server: return_progress to also report 0% processing state (#18305)

tools/server/server-context.cpp

@@ -2313,6 +2313,12 @@ private:
                         slot.n_prompt_tokens_processed = 0;
 
                         slot.prompt.tokens.keep_first(n_past);
+
+                        // send initial 0% progress update if needed
+                        // this is to signal the client that the request has started processing
+                        if (slot.task->params.stream && slot.task->params.return_progress) {
+                            send_partial_response(slot, {}, true);
+                        }
                     }
 
                     if (!slot.can_split()) {

tools/server/tests/unit/test_chat_completion.py

@@ -434,8 +434,8 @@ def test_context_size_exceeded_stream():
 @pytest.mark.parametrize(
     "n_batch,batch_count,reuse_cache",
     [
-        (64, 3, False),
-        (64, 1, True),
+        (64, 4, False),
+        (64, 2, True),
     ]
 )
 def test_return_progress(n_batch, batch_count, reuse_cache):
@@ -462,10 +462,18 @@ def test_return_progress(n_batch, batch_count, reuse_cache):
     res = make_cmpl_request()
     last_progress = None
     total_batch_count = 0
+
     for data in res:
         cur_progress = data.get("prompt_progress", None)
         if cur_progress is None:
             continue
+        if total_batch_count == 0:
+            # first progress report must have n_cache == n_processed
+            assert cur_progress["total"] > 0
+            assert cur_progress["cache"] == cur_progress["processed"]
+            if reuse_cache:
+                # when reusing cache, we expect some cached tokens
+                assert cur_progress["cache"] > 0
         if last_progress is not None:
             assert cur_progress["total"] == last_progress["total"]
             assert cur_progress["cache"] == last_progress["cache"]
@@ -473,6 +481,7 @@ def test_return_progress(n_batch, batch_count, reuse_cache):
         total_batch_count += 1
         last_progress = cur_progress
 
+    # last progress should indicate completion (all tokens processed)
     assert last_progress is not None
     assert last_progress["total"] > 0
     assert last_progress["processed"] == last_progress["total"]