server: leave a busy slot untouched when a request pins it (#30295)

A request asking for a busy id_slot still ran the prompt cache update
on that slot before being deferred. When the RAM cache held a better
match, it was loaded into the slot while another request was still
generating there, and that generation continued on the wrong context.

The busy slot is now returned as is and the request waits for it.

tools/server/server-context.cpp

@@ -1686,6 +1686,11 @@ private:
         if (task.id_slot != -1) {
             ret = get_slot_by_id(task.id_slot);
             if (ret) {
+                // a busy slot is returned untouched, the caller defers the task
+                if (ret->is_processing()) {
+                    return ret;
+                }
+
                 SLT_INF(*ret, "selected slot by id (%d)\n", task.id_slot);
             }
         }

tools/server/tests/unit/test_completion.py

@@ -237,6 +237,29 @@ def test_nocache_long_input_prompt():
     })
     assert res.status_code == 400
 
+
+# a request pinned to a busy slot leaves the generation running on it untouched
+def test_pinned_request_on_busy_slot():
+    global server
+    server.n_ctx = 4096
+    server.start()
+    story = "Once upon a time a dragon named Ember guarded a golden key in a deep cave. " * 8
+
+    def run(pin_busy: bool) -> str:
+        server.make_request("POST", "/completion", data={"prompt": story, "id_slot": 0, "n_predict": 4, "temperature": 0.0})
+        res = server.make_stream_request("POST", "/completion", data={
+            "prompt": "To bake bread, mix flour, water and salt, then",
+            "id_slot": 0, "n_predict": 1024, "ignore_eos": True, "temperature": 0.0, "stream": True,
+        })
+        content = next(res)["content"]
+        if pin_busy:
+            server.make_request("POST", "/completion", data={"prompt": story + "The dragon", "id_slot": 0, "n_predict": 4, "temperature": 0.0})
+        return content + "".join(chunk["content"] for chunk in res)
+
+    baseline = run(pin_busy=False)
+    assert run(pin_busy=True) == baseline
+
+
 def test_json_prompt_no_mtmd():
     global server
     server.start()