nghttpx: Write PID in temporary file then rename

Write PID in temporary file first.  Then rename it as the real
destination.  It will avoid the issue that the external process may
read the empty PID file because of race condition.

src/shrpx.cc

@@ -200,18 +200,58 @@ int chown_to_running_user(const char *path) {
 
 namespace {
 void save_pid() {
-  std::ofstream out(get_config()->pid_file.c_str(), std::ios::binary);
-  out << get_config()->pid << "\n";
-  out.close();
-  if (!out) {
-    LOG(ERROR) << "Could not save PID to file " << get_config()->pid_file;
+  constexpr auto SUFFIX = StringRef::from_lit(".XXXXXX");
+  auto &pid_file = get_config()->pid_file;
+
+  std::vector<char> temp_path;
+  temp_path.reserve(get_config()->pid_file.size() + SUFFIX.size() + 1);
+
+  auto p = std::copy(std::begin(pid_file), std::end(pid_file),
+                     std::begin(temp_path));
+
+  p = std::copy(std::begin(SUFFIX), std::end(SUFFIX), p);
+  *p = '\0';
+
+  auto fd = mkstemp(temp_path.data());
+  if (fd == -1) {
+    auto error = errno;
+    LOG(ERROR) << "Could not save PID to file " << pid_file << ": "
+               << strerror(error);
+    exit(EXIT_FAILURE);
+  }
+
+  auto content = util::utos(get_config()->pid) + '\n';
+
+  if (write(fd, content.c_str(), content.size()) == -1) {
+    auto error = errno;
+    LOG(ERROR) << "Could not save PID to file " << pid_file << ": "
+               << strerror(error);
+    exit(EXIT_FAILURE);
+  }
+
+  if (fsync(fd) == -1) {
+    auto error = errno;
+    LOG(ERROR) << "Could not save PID to file " << pid_file << ": "
+               << strerror(error);
+    exit(EXIT_FAILURE);
+  }
+
+  close(fd);
+
+  if (rename(temp_path.data(), pid_file.c_str()) == -1) {
+    auto error = errno;
+    LOG(ERROR) << "Could not save PID to file " << pid_file << ": "
+               << strerror(error);
+
+    unlink(temp_path.data());
+
     exit(EXIT_FAILURE);
   }
 
   if (get_config()->uid != 0) {
-    if (chown_to_running_user(get_config()->pid_file.c_str()) == -1) {
+    if (chown_to_running_user(pid_file.c_str()) == -1) {
       auto error = errno;
-      LOG(WARN) << "Changing owner of pid file " << get_config()->pid_file
+      LOG(WARN) << "Changing owner of pid file " << pid_file
                 << " failed: " << strerror(error);
     }
   }