IndexFlat: Reconstruct validate if key < ntotal (#4415)

Summary:
Pull Request resolved: https://github.com/facebookresearch/faiss/pull/4415

https://github.com/facebookresearch/faiss/issues/4413: Customer report an issue when index.reconstruct resulted in an undefined behavior.

Reviewed By: pankajsingh88

Differential Revision: D77549128

fbshipit-source-id: d588380c7315a92376f64cef174f1128a856b2e7

faiss/IndexFlat.cpp

@@ -239,6 +239,7 @@ FlatCodesDistanceComputer* IndexFlat::get_FlatCodesDistanceComputer() const {
 }
 
 void IndexFlat::reconstruct(idx_t key, float* recons) const {
+    FAISS_THROW_IF_NOT(key < ntotal);
     memcpy(recons, &(codes[key * code_size]), code_size);
 }
 

tests/test_index.py

@@ -646,7 +646,7 @@ class TestReconsException(unittest.TestCase):
             index.reconstruct, 100001
         )
 
-    def test_reconstuct_after_add(self):
+    def test_reconstruct_after_add(self):
         index = faiss.index_factory(10, 'IVF5,SQfp16')
         index.train(faiss.randn((100, 10), 123))
         index.add(faiss.randn((100, 10), 345))
@@ -657,6 +657,26 @@ class TestReconsException(unittest.TestCase):
         index.reconstruct(5)
         index.reconstruct(150)
 
+    def test_reconstruct_larger_ntotal(self):
+        vect_dim = 5
+        n_vectors = 10
+
+        # Create the dataset
+        data = np.random.randint(100, size=(n_vectors, vect_dim))
+
+        # Build index
+        index = faiss.IndexFlatL2(vect_dim)
+        index.add(data)
+
+        # Reconstruct < ntotal (10) without an issue
+        index.reconstruct(9)
+
+        # Reconstruct >= ntotal (10), assert exception raise
+        self.assertRaises(
+            RuntimeError,
+            index.reconstruct, 10
+        )
+
 
 class TestReconsHash(unittest.TestCase):