util.cc: refactor read_file (#21350)

selfdrive/common/util.cc

@@ -54,26 +54,26 @@ 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();
-    if (pos > 0) {
-      std::string result;
-      result.resize(pos);
-      ifs.seekg(0, std::ios::beg);
-      ifs.read(result.data(), pos);
-      if (ifs) {
+  std::ifstream f(fn, std::ios::binary | std::ios::in);
+  if (f.is_open()) {
+    f.seekg(0, std::ios::end);
+    int size = f.tellg();
+    if (f.good() && size > 0) {
+      std::string result(size, '\0');
+      f.seekg(0, std::ios::beg);
+      f.read(result.data(), size);
+      // return either good() or has reached end-of-file (e.g. /sys/power/wakeup_count)
+      if (f.good() || f.eof()) {
+        result.resize(f.gcount());
         return result;
       }
     }
-  }
-  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) {