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, 4 insertions, 0 deletions

diff --git a/changes/bug21540 b/changes/bug21540
new file mode 100644
index 0000000000..0cf684b7f2
--- /dev/null
+++ b/changes/bug21540
@@ -0,0 +1,4 @@
+  o Minor bugfixes (windows, relay):
+    - Resolve "Failure from drain_fd: No error" warnings on Windows
+      relays. Fixes bug 21540; bugfix on 0.2.6.3-alpha.
+