test: wallet, coverage for concurrent db transactions

Verifying that a database handler can't commit/abort changes
occurring in a different database handler.

src/wallet/test/db_tests.cpp

@@ -279,8 +279,48 @@ BOOST_AUTO_TEST_CASE(txn_close_failure_dangling_txn)
     BOOST_CHECK(!batch2->Exists(key));
 }
 
-#endif // USE_SQLITE
+BOOST_AUTO_TEST_CASE(concurrent_txn_dont_interfere)
+{
+    std::string key = "key";
+    std::string value = "value";
+    std::string value2 = "value_2";
 
+    DatabaseOptions options;
+    DatabaseStatus status;
+    bilingual_str error;
+    const auto& database = MakeSQLiteDatabase(m_path_root / "sqlite", options, status, error);
+
+    std::unique_ptr<DatabaseBatch> handler = Assert(database)->MakeBatch();
+
+    // Verify concurrent db transactions does not interfere between each other.
+    // Start db txn, write key and check the key does exist within the db txn.
+    BOOST_CHECK(handler->TxnBegin());
+    BOOST_CHECK(handler->Write(key, value));
+    BOOST_CHECK(handler->Exists(key));
+
+    // But, the same key, does not exist in another handler
+    std::unique_ptr<DatabaseBatch> handler2 = Assert(database)->MakeBatch();
+    BOOST_CHECK(handler2->Exists(key));
+
+    // Attempt to commit the handler txn calling the handler2 methods.
+    // Which, must not be possible.
+    BOOST_CHECK(!handler2->TxnCommit());
+    BOOST_CHECK(!handler2->TxnAbort());
+
+    // Only the first handler can commit the changes.
+    BOOST_CHECK(handler->TxnCommit());
+    // And, once commit is completed, handler2 can read the record
+    std::string read_value;
+    BOOST_CHECK(handler2->Read(key, read_value));
+    BOOST_CHECK_EQUAL(read_value, value);
+
+    // Also, once txn is committed, single write statements are re-enabled.
+    // Which means that handler2 can read the record changes directly.
+    BOOST_CHECK(handler->Write(key, value2, /*fOverwrite=*/true));
+    BOOST_CHECK(handler2->Read(key, read_value));
+    BOOST_CHECK_EQUAL(read_value, value2);
+}
+#endif // USE_SQLITE
 
 BOOST_AUTO_TEST_SUITE_END()
 } // namespace wallet