Correctly handle fd-drain errors on windows workqueues

Windows doesn't let you check the socket error for a socket with
WSAGetLastError() and getsockopt(SO_ERROR).  But
getsockopt(SO_ERROR) clears the error on the socket, so you can't
call it more than once per error.

When we introduced recv_ni to help drain alert sockets, back in
0.2.6.3-alpha, we had the failure path for recv_ni call getsockopt()
twice, though: once to check for EINTR and one to check for EAGAIN.
Of course, we never got the eagain, so we treated it as an error,
and warned about: "No error".

The fix here is to have these functions return -errno on failure.

Fixes bug 21540; bugfix on 0.2.6.3-alpha.

1 files changed, 3 insertions, 2 deletions

diff --git a/src/common/workqueue.c b/src/common/workqueue.c
index e1fb663a2a..ec6e257b4c 100644
--- a/src/common/workqueue.c
+++ b/src/common/workqueue.c
@@ -510,12 +510,13 @@ replyqueue_get_socket(replyqueue_t *rq)
 void
 replyqueue_process(replyqueue_t *queue)
 {
-  if (queue->alert.drain_fn(queue->alert.read_fd) < 0) {
+  int r = queue->alert.drain_fn(queue->alert.read_fd);
+  if (r < 0) {
     //LCOV_EXCL_START
     static ratelim_t warn_limit = RATELIM_INIT(7200);
     log_fn_ratelim(&warn_limit, LOG_WARN, LD_GENERAL,
                  "Failure from drain_fd: %s",
-                 tor_socket_strerror(tor_socket_errno(queue->alert.read_fd)));
+                   tor_socket_strerror(-r));
     //LCOV_EXCL_STOP
   }