
+        value_iterator(MDB_cursor* cur)
+          : cur(cur), values()
+        {
+            if (cur)
+                values = lmdb::stream::get(*cur, MDB_GET_CURRENT, 0, sizeof(T)).second;
+        }
+
+        value_iterator(value_iterator const&) = default;
+        ~value_iterator() = default;
+        value_iterator& operator=(value_iterator const&) = default;
+
+        //! \return True if `this` is one-past the last value.
+        bool is_end() const noexcept { return values.empty(); }
+
+        //! \return True iff `rhs` is referencing `this` value.
+        bool equal(value_iterator const& rhs) const noexcept
+        {
+            return
+                (values.empty() && rhs.values.empty()) ||
+                values.data() == rhs.values.data();
+        }
+
+        //! Invalidates all prior copies of the iterator.
+        value_iterator& operator++()
+        {
+            increment();
+            return *this;
+        }
+
+        //! \return A copy that is already invalidated, ignore
+        value_iterator operator++(int)
+        {
+            value_iterator out{*this};
+            increment();
+            return out;
+        }
+
+        /*!
+            Get a specific field within `F`. Default behavior is to return
+            the entirety of `U`, despite the filtering logic of `operator*`.
+
+            \pre `!is_end()`
+
+            \tparam U must match `T`, used for `MONERO_FIELD` sanity checking.
+            \tparam G field type to extract from the value
+            \tparam uoffset to `G` type, or `0` when `std::is_same<U, G>()`.
+
+            \return The field `G`, at `uoffset` within `U`.
+        */
+        template<typename U, typename G = U, std::size_t uoffset = 0>
+        G get_value() const noexcept
+        {
+            static_assert(std::is_same<U, T>(), "bad MONERO_FIELD usage?");
+            static_assert(std::is_pod<U>(), "value type must be pod");
+            static_assert(std::is_pod<G>(), "field type must be pod");
+            static_assert(sizeof(G) + uoffset <= sizeof(U), "bad field and/or offset");
+            assert(sizeof(G) + uoffset <= values.size());
+            assert(!is_end());
+
+            G value;
+            std::memcpy(std::addressof(value), values.data() + uoffset, sizeof(value));
+            return value;
+        }
+
+        //! \pre `!is_end()` \return The field `F`, at `offset`, within `T`.
+        value_type operator*() const noexcept { return get_value<T, F, offset>(); }
+    };
+ 
+    /*!
+        C++ wrapper for a LMDB read-only cursor on a fixed-sized value `T`.
+
+        \tparam T value type being stored by each record.
+        \tparam D cleanup functor for the cursor; usually unique per db/table.
+    */
+    template<typename T, typename D>
+    class value_stream
+    {
+        std::unique_ptr<MDB_cursor, D> cur;
+    public:
+
+        //! Take ownership of `cur` without changing position. `nullptr` valid.
+        explicit value_stream(std::unique_ptr<MDB_cursor, D> cur)
+          : cur(std::move(cur))
+        {}
+
+        value_stream(value_stream&&) = default;
+        value_stream(value_stream const&) = delete;
+        ~value_stream() = default;
+        value_stream& operator=(value_stream&&) = default;
+        value_stream& operator=(value_stream const&) = delete;
+
+        /*!
+            Give up ownership of the cursor. `count()`, `make_iterator()` and
+            `make_range()` can still be invoked, but return the empty set.
+
+            \return Currently owned LMDB cursor.
+        */
+        std::unique_ptr<MDB_cursor, D> give_cursor() noexcept
+        {
+            return {std::move(cur)};
+        }
+
+        /*!
+            Place the stream back at the first value. Newly created iterators
+            will start at the first value again.
+
+            \note Invalidates all current iterators from `this`, including
+                those created with `make_iterator` or `make_range`.
+        */
+        void reset()
+        {
+            if (cur)
+                lmdb::stream::get(*cur, MDB_FIRST_DUP, 0, 0);
+        }
+
+        /*!
+            \throw std::system_error if LMDB has unexpected errors.
+            \return Number of values at this key.
+        */
+        std::size_t count() const
+        {
+            return lmdb::stream::count(cur.get());
+        }
+
+        /*!
+            Return a C++ iterator over database values from current cursor
+            position that will reach `.is_end()` after the last duplicate key
+            record. Calling `make_iterator()` will return an iterator whose
+            `operator*` will return entire value (`T`).
+            `make_iterator<MONERO_FIELD(account, id)>()` will return an
+            iterator whose `operator*` will return a `decltype(account.id)`
+            object - the other fields in the struct `account` are never copied
+            from the database.
+
+            \throw std::system_error if LMDB has unexpected errors.
+            \return C++ iterator starting at current cursor position.
+        */
+        template<typename U = T, typename F = U, std::size_t offset = 0>
+        value_iterator<U, F, offset> make_iterator() const
+        {
+            static_assert(std::is_same<U, T>(), "was MONERO_FIELD used with wrong type?");
+            return {cur.get()};
+        }
+
+        /*!
+            Return a range from current cursor position until last duplicate
+            key record. Useful in for-each range loops or in templated code
+            expecting a range of elements. Calling `make_range()` will return
+            a range of `T` objects. `make_range<MONERO_FIELD(account, id)>()`
+            will return a range of `decltype(account.id)` objects - the other
+            fields in the struct `account` are never copied from the database.
+
+            \throw std::system_error if LMDB has unexpected errors.
+            \return An InputIterator range over values at cursor position.
+        */
+        template<typename U = T, typename F = U, std::size_t offset = 0>
+        boost::iterator_range<value_iterator<U, F, offset>> make_range() const
+        {
+            return {make_iterator<U, F, offset>(), value_iterator<U, F, offset>{}};
+        }
+    };
+
+    template<typename T, typename F, std::size_t offset>
+    inline
+    bool operator==(value_iterator<T, F, offset> const& lhs, value_iterator<T, F, offset> const& rhs) noexcept
+    {
+        return lhs.equal(rhs);
+    }
+
+    template<typename T, typename F, std::size_t offset>
+    inline
+    bool operator!=(value_iterator<T, F, offset> const& lhs, value_iterator<T, F, offset> const& rhs) noexcept
+    {
+        return !lhs.equal(rhs);
+    }
+} // lmdb
+