Stop returning an error after successful send of zero length UDP packets

A zero-length send is invalid with TCP, but well defined with UDP.
udp:send"" was returning (nil,"refused"), indicating that it failed when
the packet was actually sent. The test script reproduces the bug, and
includes a tcpdump of the zero length packet being sent.

2 files changed, 30 insertions, 6 deletions

diff --git a/src/usocket.c b/src/usocket.c
index ef275b4..97f8b4f 100644
--- a/src/usocket.c
+++ b/src/usocket.c
@@ -213,14 +213,13 @@ int socket_send(p_socket ps, const char *data, size_t count,
     for ( ;; ) {
         long put = (long) send(*ps, data, count, 0);
         /* if we sent anything, we are done */
-        if (put > 0) {
+        if (put >= 0) {
             *sent = put;
             return IO_DONE;
         }
         err = errno;
-        /* send can't really return 0, but EPIPE means the connection was 
-           closed */
-        if (put == 0 || err == EPIPE) return IO_CLOSED;
+        /* EPIPE means the connection was closed */
+        if (err == EPIPE) return IO_CLOSED;
         /* we call was interrupted, just try again */
         if (err == EINTR) continue;
         /* if failed fatal reason, report error */
@@ -243,12 +242,12 @@ int socket_sendto(p_socket ps, const char *data, size_t count, size_t *sent,
     if (*ps == SOCKET_INVALID) return IO_CLOSED;
     for ( ;; ) {
         long put = (long) sendto(*ps, data, count, 0, addr, len);  
-        if (put > 0) {
+        if (put >= 0) {
             *sent = put;
             return IO_DONE;
         }
         err = errno;
-        if (put == 0 || err == EPIPE) return IO_CLOSED;
+        if (err == EPIPE) return IO_CLOSED;
         if (err == EINTR) continue;
         if (err != EAGAIN) return err;
         if ((err = socket_waitfd(ps, WAITFD_W, tm)) != IO_DONE) return err;
diff --git a/test/udp-zero-length-send b/test/udp-zero-length-send
new file mode 100755
index 0000000..a594944
--- /dev/null
+++ b/test/udp-zero-length-send
@@ -0,0 +1,25 @@
+#!/usr/bin/lua
+
+--[[
+Show that luasocket returns an error message on zero-length UDP sends,
+even though the send is valid, and in fact the UDP packet is sent
+to the peer:
+
+% sudo tcpdump -i lo -n
+tcpdump: verbose output suppressed, use -v or -vv for full protocol decode
+listening on lo, link-type EN10MB (Ethernet), capture size 65535 bytes
+13:40:16.652808 IP 127.0.0.1.56573 > 127.0.0.1.5432: UDP, length 0
+
+]]
+
+require"socket"
+
+s = assert(socket.udp())
+r = assert(socket.udp())
+assert(r:setsockname("*", 5432))
+assert(s:setpeername("127.0.0.1", 5432))
+
+ssz, emsg = s:send("")
+
+print(ssz == 0 and "OK" or "FAIL",[[send:("")]], ssz, emsg)
+