server: add auto-sleep after N seconds of idle (#18228)

* implement sleeping at queue level

* implement server-context suspend

* add test

* add docs

* optimization: add fast path

* make sure to free llama_init

* nits

* fix use-after-free

* allow /models to be accessed during sleeping, fix use-after-free

* don't allow accessing /models during sleep, it is not thread-safe

* fix data race on accessing props and model_meta

* small clean up

* trailing whitespace

* rm outdated comments

tools/server/tests/unit/test_sleep.py

@@ -0,0 +1,39 @@
+import pytest
+import time
+from utils import *
+
+server = ServerPreset.tinyllama2()
+
+
+@pytest.fixture(autouse=True)
+def create_server():
+    global server
+    server = ServerPreset.tinyllama2()
+
+
+def test_server_sleep():
+    global server
+    server.sleep_idle_seconds = 1
+    server.start()
+
+    # wait a bit so that server can go to sleep
+    time.sleep(2)
+
+    # make sure these endpoints are still responsive after sleep
+    res = server.make_request("GET", "/health")
+    assert res.status_code == 200
+    res = server.make_request("GET", "/props")
+    assert res.status_code == 200
+    assert res.body["is_sleeping"] == True
+
+    # make a generation request to wake up the server
+    res = server.make_request("POST", "/completion", data={
+        "n_predict": 1,
+        "prompt": "Hello",
+    })
+    assert res.status_code == 200
+
+    # it should no longer be sleeping
+    res = server.make_request("GET", "/props")
+    assert res.status_code == 200
+    assert res.body["is_sleeping"] == False