Introduce new DB interface

Makefile

@@ -147,6 +147,7 @@ cosmovisor:
 mocks: $(MOCKS_DIR)
 	mockgen -source=client/account_retriever.go -package mocks -destination tests/mocks/account_retriever.go
 	mockgen -package mocks -destination tests/mocks/tendermint_tm_db_DB.go github.com/tendermint/tm-db DB
+	mockgen -package mocks -destination tests/mocks/db/db_DB.go github.com/cosmos/cosmos-sdk/db DBReadWriter
 	mockgen -source=types/module/module.go -package mocks -destination tests/mocks/types_module_module.go
 	mockgen -source=types/invariant.go -package mocks -destination tests/mocks/types_invariant.go
 	mockgen -source=types/router.go -package mocks -destination tests/mocks/types_router.go

db/adapter.go

@@ -0,0 +1,30 @@
+package db
+
+import "errors"
+
+type readerRWAdapter struct{ DBReader }
+
+var (
+	// ErrReadOnly is returned when a write operation is attempted on a read-only transaction.
+	ErrReadOnly = errors.New("Cannot modify read-only transaction")
+)
+
+// Returns a ReadWriter that forwards to a reader and errors if writes are
+// attempted. Can be used to pass a Reader when a ReadWriter is expected
+// but no writes will actually occur.
+func NewReadWriterFromReader(r DBReader) DBReadWriter {
+	return readerRWAdapter{r}
+}
+
+func (readerRWAdapter) Set([]byte, []byte) error {
+	return ErrReadOnly
+}
+
+func (readerRWAdapter) Delete([]byte) error {
+	return ErrReadOnly
+}
+
+func (rw readerRWAdapter) Commit() error {
+	rw.Discard()
+	return nil
+}