server-stream: follow-up on SSE Replay Buffer (#23226) (#25047)

* server-stream : pimpl

* server-stream: prefix free functions with server_stream_

address review from ggerganov: scope the public stream functions under the
server_stream_ prefix, matching server_stream_session_manager_start/stop.

* server-stream: guard session and manager state with the mutex

address review from ggerganov: make done, completed_ts and the GC running flag plain members under their
mutex and set the condvar predicates under the lock. keep cancelled atomic for
the lock-free should_stop poll.

* server-stream: trim comments to the non-obvious

address review from ggerganov: drop comments that restate the code, keep the
concurrency, lifetime and ordering rationale. de-stale a few comments left by the
pimpl: g_stream_sessions is now internal and the /v1/streams listing is gone.

* server-stream: update dev docs for the pimpl and prefix

reflect server_stream_session_manager_start/stop and the server_stream_ prefix,
note the manager is now a file-static singleton hidden in the .cpp

* server-stream: move stream traces to debug level

keep the bring-up traces for diagnostics but off the default log: skip
drain, draining, drain ended, DELETE evict, attach_pipe, and the router
stream resume proxy.

* server-stream: align router stream resume proxy trace with upstream

the child-side bring-up traces are already SRV_TRC on master, move the
router stream resume proxy trace to the same level.

* server-stream: move stream_read_status enum to the cpp

it is only used by the hidden session and consumer types, so it belongs
with them behind the pimpl boundary, not on the public header surface.

---------

Co-authored-by: Georgi Gerganov <ggerganov@gmail.com>

tools/server/server.cpp

@@ -85,7 +85,7 @@ int llama_server(int argc, char ** argv) {
 
     // start the stream session manager GC right after common init, before any HTTP route can
     // touch it. lifecycle is symmetric, stop_gc() runs in clean_up() before backend free
-    g_stream_sessions.start_gc();
+    server_stream_session_manager_start();
 
     if (!common_params_parse(argc, argv, params, LLAMA_EXAMPLE_SERVER)) {
         return 1;
@@ -245,8 +245,8 @@ int llama_server(int argc, char ** argv) {
     ctx_http.post("/slots/:id_slot",           ex_wrapper(routes.post_slots));
 
     // resumable streaming, the conversation_id is the session identity end to end. router and
-    // child wire different handlers under the same paths: a child binds the local g_stream_sessions
-    // backed factories, the router binds proxies that resolve the owning child through the
+    // child wire different handlers under the same paths: a child binds the local session
+    // factories, the router binds proxies that resolve the owning child through the
     // conv_id -> model map
     server_http_context::handler_t stream_get_h;
     server_http_context::handler_t streams_lookup_h;
@@ -256,9 +256,9 @@ int llama_server(int argc, char ** argv) {
         streams_lookup_h = models_routes->router_streams_lookup;
         stream_delete_h  = models_routes->router_stream_delete;
     } else {
-        stream_get_h     = make_stream_get_handler();
-        streams_lookup_h = make_streams_lookup_handler();
-        stream_delete_h  = make_stream_delete_handler();
+        stream_get_h     = server_stream_make_get_handler();
+        streams_lookup_h = server_stream_make_lookup_handler();
+        stream_delete_h  = server_stream_make_delete_handler();
     }
     ctx_http.get ("/v1/stream/:conv_id",       ex_wrapper(stream_get_h));
     // POST /v1/streams/lookup with body {"conversation_ids": [...]}. you can only ask for ids
@@ -343,7 +343,7 @@ int llama_server(int argc, char ** argv) {
         clean_up = [&models_routes]() {
             SRV_INF("%s: cleaning up before exit...\n", __func__);
             // stop the session GC first, it finalizes live sessions and wakes pending readers
-            g_stream_sessions.stop_gc();
+            server_stream_session_manager_stop();
             if (models_routes.has_value()) {
                 models_routes->stopping.store(true); // maybe redundant, but just to be safe
                 models_routes->models.unload_all();
@@ -371,7 +371,7 @@ int llama_server(int argc, char ** argv) {
         clean_up = [&ctx_http, &ctx_server]() {
             SRV_INF("%s: cleaning up before exit...\n", __func__);
             // stop the session GC first, it finalizes live sessions and wakes pending readers
-            g_stream_sessions.stop_gc();
+            server_stream_session_manager_stop();
             ctx_http.stop();
             ctx_server.terminate();
             llama_backend_free();