server : handle unsupported cases

src/llama-sampling.cpp

@@ -1226,16 +1226,16 @@ static void llama_sampler_top_k_backend_apply(
     struct ggml_tensor * top_k = ggml_top_k(ctx, data->logits, sctx->k);
     ggml_set_name(top_k, "top_k");
 
-    struct ggml_tensor * logits_rows = ggml_reshape_2d(ctx, data->logits, 1, data->logits->ne[0]);
-    struct ggml_tensor * top_k_rows = ggml_get_rows(ctx, logits_rows, top_k);
-    ggml_set_name(top_k_rows, "top_k_rows");
-
     if (data->candidates) {
         data->candidates = ggml_get_rows(ctx, data->candidates, top_k);
     } else {
         data->candidates = top_k;
     }
 
+    struct ggml_tensor * logits_rows = ggml_reshape_2d(ctx, data->logits, 1, data->logits->ne[0]);
+    struct ggml_tensor * top_k_rows = ggml_get_rows(ctx, logits_rows, top_k);
+    ggml_set_name(top_k_rows, "top_k_rows");
+
     data->logits = ggml_reshape_1d(ctx, top_k_rows, sctx->k);
 
     GGML_UNUSED(gf);

tools/server/server-context.cpp

@@ -1056,11 +1056,20 @@ struct server_context_impl {
                 return false;
             }
 
-            // TODO: getting post/pre sampling logits is not yet supported with backend sampling
             const bool need_logits = task.params.sampling.n_probs > 0;
 
+            bool backend_sampling = true;
+
+            backend_sampling &= task.params.sampling.backend_sampling;
+
+            // TODO: speculative decoding requires multiple samples per batch - not supported yet
+            backend_sampling &= !(slot.ctx_dft && task.params.speculative.n_max > 0);
+
+            // TODO: getting post/pre sampling logits is not yet supported with backend sampling
+            backend_sampling &= !need_logits;
+
             // TODO: tmp until backend sampling is fully implemented
-            if (task.params.sampling.backend_sampling && !need_logits) {
+            if (backend_sampling) {
                 llama_set_sampler(ctx, slot.id, common_sampler_get(slot.smpl.get()));
             } else {
                 llama_set_sampler(ctx, slot.id, nullptr);