Merge bitcoin/bitcoin#27556: wallet: fix deadlock in bdb read write operation

69d43905b7 test: add coverage for wallet read write db deadlock (furszy)
12daf6fcdc walletdb: scope bdb::EraseRecords under a single db txn (furszy)
043fcb0b05 wallet: bugfix, GetNewCursor() misses to provide batch ptr to BerkeleyCursor (furszy)

Pull request description:

  Decoupled from #26644 so it can closed in favor of #26715.

  Basically, with bdb, we can't make a write operation while we are traversing the db with the same db handler. These two operations are performed in different txn contexts and cause a deadlock.

  Added coverage by using `EraseRecords()` which is the simplest function that executes this process.

  To replicate it, need bdb support and drop the first commit. The test will run forever.

ACKs for top commit:
  achow101:
    ACK 69d43905b7
  hebasto:
    re-ACK 69d43905b7

Tree-SHA512: b3773be78925f674e962f4a5c54b398a9d0cfe697148c01c3ec0d68281cc5c1444b38165960d219ef3cf1a57c8ce6427f44a876275958d49bbc0808486e19d7d

src/wallet/walletdb.cpp

@@ -1136,6 +1136,9 @@ bool WalletBatch::WriteWalletFlags(const uint64_t flags)
 
 bool WalletBatch::EraseRecords(const std::unordered_set<std::string>& types)
 {
+    // Begin db txn
+    if (!m_batch->TxnBegin()) return false;
+
     // Get cursor
     std::unique_ptr<DatabaseCursor> cursor = m_batch->GetNewCursor();
     if (!cursor)
@@ -1144,8 +1147,7 @@ bool WalletBatch::EraseRecords(const std::unordered_set<std::string>& types)
     }
 
     // Iterate the DB and look for any records that have the type prefixes
-    while (true)
-    {
+    while (true) {
         // Read next record
         DataStream key{};
         DataStream value{};
@@ -1153,6 +1155,8 @@ bool WalletBatch::EraseRecords(const std::unordered_set<std::string>& types)
         if (status == DatabaseCursor::Status::DONE) {
             break;
         } else if (status == DatabaseCursor::Status::FAIL) {
+            cursor.reset(nullptr);
+            m_batch->TxnAbort(); // abort db txn
             return false;
         }
 
@@ -1163,10 +1167,16 @@ bool WalletBatch::EraseRecords(const std::unordered_set<std::string>& types)
         key >> type;
 
         if (types.count(type) > 0) {
-            m_batch->Erase(key_data);
+            if (!m_batch->Erase(key_data)) {
+                cursor.reset(nullptr);
+                m_batch->TxnAbort();
+                return false; // erase failed
+            }
         }
     }
-    return true;
+    // Finish db txn
+    cursor.reset(nullptr);
+    return m_batch->TxnCommit();
 }
 
 bool WalletBatch::TxnBegin()