mtmd: fix silent prompt truncation on embedded NUL (#25548)

* mtmd: fix silent prompt truncation on embedded NUL

mtmd_input_text carried the prompt as a bare const char* with no
length, so a NUL byte in message content cut the prompt at the
tokenizer boundary and dropped every later message plus the assistant
marker, with no log. Add an explicit text_len and thread it through,
matching llama_tokenize and the text only path.

* cleanup

---------

Co-authored-by: Xuan Son Nguyen <son@huggingface.co>

common/common.h

@@ -1081,6 +1081,9 @@ enum ggml_opt_optimizer_type common_opt_get_optimizer(const char *);
 struct common_prompt_checkpoint {
     int64_t n_tokens;
 
+    // (optional) id of the task that created the checkpoint
+    int id_task = -1;
+
     llama_pos pos_min;
     llama_pos pos_max;
 

tools/mtmd/mtmd-cli.cpp

@@ -250,7 +250,8 @@ static int eval_message(mtmd_cli_context & ctx, common_chat_msg & msg) {
     LOG_DBG("formatted_chat.prompt: %s\n", formatted_chat.c_str());
 
     mtmd_input_text text;
-    text.text          = formatted_chat.c_str();
+    text.text          = formatted_chat.data();
+    text.text_len      = formatted_chat.size();
     text.add_special   = add_bos;
     text.parse_special = true;
 

tools/mtmd/mtmd.cpp

@@ -809,7 +809,7 @@ void mtmd_free(mtmd_context * ctx) {
 struct mtmd_tokenizer {
     mtmd_context * ctx;
 
-    std::string input_text;
+    std::string input_text; // note: can contain null bytes; do not use c_str()
     bool add_special;
     bool parse_special;
     const llama_vocab * vocab;
@@ -839,9 +839,10 @@ struct mtmd_tokenizer {
             size_t n_bitmaps) : ctx(ctx) {
         add_special   = text->add_special;
         parse_special = text->parse_special;
-        input_text    = text->text;
         vocab         = ctx->vocab;
 
+        input_text.assign(text->text, text->text_len);
+
         std::vector<const mtmd_bitmap *> bitmaps(bmps, bmps + n_bitmaps);
         auto parts_str = split_text(input_text, ctx->media_marker);
         size_t i_bm = 0;

tools/mtmd/mtmd.h

@@ -67,6 +67,7 @@ struct mtmd_batch;
 
 struct mtmd_input_text {
     const char * text;
+    size_t text_len;
     bool add_special;
     bool parse_special;
 };

tools/server/server-common.cpp

@@ -705,7 +705,8 @@ server_tokens process_mtmd_prompt(mtmd_context * mctx, const std::string & promp
     std::vector<server_tokens> inputs;
     // multimodal
     mtmd_input_text inp_txt = {
-        prompt.c_str(),
+        prompt.data(),
+        prompt.size(),
         /* add_special */   true,
         /* parse_special */ true,
     };

tools/server/server-context.cpp

@@ -2290,6 +2290,24 @@ private:
 
     // n_tokens_cur: the number of tokens added to the batch for the current slot
     void create_checkpoint(server_slot & slot, const int64_t n_tokens_cur, llama_pos pos_min, llama_pos pos_max) {
+        const int id_task = slot.task->id;
+
+        // evict checkpoints within min-step of a previous checkpoint, unless they were
+        // created by the current task
+        int64_t last = -1;
+        for (auto it = slot.prompt.checkpoints.begin(); it != slot.prompt.checkpoints.end(); ) {
+            if (it->id_task != id_task && last >= 0 && it->n_tokens <= last + params_base.checkpoint_min_step) {
+                SLT_TRC(slot, "erasing context checkpoint too close to an earlier one (pos_min = %d, pos_max = %d, n_tokens = %" PRId64 ", size = %.3f MiB)\n",
+                        it->pos_min, it->pos_max, it->n_tokens, (float) it->size() / 1024 / 1024);
+
+                it = slot.prompt.checkpoints.erase(it);
+                continue;
+            }
+
+            last = it->n_tokens;
+            ++it;
+        }
+
         while (slot.prompt.checkpoints.size() >= (size_t) params_base.n_ctx_checkpoints) {
             // make room for the new checkpoint, if needed
             const auto & cur = slot.prompt.checkpoints.front();
@@ -2302,6 +2320,8 @@ private:
 
         auto & cur = slot.prompt.checkpoints.emplace_back();
 
+        cur.id_task = id_task;
+
         // [TAG_CHECKPOINTS_FIX_POS_MIN]
         // TODO: here we incorrectly deterimne that the saved checkpoint data covers the [pos_min, pos_max] range
         //       this is not true for SWA models: https://github.com/ggml-org/llama.cpp/pull/24411#issuecomment-4677983225
@@ -3511,7 +3531,10 @@ private:
                     do_checkpoint = do_checkpoint && !has_mtmd;
 
                     // no need to create checkpoints that are too close together, unless it's the last user message
-                    do_checkpoint = do_checkpoint && (slot.prompt.checkpoints.empty() || is_last_user_message || n_tokens_start > slot.prompt.checkpoints.back().n_tokens + params_base.checkpoint_min_step);
+                    do_checkpoint = do_checkpoint && (
+                            slot.prompt.checkpoints.empty() ||
+                            is_last_user_message || near_prompt_end ||
+                            n_tokens_start > slot.prompt.checkpoints.back().n_tokens + params_base.checkpoint_min_step);
                     SLT_DBG(slot, "main/do_checkpoint = %s, pos_min = %d, pos_max = %d\n", do_checkpoint ? "yes" : "no", pos_min, pos_max);
 
                     // note: we create the checkpoint before calling llama_decode(), so the current batch is not