server: allow accessing /metrics and /slots during llama_decode() (#27041)

* server_queue::worker

* call llama_decode inside yield_to_queue

* also handle process_mtmd_chunk

* clean up

* nits

* rm test

tools/server/server-queue.h

@@ -4,7 +4,9 @@
 
 #include <condition_variable>
 #include <deque>
+#include <exception>
 #include <mutex>
+#include <thread>
 #include <vector>
 #include <unordered_set>
 
@@ -21,16 +23,32 @@ private:
     // queues
     std::deque<server_task> queue_tasks;
     std::deque<server_task> queue_tasks_deferred;
+    // tasks declined while yielding, put back in queue_tasks once the yield is done
+    // note: kept as a member so that cleanup_pending_task() can also reach them
+    std::deque<server_task> queue_tasks_unhandled;
 
     std::mutex mutex_tasks;
     std::condition_variable condition_tasks;
 
+    // used by yield_to_queue, all fields are guarded by mutex_tasks
+    struct worker_t {
+        std::thread             thread;
+        std::condition_variable cv;   // the worker sleeps on this until there is work
+        std::function<void()>   work; // pending work, picked up by the thread
+        std::exception_ptr      exception; // exception thrown by work(), if any
+        bool stop = false;
+        bool busy = false;
+    };
+    worker_t worker;
+
     // callback functions
-    std::function<void(server_task &&)> callback_new_task;
-    std::function<void(void)>           callback_update_slots;
-    std::function<void(bool)>           callback_sleeping_state;
+    std::function<bool(server_task &&, bool)> callback_new_task;
+    std::function<void(void)>                 callback_update_slots;
+    std::function<void(bool)>                 callback_sleeping_state;
 
 public:
+    ~server_queue() { worker_stop(); }
+
     // Add a new task to the end of the queue
     int post(server_task && task, bool front = false);
 
@@ -75,6 +93,15 @@ public:
      */
     void start_loop(int64_t idle_sleep_ms = -1);
 
+    // run work() on a separate thread, while the current thread calls process_new_tasks
+    // returns once work() is done (may throw exceptions)
+    // must be called from start_loop() thread (ideally inside callback_update_slots)
+    // use case: return metrics while encode/decode is running
+    // ref: https://github.com/ggml-org/llama.cpp/pull/27041
+    //
+    // tasks declined by callback_new_task are put back in the queue once this returns
+    void yield_to_queue(std::function<void()> && work);
+
     // for metrics
     size_t queue_tasks_deferred_size() {
         std::unique_lock<std::mutex> lock(mutex_tasks);
@@ -86,7 +113,10 @@ public:
     //
 
     // Register function to process a new task
-    void on_new_task(std::function<void(server_task &&)> callback) {
+    // the second argument tells whether the queue is currently yielding (see yield_to_queue)
+    // only then may the callback return false to decline the task, and it must leave it
+    // untouched, so that it can be put back in the queue later
+    void on_new_task(std::function<bool(server_task &&, bool)> callback) {
         callback_new_task = std::move(callback);
     }
 
@@ -112,6 +142,15 @@ public:
 
 private:
     void cleanup_pending_task(int id_target);
+
+    // process all pending tasks in the queue
+    // returns true if the queue is terminated, false if there is no more task to process
+    // while yielding, declined tasks are moved to queue_tasks_unhandled
+    bool process_new_tasks(bool is_yielding);
+
+    // for worker_t
+    void worker_loop();
+    void worker_stop();
 };
 
 // struct for managing server responses