wallet: add dummy BerkeleyRODatabase and BerkeleyROBatch classes

BerkeleyRODatabase and BerkeleyROBatch will be used to access a BDB file
without the use of BDB. For now, these are dummy classes.

src/Makefile.am

@@ -347,6 +347,7 @@ BITCOIN_CORE_H = \
   wallet/feebumper.h \
   wallet/fees.h \
   wallet/load.h \
+  wallet/migrate.h \
   wallet/receive.h \
   wallet/rpc/util.h \
   wallet/rpc/wallet.h \
@@ -507,6 +508,7 @@ libbitcoin_wallet_a_SOURCES = \
   wallet/fees.cpp \
   wallet/interfaces.cpp \
   wallet/load.cpp \
+  wallet/migrate.cpp \
   wallet/receive.cpp \
   wallet/rpc/addresses.cpp \
   wallet/rpc/backup.cpp \

src/wallet/migrate.cpp

@@ -0,0 +1,37 @@
+// Distributed under the MIT software license, see the accompanying
+// file COPYING or http://www.opensource.org/licenses/mit-license.php.
+
+#include <wallet/migrate.h>
+
+namespace wallet {
+
+void BerkeleyRODatabase::Open()
+{
+}
+
+std::unique_ptr<DatabaseBatch> BerkeleyRODatabase::MakeBatch(bool flush_on_close)
+{
+    return std::make_unique<BerkeleyROBatch>(*this);
+}
+
+bool BerkeleyRODatabase::Backup(const std::string& dest) const
+{
+    return false;
+}
+
+bool BerkeleyROBatch::ReadKey(DataStream&& key, DataStream& value)
+{
+    return false;
+}
+
+bool BerkeleyROBatch::HasKey(DataStream&& key)
+{
+    return false;
+}
+
+DatabaseCursor::Status BerkeleyROCursor::Next(DataStream& ssKey, DataStream& ssValue)
+{
+    return DatabaseCursor::Status::FAIL;
+}
+
+} // namespace wallet

src/wallet/migrate.h

@@ -0,0 +1,108 @@
+// Copyright (c) 2021 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_WALLET_MIGRATE_H
+#define BITCOIN_WALLET_MIGRATE_H
+
+#include <wallet/db.h>
+
+#include <optional>
+
+namespace wallet {
+/**
+ * A class representing a BerkeleyDB file from which we can only read records.
+ * This is used only for migration of legacy to descriptor wallets
+ */
+class BerkeleyRODatabase : public WalletDatabase
+{
+private:
+    const fs::path m_filepath;
+
+public:
+    /** Create DB handle */
+    BerkeleyRODatabase(const fs::path& filepath, bool open = true) : WalletDatabase(), m_filepath(filepath)
+    {
+        if (open) Open();
+    }
+    ~BerkeleyRODatabase(){};
+
+    /** Open the database if it is not already opened. */
+    void Open() override;
+
+    /** Indicate the a new database user has began using the database. Increments m_refcount */
+    void AddRef() override {}
+    /** Indicate that database user has stopped using the database and that it could be flushed or closed. Decrement m_refcount */
+    void RemoveRef() override {}
+
+    /** Rewrite the entire database on disk, with the exception of key pszSkip if non-zero
+     */
+    bool Rewrite(const char* pszSkip = nullptr) override { return false; }
+
+    /** Back up the entire database to a file.
+     */
+    bool Backup(const std::string& strDest) const override;
+
+    /** Make sure all changes are flushed to database file.
+     */
+    void Flush() override {}
+    /** Flush to the database file and close the database.
+     *  Also close the environment if no other databases are open in it.
+     */
+    void Close() override {}
+    /* flush the wallet passively (TRY_LOCK)
+       ideal to be called periodically */
+    bool PeriodicFlush() override { return false; }
+
+    void IncrementUpdateCounter() override {}
+
+    void ReloadDbEnv() override {}
+
+    /** Return path to main database file for logs and error messages. */
+    std::string Filename() override { return fs::PathToString(m_filepath); }
+
+    std::string Format() override { return "bdb_ro"; }
+
+    /** Make a DatabaseBatch connected to this database */
+    std::unique_ptr<DatabaseBatch> MakeBatch(bool flush_on_close = true) override;
+};
+
+class BerkeleyROCursor : public DatabaseCursor
+{
+public:
+    Status Next(DataStream& key, DataStream& value) override;
+};
+
+/** RAII class that provides access to a BerkeleyRODatabase */
+class BerkeleyROBatch : public DatabaseBatch
+{
+private:
+    const BerkeleyRODatabase& m_database;
+
+    bool ReadKey(DataStream&& key, DataStream& value) override;
+    // WriteKey returns true since various automatic upgrades for older wallets will expect writing to not fail.
+    // It is okay for this batch type to not actually write anything as those automatic upgrades will occur again after migration.
+    bool WriteKey(DataStream&& key, DataStream&& value, bool overwrite = true) override { return true; }
+    bool EraseKey(DataStream&& key) override { return false; }
+    bool HasKey(DataStream&& key) override;
+    bool ErasePrefix(Span<const std::byte> prefix) override { return false; }
+
+public:
+    explicit BerkeleyROBatch(const BerkeleyRODatabase& database) : m_database(database) {}
+    ~BerkeleyROBatch() {}
+
+    BerkeleyROBatch(const BerkeleyROBatch&) = delete;
+    BerkeleyROBatch& operator=(const BerkeleyROBatch&) = delete;
+
+    void Flush() override {}
+    void Close() override {}
+
+    std::unique_ptr<DatabaseCursor> GetNewCursor() override { return std::make_unique<BerkeleyROCursor>(); }
+    std::unique_ptr<DatabaseCursor> GetNewPrefixCursor(Span<const std::byte> prefix) override { return std::make_unique<BerkeleyROCursor>(); }
+    bool TxnBegin() override { return false; }
+    bool TxnCommit() override { return false; }
+    bool TxnAbort() override { return false; }
+};
+} // namespace wallet
+
+#endif // BITCOIN_WALLET_MIGRATE_H