util.cc: refactor read_file (#21295)

* mode read & open once

* change return statement for better readibility

* apply reviews

* Update selfdrive/common/util.cc

* fix

Co-authored-by: Adeeb Shihadeh <adeebshihadeh@gmail.com>
old-commit-hash: 190a845505

selfdrive/common/util.cc

@@ -54,26 +54,24 @@ 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);
+  std::ifstream f(fn, std::ios::binary | std::ios::in | std::ios::ate);
-  if (ifs) {
+  if (f) {
-    int pos = ifs.tellg();
+    int pos = f.tellg();
     if (pos > 0) {
       std::string result;
       result.resize(pos);
-      ifs.seekg(0, std::ios::beg);
+      f.seekg(0, std::ios::beg);
-      ifs.read(result.data(), pos);
+      if (f.read(result.data(), pos)) {
-      if (ifs) {
         return result;
       }
-    }
+    } else {
-  }
-  ifs.close();
-
       // 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) {