fix(opensearch): keep the BM25 leg in hybrid search (#15760)

### What problem does this PR solve?

Fixes the OpenSearch side of #10747: hybrid search drops the keyword
(BM25) leg and
ends up doing plain vector search.

When a search has both a text and a vector leg, `OSConnection.search()`
throws the text
query away:

    del q["query"]
    q["query"] = {"knn": knn_query}

The text clause only stays on as a filter inside the knn query, so it
narrows the
candidate set but doesn't count towards scoring. So hybrid search on
OpenSearch behaves
like plain vector search, unlike the Elasticsearch backend.

What I changed:

- when both legs are present, send a real hybrid query
`{"hybrid": {"queries": [bm25, {"knn": ...}]}}` and let a
normalization-processor
  search pipeline score and combine the two legs
- only the actual filters (kb_id, available_int, ...) go in the knn
filter, not the
  text must clause
- create the pipeline on startup if it's missing, so there's no separate
provisioning
step. name and weights can be set under `os:` in service_conf.yaml, or
via
`OS_HYBRID_PIPELINE`; defaults are `ragflow_hybrid_pipeline` and `[0.5,
0.5]`
- normalization-processor needs OpenSearch 2.10+. on older clusters, or
when the
pipeline can't be created, log a warning and fall back to vector-only
instead of
  pointing at a pipeline that doesn't exist

This is only the hybrid-search fix; `create_doc_meta_idx` is already on
main.

Testing (there's no OpenSearch path in CI): added a unit test
(`test/unit_test/rag/utils/test_opensearch_hybrid_search.py`, no
services needed) that
checks the query built in each case — hybrid + pipeline param for
text+vector, plain knn
for vector-only, plain bool for text-only, the knn filter never carrying
the text
query_string, and the vector-only fallback when the pipeline isn't
available. Also ran
it against a real OpenSearch 2.19.1 container with a doc that matches
the keyword but
sits outside the knn top-k: pure knn returns `['D1','D2','D5']` (keyword
doc missing),
the hybrid query returns `['A','D1','D2','D5']` (keyword doc present).

### Type of change

- [x] Bug Fix (non-breaking change which fixes an issue)

