fix(server): handle running script load inside multi (#4074)

Signed-off-by: adi_holden <adi@dragonflydb.io>

src/server/common.h

@@ -47,6 +47,8 @@ using RdbTypeFreqMap = absl::flat_hash_map<unsigned, size_t>;
 class CommandId;
 class Transaction;
 class EngineShard;
+class ConnectionState;
+class Interpreter;
 
 struct LockTagOptions {
   bool enabled = false;
@@ -353,6 +355,29 @@ template <typename Mutex> class ABSL_SCOPED_LOCKABLE SharedLock {
   bool is_locked_;
 };
 
+// Ensures availability of an interpreter for EVAL-like commands and it's automatic release.
+// If it's part of MULTI, the preborrowed interpreter is returned, otherwise a new is acquired.
+struct BorrowedInterpreter {
+  BorrowedInterpreter(Transaction* tx, ConnectionState* state);
+
+  ~BorrowedInterpreter();
+
+  // Give up ownership of the interpreter, it must be returned manually.
+  Interpreter* Release() && {
+    DCHECK(owned_);
+    owned_ = false;
+    return interpreter_;
+  }
+
+  operator Interpreter*() {
+    return interpreter_;
+  }
+
+ private:
+  Interpreter* interpreter_ = nullptr;
+  bool owned_ = false;
+};
+
 extern size_t serialization_max_chunk_size;
 
 }  // namespace dfly