Fix vrand ping pong (#14183)

VRANDMEMBER had a bug when exactly two elements where present in the
vector set: we selected a fixed number of random paths to take, and this
will lead always to the same element. This PR should be kindly
back-ported to Redis 8.x.

modules/vector-sets/hnsw.c

@@ -1975,6 +1975,13 @@ hnswNode *hnsw_random_node(HNSW *index, int slot) {
     double logN = log2(index->node_count + 1);
     uint32_t num_walks = (uint32_t)(logN * c);
 
+    /* Avoid the ping-pong effect: imagine there are just two nodes and
+     * the number of walks selected is even. We will select always the
+     * first element of the graph; conversely, if it is odd, we will always
+     * select the other element. One way to add more selection randomness is
+     * to randomly add '1' or '0' to the number of walks to perform. */
+    num_walks += rand() & 1;
+
     // Perform random walk at level 0.
     for (uint32_t i = 0; i < num_walks; i++) {
         if (current->layers[0].num_links == 0) return current;

modules/vector-sets/tests/persistence.py

@@ -82,5 +82,5 @@ class HNSWPersistence(TestCase):
                 f"Projected vectors: Score mismatch for {key}: " + \
                 f"before={initial_projected[key]:.6f}, after={reloaded_projected[key]:.6f}"
 
-        self.redis.del(f"{self.test_key}:normal")
-        self.redis.del(f"{self.test_key}:projected")
+        self.redis.delete(f"{self.test_key}:normal")
+        self.redis.delete(f"{self.test_key}:projected")

modules/vector-sets/tests/vrand-ping-pong.py

@@ -0,0 +1,35 @@
+from test import TestCase, generate_random_vector
+import struct
+
+class VRANDMEMBERPingPongRegressionTest(TestCase):
+    def getname(self):
+        return "[regression] VRANDMEMBER ping-pong"
+
+    def test(self):
+        """
+        This test ensures that when only two vectors exist, VRANDMEMBER
+        does not get stuck returning only one of them due to the "ping-pong" issue.
+        """
+        self.redis.delete(self.test_key) # Clean up before test
+        dim = 4
+
+        # Add exactly two vectors
+        vec1_name = "vec1"
+        vec1_data = generate_random_vector(dim)
+        self.redis.execute_command('VADD', self.test_key, 'VALUES', dim, *vec1_data, vec1_name)
+
+        vec2_name = "vec2"
+        vec2_data = generate_random_vector(dim)
+        self.redis.execute_command('VADD', self.test_key, 'VALUES', dim, *vec2_data, vec2_name)
+
+        # Call VRANDMEMBER many times and check for distribution
+        iterations = 100
+        results = []
+        for _ in range(iterations):
+            member = self.redis.execute_command('VRANDMEMBER', self.test_key)
+            results.append(member.decode())
+
+        # Verify that both members were returned, proving it's not stuck
+        unique_results = set(results)
+
+        assert len(unique_results) == 2, f"Ping-pong test failed: should have returned 2 unique members, but got {len(unique_results)}."