fix: [2.5] use dummy call for search iterator v2 compatibility (#2570)

issue: milvus-io/milvus#39117
pr: #2569

Signed-off-by: Patrick Weizhi Xu <[email protected]>
(cherry picked from commit 8aaa527)

pymilvus/client/search_iterator.py

@@ -22,9 +22,6 @@
 
 
 class SearchIteratorV2:
-    # for compatibility, save the first result during init
-    _saved_first_res = None
-    _is_saved = False
     # for compatibility, track the number of total results left
     _left_res_cnt = None
 
@@ -51,7 +48,7 @@ def __init__(
             self._left_res_cnt = limit
 
         self._conn = connection
-        self.__set_up_collection_id(collection_name)
+        self._set_up_collection_id(collection_name)
         kwargs[COLLECTION_ID] = self._collection_id
         self._params = {
             "collection_name": collection_name,
@@ -70,35 +67,41 @@ def __init__(
             GUARANTEE_TIMESTAMP: 0,
             **kwargs,
         }
-        # this raises MilvusException if the server does not support V2
-        self._saved_first_res = self.next()
-        self._is_saved = True
+        self._probe_for_compability(self._params)
 
-    def __set_up_collection_id(self, collection_name: str):
+    def _set_up_collection_id(self, collection_name: str):
         res = self._conn.describe_collection(collection_name)
         self._collection_id = res[COLLECTION_ID]
 
+    def _check_token_exists(self, token: Union[str, None]):
+        if token is None or token == "":
+            raise ServerVersionIncompatibleException(
+                message=ExceptionsMessage.SearchIteratorV2FallbackWarning
+            )
+
+    # this detects whether the server supports search_iterator_v2 and is for compatibility only
+    # if the server holds iterator states, this implementation needs to be reconsidered
+    def _probe_for_compability(self, params: Dict):
+        dummy_params = deepcopy(params)
+        dummy_batch_size = 1
+        dummy_params["limit"] = dummy_batch_size
+        dummy_params[ITER_SEARCH_BATCH_SIZE_KEY] = dummy_batch_size
+        iter_info = self._conn.search(**dummy_params).get_search_iterator_v2_results_info()
+        self._check_token_exists(iter_info.token)
+
     def next(self):
-        # for compatibility
-        if self._is_saved:
-            self._is_saved = False
-            return self._saved_first_res
-        self._saved_first_res = None
         if self._left_res_cnt is not None and self._left_res_cnt <= 0:
             return SearchPage(None)
 
         res = self._conn.search(**self._params)
         iter_info = res.get_search_iterator_v2_results_info()
+        self._check_token_exists(iter_info.token)
         self._params[ITER_SEARCH_LAST_BOUND_KEY] = iter_info.last_bound
 
         # patch token and guarantee timestamp for the first next() call
         if ITER_SEARCH_ID_KEY not in self._params:
-            if iter_info.token is not None and iter_info.token != "":
-                self._params[ITER_SEARCH_ID_KEY] = iter_info.token
-            else:
-                raise ServerVersionIncompatibleException(
-                    message=ExceptionsMessage.SearchIteratorV2FallbackWarning
-                )
+            # the token should not change during the lifetime of the iterator
+            self._params[ITER_SEARCH_ID_KEY] = iter_info.token
         if self._params[GUARANTEE_TIMESTAMP] <= 0:
             if res.get_session_ts() > 0:
                 self._params[GUARANTEE_TIMESTAMP] = res.get_session_ts()