Merge bitcoin/bitcoin#26690: wallet: Refactor database cursor into its own object with proper return codes

4aebd832a405090c2608e4b60bb4f34501bcea61 db: Change DatabaseCursor::Next to return status enum (Andrew Chow)
d79e8dcf2981ef1964a2fde8c472b5de1ca1c963 wallet: Have cursor users use DatabaseCursor directly (Andrew Chow)
7a198bba0a1d0a0f0fd4ca947955cb52b84bdd4b wallet: Introduce DatabaseCursor RAII class for managing cursor (Andrew Chow)
69efbc011bb74fcd8dd9ed2a8a5d31bc9e323c10 Move SafeDbt out of BerkeleyBatch (Andrew Chow)

Pull request description:

  Instead of having database cursors be tied to a particular `DatabaseBatch` object and requiring its setup and teardown be separate functions in that batch, we can have cursors be separate RAII classes. This makes it easier to create and destroy cursors as well as having cursors that have slightly different behaviors.

  Additionally, since reading data from a cursor is a tri-state, this PR changes the return value of the `Next` function (formerly `ReadAtCursor`) to return an Enum rather than the current system of 2 booleans. This greatly simplifies and unifies the code that deals with cursors as now there is no confusion as to what the function returns when there are no records left to be read.

  Extracted from #24914

ACKs for top commit:
  furszy:
    diff ACK 4aebd83
  theStack:
    Code-review ACK 4aebd832a405090c2608e4b60bb4f34501bcea61

Tree-SHA512: 5d0be56a18de5b08c777dd5a73ba5a6ef1e696fdb07d1dca952a88ded07887b7c5c04342f9a76feb2f6fe24a45dc31f094f1f5d9500e6bdf4a44f4edb66dcaa1

1 files changed, 31 insertions, 22 deletions

diff --git a/src/wallet/bdb.h b/src/wallet/bdb.h
index 40a1031c8e..3881419d45 100644
--- a/src/wallet/bdb.h
+++ b/src/wallet/bdb.h
@@ -165,29 +165,41 @@ public:
     std::unique_ptr<DatabaseBatch> MakeBatch(bool flush_on_close = true) override;
 };
 
-/** RAII class that provides access to a Berkeley database */
-class BerkeleyBatch : public DatabaseBatch
+/** RAII class that automatically cleanses its data on destruction */
+class SafeDbt final
 {
-    /** RAII class that automatically cleanses its data on destruction */
-    class SafeDbt final
-    {
-        Dbt m_dbt;
+    Dbt m_dbt;
 
-    public:
-        // construct Dbt with internally-managed data
-        SafeDbt();
-        // construct Dbt with provided data
-        SafeDbt(void* data, size_t size);
-        ~SafeDbt();
+public:
+    // construct Dbt with internally-managed data
+    SafeDbt();
+    // construct Dbt with provided data
+    SafeDbt(void* data, size_t size);
+    ~SafeDbt();
+
+    // delegate to Dbt
+    const void* get_data() const;
+    uint32_t get_size() const;
+
+    // conversion operator to access the underlying Dbt
+    operator Dbt*();
+};
 
-        // delegate to Dbt
-        const void* get_data() const;
-        uint32_t get_size() const;
+class BerkeleyCursor : public DatabaseCursor
+{
+private:
+    Dbc* m_cursor;
 
-        // conversion operator to access the underlying Dbt
-        operator Dbt*();
-    };
+public:
+    explicit BerkeleyCursor(BerkeleyDatabase& database);
+    ~BerkeleyCursor() override;
+
+    Status Next(CDataStream& key, CDataStream& value) override;
+};
 
+/** RAII class that provides access to a Berkeley database */
+class BerkeleyBatch : public DatabaseBatch
+{
 private:
     bool ReadKey(CDataStream&& key, CDataStream& value) override;
     bool WriteKey(CDataStream&& key, CDataStream&& value, bool overwrite = true) override;
@@ -198,7 +210,6 @@ protected:
     Db* pdb;
     std::string strFile;
     DbTxn* activeTxn;
-    Dbc* m_cursor;
     bool fReadOnly;
     bool fFlushOnClose;
     BerkeleyEnvironment *env;
@@ -214,9 +225,7 @@ public:
     void Flush() override;
     void Close() override;
 
-    bool StartCursor() override;
-    bool ReadAtCursor(CDataStream& ssKey, CDataStream& ssValue, bool& complete) override;
-    void CloseCursor() override;
+    std::unique_ptr<DatabaseCursor> GetNewCursor() override;
     bool TxnBegin() override;
     bool TxnCommit() override;
     bool TxnAbort() override;