Signed-off-by: Danut Matei <matei.danut.dm@gmail.com>

test/unit_test/rag/utils/test_opensearch_hybrid_search.py

@@ -0,0 +1,219 @@
+#
+#  Copyright 2025 The InfiniFlow Authors. All Rights Reserved.
+#
+#  Licensed under the Apache License, Version 2.0 (the "License");
+#  you may not use this file except in compliance with the License.
+#  You may obtain a copy of the License at
+#
+#      http://www.apache.org/licenses/LICENSE-2.0
+#
+#  Unless required by applicable law or agreed to in writing, software
+#  distributed under the License is distributed on an "AS IS" BASIS,
+#  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+#  See the License for the specific language governing permissions and
+#  limitations under the License.
+#
+"""
+Tests for the query OSConnection.search() builds for hybrid search.
+
+#10747: when a query had both a text and a vector leg, the text leg got dropped
+(del q["query"]; q["query"] = {"knn": ...}) and only survived as a knn filter,
+so hybrid search on OpenSearch was effectively vector-only. The Elasticsearch
+backend doesn't have this problem.
+
+These check the request body/params for each text/vector combination with the
+client mocked, so no cluster is needed.
+"""
+
+from __future__ import annotations
+
+import sys
+import types
+from unittest.mock import MagicMock
+
+import pytest
+
+
+# Importing OSConnection touches opensearchpy at module load, so guard for
+# environments where the package isn't installed.
+opensearchpy = pytest.importorskip("opensearchpy")
+
+
+def _install_module(name: str, **attrs) -> types.ModuleType:
+    mod = sys.modules.get(name)
+    if mod is None:
+        mod = types.ModuleType(name)
+        sys.modules[name] = mod
+    for key, value in attrs.items():
+        if not hasattr(mod, key):
+            setattr(mod, key, value)
+    return mod
+
+
+def _install_module_stubs() -> None:
+    """Replace the heavy modules opensearch_conn imports at load time.
+
+    ``rag.utils.opensearch_conn`` imports ``common.settings`` (which pulls every
+    storage backend) and ``rag.nlp`` at module load. We stub just those so the
+    real ``OSConnection`` class can be imported without a live environment.
+    """
+    _install_module(
+        "common.settings",
+        OS={"hosts": "stub", "username": "u", "password": "p"},
+        ES={},
+        DOC_ENGINE_INFINITY=False,
+        DOC_ENGINE_OCEANBASE=False,
+        DOC_ENGINE="opensearch",
+        docStoreConn=None,
+    )
+    _install_module(
+        "rag.nlp",
+        is_english=lambda *_args, **_kwargs: False,
+        rag_tokenizer=MagicMock(),
+    )
+
+
+_install_module_stubs()
+
+from common.doc_store.doc_store_base import (  # noqa: E402
+    FusionExpr,
+    MatchDenseExpr,
+    MatchTextExpr,
+)
+
+
+def _resolve_os_connection_class():
+    """Return the real OSConnection class.
+
+    ``@singleton`` wraps the class in a closure that returns a cached instance
+    on call, so ``opensearch_conn.OSConnection`` at module scope is a function,
+    not a type. Unwrap it so we can ``__new__`` an instance directly and bypass
+    the network-dependent ``__init__``.
+    """
+    from rag.utils import opensearch_conn
+
+    candidate = opensearch_conn.OSConnection
+    if isinstance(candidate, type):
+        return candidate
+    closure = getattr(candidate, "__closure__", None) or ()
+    for cell in closure:
+        contents = cell.cell_contents
+        if isinstance(contents, type):
+            return contents
+    raise RuntimeError("Could not locate the OSConnection class in module scope")
+
+
+def _make_os_connection(hybrid_search_enabled: bool = True):
+    """Build an OSConnection without invoking its real ``__init__``."""
+    cls = _resolve_os_connection_class()
+    conn = cls.__new__(cls)
+    conn.os = MagicMock()
+    conn.os.search.return_value = {
+        "hits": {"total": {"value": 0}, "hits": []},
+        "timed_out": False,
+    }
+    conn.info = {"version": {"number": "2.18.0"}}
+    conn.hybrid_search_enabled = hybrid_search_enabled
+    conn._hybrid_pipeline = "ragflow_hybrid_pipeline"
+    return conn
+
+
+def _text_expr():
+    return MatchTextExpr(fields=["content_ltks"], matching_text="what is kubernetes", topn=10, extra_options={})
+
+
+def _dense_expr():
+    return MatchDenseExpr(
+        vector_column_name="q_1024_vec",
+        embedding_data=[0.1] * 8,
+        embedding_data_type="float",
+        distance_type="cosine",
+        topn=5,
+        extra_options={"similarity": 0.0},
+    )
+
+
+def _fusion_expr():
+    return FusionExpr(method="weighted_sum", topn=5, fusion_params={"weights": "0.5,0.5"})
+
+
+def _call_search(conn, match_expressions):
+    """Call search() and return (body, params) handed to the OpenSearch client."""
+    conn.search(
+        select_fields=["content_ltks"],
+        highlight_fields=[],
+        condition={},
+        match_expressions=match_expressions,
+        order_by=None,
+        offset=0,
+        limit=10,
+        index_names=["idx1"],
+        knowledgebase_ids=["kb1"],
+    )
+    call = conn.os.search.call_args
+    return call.kwargs.get("body"), call.kwargs.get("params")
+
+
+class TestHybridSearchDSL:
+    def test_hybrid_query_structure(self):
+        """text + vector must produce a {"hybrid": {"queries": [bool, {"knn": ...}]}}."""
+        conn = _make_os_connection()
+        body, _ = _call_search(conn, [_text_expr(), _dense_expr(), _fusion_expr()])
+
+        assert "hybrid" in body["query"], "hybrid query not present"
+        queries = body["query"]["hybrid"]["queries"]
+        assert len(queries) == 2, "hybrid must have exactly two sub-queries"
+        keyword_q, knn_q = queries
+        assert "bool" in keyword_q, "first hybrid leg must be the keyword bool query"
+        assert "knn" in knn_q, "second hybrid leg must be the knn query"
+
+    def test_hybrid_passes_search_pipeline_param(self):
+        conn = _make_os_connection()
+        _, params = _call_search(conn, [_text_expr(), _dense_expr(), _fusion_expr()])
+
+        assert params is not None, "search_pipeline params must be passed for hybrid search"
+        assert params.get("search_pipeline") == "ragflow_hybrid_pipeline"
+
+    def test_knn_only_query_structure(self):
+        """vector only must stay a pure knn query with no pipeline param."""
+        conn = _make_os_connection()
+        body, params = _call_search(conn, [_dense_expr()])
+
+        assert "knn" in body["query"], "knn-only search must use a knn query"
+        assert "hybrid" not in body["query"], "knn-only must not be hybrid"
+        assert params is None, "knn-only must not pass a search_pipeline"
+
+    def test_text_only_query_structure(self):
+        """text only must stay a bool query with no knn/hybrid and no pipeline."""
+        conn = _make_os_connection()
+        body, params = _call_search(conn, [_text_expr()])
+
+        assert "knn" not in body.get("query", {}), "text-only must not use knn"
+        assert "hybrid" not in body.get("query", {}), "text-only must not use hybrid"
+        assert params is None, "text-only must not pass a search_pipeline"
+
+    def test_knn_filter_excludes_text_must_clause(self):
+        """The KNN pre-filter must carry only filter conditions, never the
+        text query_string must-clause (the root cause of #10747)."""
+        conn = _make_os_connection()
+        body, _ = _call_search(conn, [_text_expr(), _dense_expr(), _fusion_expr()])
+
+        knn_clause = body["query"]["hybrid"]["queries"][1]["knn"]
+        vec_params = next(iter(knn_clause.values()))
+        knn_filter = vec_params.get("filter", {})
+        assert "query_string" not in str(knn_filter), "knn filter must not include the text query_string clause"
+
+    def test_falls_back_to_knn_when_pipeline_unavailable(self):
+        """When the normalization pipeline could not be provisioned (e.g. cluster
+        < 2.10 or insufficient privileges), a text+vector query must degrade to a
+        pure knn query rather than reference a non-existent pipeline."""
+        conn = _make_os_connection(hybrid_search_enabled=False)
+        body, params = _call_search(conn, [_text_expr(), _dense_expr(), _fusion_expr()])
+
+        assert "hybrid" not in body["query"], "must not build a hybrid query without a pipeline"
+        assert "knn" in body["query"], "must fall back to a pure knn query"
+        assert params is None, "must not reference a search_pipeline when disabled"
+
+
+if __name__ == "__main__":
+    raise SystemExit(pytest.main([__file__, "-v"]))