Merge bitcoin/bitcoin#27790: walletdb: Add PrefixCursor

ba616b932cb9e9adb7eb9f1826caa62ce422a22d wallet: Add GetPrefixCursor to DatabaseBatch (Andrew Chow)
1d858b055daeea363e0450f327672658548be4c6 walletdb: Handle when database keys are empty (Ryan Ofsky)
84b2f353bbefb9264284e7430863b2fa1d796d38 walletdb: Consistently clear key and value streams before writing (Andrew Chow)

Pull request description:

  Split from #24914 as suggested in https://github.com/bitcoin/bitcoin/pull/24914#pullrequestreview-1442091917

  This PR adds a wallet database cursor that gives a view over all of the records beginning with the same prefix.

ACKs for top commit:
  ryanofsky:
    Code review ACK ba616b932cb9e9adb7eb9f1826caa62ce422a22d. Just suggested changes since last review
  furszy:
    ACK ba616b93

Tree-SHA512: 38a61849f108d8003d28c599b1ad0421ac9beb3afe14c02f1253e7b4efc3d4eef483e32647a820fc6636bca3f9efeff9fe062b6b602e0cded69f21f8b26af544

1 files changed, 24 insertions, 5 deletions

diff --git a/src/wallet/bdb.cpp b/src/wallet/bdb.cpp
index 6dce51fc12..68abdcd81e 100644
--- a/src/wallet/bdb.cpp
+++ b/src/wallet/bdb.cpp
@@ -668,7 +668,8 @@ void BerkeleyDatabase::ReloadDbEnv()
     env->ReloadDbEnv();
 }
 
-BerkeleyCursor::BerkeleyCursor(BerkeleyDatabase& database, const BerkeleyBatch& batch)
+BerkeleyCursor::BerkeleyCursor(BerkeleyDatabase& database, const BerkeleyBatch& batch, Span<const std::byte> prefix)
+    : m_key_prefix(prefix.begin(), prefix.end())
 {
     if (!database.m_db.get()) {
         throw std::runtime_error(STR_INTERNAL_BUG("BerkeleyDatabase does not exist"));
@@ -685,19 +686,30 @@ DatabaseCursor::Status BerkeleyCursor::Next(DataStream& ssKey, DataStream& ssVal
 {
     if (m_cursor == nullptr) return Status::FAIL;
     // Read at cursor
-    SafeDbt datKey;
+    SafeDbt datKey(m_key_prefix.data(), m_key_prefix.size());
     SafeDbt datValue;
-    int ret = m_cursor->get(datKey, datValue, DB_NEXT);
+    int ret = -1;
+    if (m_first && !m_key_prefix.empty()) {
+        ret = m_cursor->get(datKey, datValue, DB_SET_RANGE);
+    } else {
+        ret = m_cursor->get(datKey, datValue, DB_NEXT);
+    }
+    m_first = false;
     if (ret == DB_NOTFOUND) {
         return Status::DONE;
     }
-    if (ret != 0 || datKey.get_data() == nullptr || datValue.get_data() == nullptr) {
+    if (ret != 0) {
         return Status::FAIL;
     }
 
+    Span<const std::byte> raw_key = {AsBytePtr(datKey.get_data()), datKey.get_size()};
+    if (!m_key_prefix.empty() && std::mismatch(raw_key.begin(), raw_key.end(), m_key_prefix.begin(), m_key_prefix.end()).second != m_key_prefix.end()) {
+        return Status::DONE;
+    }
+
     // Convert to streams
     ssKey.clear();
-    ssKey.write({AsBytePtr(datKey.get_data()), datKey.get_size()});
+    ssKey.write(raw_key);
     ssValue.clear();
     ssValue.write({AsBytePtr(datValue.get_data()), datValue.get_size()});
     return Status::MORE;
@@ -716,6 +728,12 @@ std::unique_ptr<DatabaseCursor> BerkeleyBatch::GetNewCursor()
     return std::make_unique<BerkeleyCursor>(m_database, *this);
 }
 
+std::unique_ptr<DatabaseCursor> BerkeleyBatch::GetNewPrefixCursor(Span<const std::byte> prefix)
+{
+    if (!pdb) return nullptr;
+    return std::make_unique<BerkeleyCursor>(m_database, *this, prefix);
+}
+
 bool BerkeleyBatch::TxnBegin()
 {
     if (!pdb || activeTxn)
@@ -777,6 +795,7 @@ bool BerkeleyBatch::ReadKey(DataStream&& key, DataStream& value)
     SafeDbt datValue;
     int ret = pdb->get(activeTxn, datKey, datValue, 0);
     if (ret == 0 && datValue.get_data() != nullptr) {
+        value.clear();
         value.write({AsBytePtr(datValue.get_data()), datValue.get_size()});
         return true;
     }