util::read_file: fix reading from sysfs and add test case (#21325)

* add sysfs test case

* Revert "util.cc: refactor read_file (#21321)"

This reverts commit 8d5c61eefe.

selfdrive/common/tests/test_util.cc

@@ -24,6 +24,10 @@ TEST_CASE("util::read_file") {
     std::string ret = util::read_file("/proc/version");
     REQUIRE(ret.find("Linux version") != std::string::npos);
   }
+  SECTION("read from sysfs") {
+    std::string ret = util::read_file("/sys/power/wakeup_count");
+    REQUIRE(!ret.empty());
+  }
   SECTION("read file") {
     char filename[] = "/tmp/test_read_XXXXXX";
     int fd = mkstemp(filename);

selfdrive/common/util.cc

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