Sleep while polling sockets on Windows (#5594)

CHANGELOG.md

@@ -9,7 +9,8 @@
 * Fix a data race when opening a flexible sync Realm (since v12.1.0).
 * Fixed a missing backlink removal when setting a Mixed from a TypedLink to null or any other non-link value. Users may have seen exception of "key not found" or assertion failures such as `mixed.hpp:165: [realm-core-12.1.0] Assertion failed: m_type` when removing the destination link object. ([#5574](https://github.com/realm/realm-core/pull/5573), since the introduction of Mixed in v11.0.0)
 * Asymmetric sync now works with embedded objects. (Issue [#5565](https://github.com/realm/realm-core/issues/5565), since 12.1.0)
-
+* Fixed an issue on Windows that would cause high CPU usage by the sync client when there are no active sync sessions. (Issue [#5591](https://github.com/realm/realm-core/issues/5591), since the introduction of Sync support for Windows)
+
 ### Breaking changes
 * `realm_sync_before_client_reset_func_t` and `realm_sync_after_client_reset_func_t` in the C API now return a boolean value to indicate whether the callback succeeded or not, which signals to the sync client that a fatal error occurred. (PR [#5564](https://github.com/realm/realm-core/pull/5564))
 

src/realm/util/network.cpp

@@ -1142,7 +1142,7 @@ bool Service::IoReactor::wait_and_advance(clock::time_point timeout, clock::time
  auto started = steady_clock::now();
  int ret = 0;
 
- do {
+ for (;;) {
  if (m_pollfd_slots.size() > 1) {
  // Poll all network sockets
  ret = WSAPoll(LPWSAPOLLFD(&m_pollfd_slots[1]), ULONG(m_pollfd_slots.size() - 1),
@@ -1155,8 +1155,15 @@ bool Service::IoReactor::wait_and_advance(clock::time_point timeout, clock::time
  ret++;
  }
 
- } while (ret == 0 &&
- (duration_cast<milliseconds>(steady_clock::now() - started).count() < max_wait_millis));
+ if (ret != 0 ||
+ (duration_cast<milliseconds>(steady_clock::now() - started).count() >= max_wait_millis)) {
+ break;
+ }
+
+ // If we don't have any sockets to poll for (m_pollfd_slots is less than 2) and no one signals
+ // the wakeup pipe, we'd be stuck busy waiting for either condition to become true.
+ std::this_thread::sleep_for(std::chrono::milliseconds(socket_poll_timeout));
+ }
 
 #else // !defined _WIN32
  int ret = ::poll(fds, nfds, max_wait_millis);