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* rm outdated comments

common/arg.cpp

@@ -2887,6 +2887,16 @@ common_params_context common_params_parser_init(common_params & params, llama_ex
             params.lora_init_without_apply = true;
         }
     ).set_examples({LLAMA_EXAMPLE_SERVER}));
+    add_opt(common_arg(
+        {"--sleep-idle-seconds"}, "SECONDS",
+        string_format("number of seconds of idleness after which the server will sleep (default: %d; -1 = disabled)", params.sleep_idle_seconds),
+        [](common_params & params, int value) {
+            if (value == 0 || value < -1) {
+                throw std::invalid_argument("invalid value: cannot be 0 or less than -1");
+            }
+            params.sleep_idle_seconds = value;
+        }
+    ).set_examples({LLAMA_EXAMPLE_SERVER}));
     add_opt(common_arg(
         {"--simple-io"},
         "use basic IO for better compatibility in subprocesses and limited consoles",