llama : require backend samplers to be of type llama_sampler_chain

include/llama.h

@@ -369,7 +369,7 @@ extern "C" {
                           // try to disable when n_seq_max > 1 for improved performance when the sequences do not share a large prefix
                           // ref: https://github.com/ggml-org/llama.cpp/pull/14363
 
-        // backend sampler chain configuration (make sure the caller keeps the sampler chains alive)
+        // backend sampler chain configuration (make sure the caller keeps the sampler chains alive) [EXPERIMENTAL]
         // note: the samplers must be sampler chains (i.e. use llama_sampler_chain_init)
         struct llama_sampler_seq_config * samplers;
         size_t                            n_samplers;
@@ -1243,7 +1243,15 @@ extern "C" {
 
     // important: takes ownership of the sampler object and will free it when llama_sampler_free is called
     LLAMA_API void                   llama_sampler_chain_add(      struct llama_sampler * chain, struct llama_sampler * smpl);
-    LLAMA_API struct llama_sampler * llama_sampler_chain_get(const struct llama_sampler * chain, int32_t i);
+
+    // return NULL if:
+    //   - the sampler is NULL
+    //   - the sampler is not a llama_sampler_chain
+    //   - the index is out of bounds, unless i == -1
+    //   - if i == -1, returns the chain itself (can be used to check if the sampler is a chain)
+    LLAMA_API struct llama_sampler * llama_sampler_chain_get(      struct llama_sampler * chain, int32_t i);
+
+    // the total number of samplers in the chain
     LLAMA_API int                    llama_sampler_chain_n  (const struct llama_sampler * chain);
 
     // after removing a sampler, the chain will no longer own it, and it will not be freed when the chain is freed

src/llama-context.cpp

@@ -66,7 +66,9 @@ llama_context::llama_context(
         for (size_t i = 0; i < params.n_samplers; ++i) {
             const auto & config = params.samplers[i];
 
-            // TODO: assert this is a llama_sampler_chain instance
+            if (llama_sampler_chain_get(config.sampler, -1) != nullptr) {
+                throw std::runtime_error("the backend samplers must be of type llama_sampler_chain");
+            }
 
             if (set_sampler(config.seq_id, config.sampler)) {
                 const int n_samplers = llama_sampler_chain_n(config.sampler);

src/llama-sampling.cpp

@@ -803,7 +803,19 @@ void llama_sampler_chain_add(struct llama_sampler * chain, struct llama_sampler
     });
 }
 
-struct llama_sampler * llama_sampler_chain_get(const struct llama_sampler * chain, int32_t i) {
+struct llama_sampler * llama_sampler_chain_get(struct llama_sampler * chain, int32_t i) {
+    if (chain == nullptr) {
+        return nullptr;
+    }
+
+    if (chain->iface != &llama_sampler_chain_i) {
+        return nullptr;
+    }
+
+    if (i == -1) {
+        return chain;
+    }
+
     const auto * p = (const llama_sampler_chain *) chain->ctx;
 
     if (i < 0 || (size_t) i >= p->samplers.size()) {