Add invlists null check to IndexBinaryIVF::search (#4980)

Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/4980

IndexBinaryIVF::search dereferences invlists via
invlists->prefetch_lists() without any prior null check. The invlists
pointer can legitimately be nullptr through multiple paths:

1. Default construction: IndexBinaryIVF::invlists is initialized to
   nullptr (IndexBinaryIVF.h:35) and the default constructor is
   defaulted, leaving it null.

2. Deserialization with IO_FLAG_SKIP_IVF_DATA: When this flag is set,
   read_InvertedLists_up returns nullptr, and the binary IVF reader
   at index_read.cpp:2130 stores it directly (ils.release() on a null
   unique_ptr yields nullptr). This is intentional — the flag allows
   loading index metadata without the (potentially large) inverted
   list data.

3. Deserialization with fourcc "il00": The serialized data explicitly
   indicates inverted lists were not stored with the IVF object
   (index_read.cpp:453), returning nullptr.

Without this check, any of these cases would segfault on the
invlists->prefetch_lists() call. With this check, the caller receives
a clear FaissException explaining the misconfiguration.

This mirrors the existing null check in the float IndexIVF::search
(IndexIVF.cpp:312) which already guards against the same condition.

Reviewed By: mnorris11

Differential Revision: D97848088

fbshipit-source-id: 406abb870dfa093ffc3ce49ba65904bebe4763a2

faiss/IndexBinaryIVF.cpp

@@ -124,6 +124,7 @@ void IndexBinaryIVF::search(
         idx_t* labels,
         const SearchParameters* params_in) const {
     FAISS_THROW_IF_NOT(k > 0);
+    FAISS_THROW_IF_NOT_MSG(invlists, "IVF index has no inverted lists");
     const IVFSearchParameters* params = nullptr;
     if (params_in) {
         params = dynamic_cast<const IVFSearchParameters*>(params_in);

tests/test_read_index_deserialize.cpp

@@ -15,6 +15,7 @@
 #include <faiss/Index.h>
 #include <faiss/IndexBinary.h>
 #include <faiss/IndexBinaryHNSW.h>
+#include <faiss/IndexBinaryIVF.h>
 #include <faiss/IndexFlat.h>
 #include <faiss/IndexHNSW.h>
 #include <faiss/IndexIVFFlat.h>
@@ -1924,3 +1925,33 @@ TEST(ReadIndexDeserialize, BinaryHNSWCagraZeroEntrypoints) {
             },
             faiss::FaissException);
 }
+
+// ---- IndexBinaryIVF runtime safety checks ----
+
+TEST(ReadIndexDeserialize, BinaryIVFNullInvlistsSearch) {
+    IndexBinaryIVF idx;
+    idx.d = 16;
+    idx.code_size = 2;
+    idx.invlists = nullptr;
+    idx.own_invlists = false;
+    std::vector<int32_t> distances(1);
+    std::vector<idx_t> labels(1);
+    std::vector<uint8_t> query(2);
+    EXPECT_THROW(
+            {
+                try {
+                    idx.search(
+                            1,
+                            query.data(),
+                            1,
+                            distances.data(),
+                            labels.data());
+                } catch (const faiss::FaissException& e) {
+                    EXPECT_NE(
+                            std::string(e.what()).find("inverted lists"),
+                            std::string::npos);
+                    throw;
+                }
+            },
+            faiss::FaissException);
+}