download: add option to skip_download (#23059)

* download: add option to skip_download

* fix

* fix 2

* if file doesn't exist, respect skip_download flag

common/download.h

@@ -52,6 +52,9 @@ struct common_download_opts {
     std::string bearer_token;
     common_header_list headers;
     bool offline = false;
+    bool skip_download = false; // if true, only validation is performed, common_skip_download_exception may be thrown if the file is missing or invalid
+    bool download_mmproj = false;
+    bool download_mtp = false;
     common_download_callback * callback = nullptr;
 };
 
@@ -62,6 +65,11 @@ struct common_download_model_result {
     std::string mtp_path;
 };
 
+// throw if the file is missing or invalid (e.g. ETag check failed)
+struct common_skip_download_exception : public std::runtime_error {
+    common_skip_download_exception() : std::runtime_error("skip download") {}
+};
+
 // Download model from HuggingFace repo or URL
 //
 // input (via model struct):
@@ -89,9 +97,7 @@ struct common_download_model_result {
 // returns result with model_path, mmproj_path and mtp_path (empty when not found / on failure)
 common_download_model_result common_download_model(
     const common_params_model & model,
-    const common_download_opts & opts = {},
-    bool download_mmproj = false,
-    bool download_mtp    = false
+    const common_download_opts & opts = {}
 );
 
 // returns list of cached models
@@ -99,6 +105,7 @@ std::vector<common_cached_model_info> common_list_cached_models();
 
 // download single file from url to local path
 // returns status code or -1 on error
+// returns -2 if the download was skipped due to ETag mismatch (file outdated, skip_download=true)
 // skip_etag: if true, don't read/write .etag files (for HF cache where filename is the hash)
 int common_download_file_single(const std::string & url,
                                 const std::string & path,