main : allow empty --prompt-cache file (#5176)

* allow empty --prompt-cache file

This allows the use of std::tmpnam(), std::tmpfile(), Python's tempfile.NamedTemporaryFile(), and similar create-empty-file API's for the user.

I switched from the C fopen API to the C++ filesystem api to get around the fact that, to the best of my knowledge, C has no portable way to get the file size above LONG_MAX, with std::ftell() returning long? fallback to std::ifstream for c++  < 17
(the project is currently targeting C++11 it seems - file_exists() and file_size() can be removed when we upgrade to c++17)

* formatting

(requested in codereview)

* remove c++17, file_is_empty

examples/main/main.cpp

@@ -39,6 +39,17 @@ static std::ostringstream       * g_output_ss;
 static std::vector<llama_token> * g_output_tokens;
 static bool is_interacting = false;
 
+static bool file_exists(const std::string &path) {
+    std::ifstream f(path.c_str());
+    return f.good();
+}
+
+static bool file_is_empty(const std::string &path) {
+    std::ifstream f;
+    f.exceptions(std::ifstream::failbit | std::ifstream::badbit);
+    f.open(path.c_str(), std::ios::in | std::ios::binary | std::ios::ate);
+    return f.tellg() == 0;
+}
 
 static void write_logfile(
     const llama_context * ctx, const gpt_params & params, const llama_model * model,
@@ -215,12 +226,12 @@ int main(int argc, char ** argv) {
 
     if (!path_session.empty()) {
         LOG_TEE("%s: attempting to load saved session from '%s'\n", __func__, path_session.c_str());
-
-        // fopen to check for existing session
-        FILE * fp = std::fopen(path_session.c_str(), "rb");
-        if (fp != NULL) {
-            std::fclose(fp);
-
+        if (!file_exists(path_session)) {
+            LOG_TEE("%s: session file does not exist, will create.\n", __func__);
+        } else if (file_is_empty(path_session)) {
+            LOG_TEE("%s: The session file is empty. A new session will be initialized.\n", __func__);
+        } else {
+            // The file exists and is not empty
             session_tokens.resize(n_ctx);
             size_t n_token_count_out = 0;
             if (!llama_load_session_file(ctx, path_session.c_str(), session_tokens.data(), session_tokens.capacity(), &n_token_count_out)) {
@@ -229,10 +240,7 @@ int main(int argc, char ** argv) {
             }
             session_tokens.resize(n_token_count_out);
             llama_set_rng_seed(ctx, params.seed);
-
-            LOG_TEE("%s: loaded a session with prompt size of %d tokens\n", __func__, (int) session_tokens.size());
-        } else {
-            LOG_TEE("%s: session file does not exist, will create\n", __func__);
+            LOG_TEE("%s: loaded a session with prompt size of %d tokens\n", __func__, (int)session_tokens.size());
         }
     }