server: add SSE ping interval (#24013)

tools/server/server-context.cpp

@@ -3693,6 +3693,7 @@ std::unique_ptr<server_res_generator> server_routes::handle_completions_impl(
     auto res = create_response();
     auto completion_id = gen_chatcmplid();
     auto & rd = res->rd;
+    auto & params = this->params;
 
     try {
         std::vector<server_task> tasks;
@@ -3828,7 +3829,7 @@ std::unique_ptr<server_res_generator> server_routes::handle_completions_impl(
         }
         res->status = 200;
         res->content_type = "text/event-stream";
-        res->next = [res_this = res.get(), res_type, &req](std::string & output) -> bool {
+        res->next = [res_this = res.get(), res_type, &req, &params](std::string & output) -> bool {
             static auto format_error = [](task_response_type res_type, const json & res_json) {
                 if (res_type == TASK_RESPONSE_TYPE_ANTHROPIC) {
                     return format_anthropic_sse({
@@ -3873,7 +3874,25 @@ std::unique_ptr<server_res_generator> server_routes::handle_completions_impl(
                 }
 
                 // receive subsequent results
-                auto result = rd.next(req.should_stop);
+                bool timeout = false;
+                int64_t start_time = ggml_time_ms();
+                auto result = rd.next([&timeout, &req, &start_time, &params]() {
+                    if (req.should_stop()) {
+                        return true; // should_stop condition met
+                    } else if (params.sse_ping_interval > 0 && ggml_time_ms() - start_time > (int64_t)params.sse_ping_interval * 1000) {
+                        timeout = true;
+                        return true; // timeout
+                    }
+                    return false;
+                });
+
+                if (timeout) {
+                    // some clients may time out (e.g. undici) will time out if no data is received for a while, so we need to send a ping to keep the connection alive
+                    SRV_DBG("%s", "sending SSE ping\n");
+                    output = ":\n\n";
+                    return true;
+                }
+
                 if (result == nullptr) {
                     SRV_DBG("%s", "stopping streaming due to should_stop condition\n");
                     GGML_ASSERT(req.should_stop());