sgl-router: experimental Rust HTTP router for SGLang worker pools (#25851)

Co-authored-by: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

experimental/sgl-router/tests/component/policies/power_of_two.rs

@@ -0,0 +1,73 @@
+// SPDX-FileCopyrightText: Copyright (c) 2026 The SGLang Authors
+// SPDX-License-Identifier: Apache-2.0
+
+use sgl_router::discovery::{ModelId, WorkerId, WorkerMode, WorkerSpec};
+use sgl_router::policies::power_of_two::PowerOfTwoChoicesPolicy;
+use sgl_router::policies::{Policy, SelectionContext};
+use sgl_router::workers::Worker;
+use std::sync::atomic::Ordering;
+use std::sync::Arc;
+
+fn worker(id: &str) -> Arc<Worker> {
+    Arc::new(Worker::new(WorkerSpec {
+        id: WorkerId(id.into()),
+        url: format!("http://{id}"),
+        mode: WorkerMode::Plain,
+        model_ids: vec![ModelId("m".into())],
+        bootstrap_port: None,
+    }))
+}
+
+#[test]
+fn selects_lower_load() {
+    let a = worker("a");
+    let b = worker("b");
+    a.active_requests.store(10, Ordering::Relaxed);
+    b.active_requests.store(2, Ordering::Relaxed);
+    let p = PowerOfTwoChoicesPolicy::new();
+    let ws = vec![a.clone(), b.clone()];
+    let model_id = ModelId("m".into());
+    let ctx = SelectionContext::new(&model_id, None);
+    let chosen = p.select(&ws, &ctx).unwrap();
+    assert_eq!(chosen.id.0, "b");
+}
+
+#[test]
+fn distribution_skews_to_lower_load() {
+    // With 3 workers and one heavily loaded, the loaded one should win
+    // significantly less than 1/3 of selections.
+    let workers = vec![worker("a"), worker("b"), worker("c")];
+    workers[2].active_requests.store(100, Ordering::Relaxed); // c is loaded
+
+    let p = PowerOfTwoChoicesPolicy::new();
+    let model_id = ModelId("m".into());
+    let ctx = SelectionContext::new(&model_id, None);
+    let mut counts = std::collections::HashMap::new();
+    for _ in 0..1000 {
+        let w = p.select(&workers, &ctx).unwrap();
+        *counts.entry(w.id.0.clone()).or_insert(0) += 1;
+    }
+    let c_picks = *counts.get("c").unwrap_or(&0);
+    assert!(
+        c_picks < 200,
+        "loaded worker should be picked < 20% of the time, got {c_picks}"
+    );
+}
+
+#[test]
+fn empty_returns_none() {
+    let p = PowerOfTwoChoicesPolicy::new();
+    let ws: Vec<Arc<Worker>> = vec![];
+    let model_id = ModelId("m".into());
+    let ctx = SelectionContext::new(&model_id, None);
+    assert!(p.select(&ws, &ctx).is_none());
+}
+
+#[test]
+fn single_worker_returns_it() {
+    let p = PowerOfTwoChoicesPolicy::new();
+    let ws = vec![worker("only")];
+    let model_id = ModelId("m".into());
+    let ctx = SelectionContext::new(&model_id, None);
+    assert_eq!(p.select(&ws, &ctx).unwrap().id.0, "only");
+}