util.cc: refactor read_file (#21321)

* mode read & open once

* change return statement for better readibility

* apply reviews

* can't open with ate flag

selfdrive/common/util.cc

@@ -54,26 +54,25 @@ int set_core_affinity(int core) {
 namespace util {
 
 std::string read_file(const std::string& fn) {
-  std::ifstream ifs(fn, std::ios::binary | std::ios::ate);
-  if (ifs) {
-    int pos = ifs.tellg();
+  std::ifstream f(fn, std::ios::binary | std::ios::in);
+  if (f) {
+    f.seekg(0, std::ios::end);
+    int pos = f.tellg();
     if (pos > 0) {
       std::string result;
       result.resize(pos);
-      ifs.seekg(0, std::ios::beg);
-      ifs.read(result.data(), pos);
-      if (ifs) {
+      f.seekg(0, std::ios::beg);
+      if (f.read(result.data(), pos)) {
         return result;
       }
-    }
-  }
-  ifs.close();
-
+    } else {
       // fallback for files created on read, e.g. procfs
-  std::ifstream f(fn);
       std::stringstream buffer;
       buffer << f.rdbuf();
       return buffer.str();
+    }
+  }
+  return std::string();
 }
 
 int read_files_in_dir(const std::string &path, std::map<std::string, std::string> *contents) {