Fix delivery of zero-byte UDP datagrams

.release-notes/5347.md

@@ -0,0 +1,6 @@
+## Fix delivery of zero-byte UDP datagrams
+
+UDP sockets now deliver zero-byte datagrams to `UDPNotify.received` with an
+empty payload instead of treating them as an error. This preserves valid UDP
+uses such as heartbeat, keepalive, and presence datagrams without changing TCP
+zero-byte read handling.

packages/net/_test.pony

@@ -27,6 +27,7 @@ actor \nodoc\ Main is TestList
     test(_TestTCPProxy)
     test(_TestTCPUnmute)
     test(_TestTCPWritev)
+    test(_TestUDPZeroByteDatagram)
 
     // Tests below exclude windows and are listed alphabetically
     ifdef not windows then
@@ -155,6 +156,80 @@ class \nodoc\ iso _TestBroadcast is UnitTest
       exclude this test by passing the --exclude="net/Broadcast" option.
     """)
 
+class \nodoc\ _TestUDPZeroByteDatagramReceiver is UDPNotify
+  let _h: TestHelper
+
+  new create(h: TestHelper) =>
+    _h = h
+
+  fun ref not_listening(sock: UDPSocket ref) =>
+    _h.fail_action("receiver listen")
+
+  fun ref listening(sock: UDPSocket ref) =>
+    _h.complete_action("receiver listen")
+
+    let ip = sock.local_address()
+    let h = _h
+    if ip.ip4() then
+      _h.dispose_when_done(
+        UDPSocket.ip4(UDPAuth(h.env.root),
+          recover _TestUDPZeroByteDatagramSender(h, ip) end))
+    else
+      _h.dispose_when_done(
+        UDPSocket.ip6(UDPAuth(h.env.root),
+          recover _TestUDPZeroByteDatagramSender(h, ip) end))
+    end
+
+  fun ref received(
+    sock: UDPSocket ref,
+    data: Array[U8] iso,
+    from: NetAddress)
+  =>
+    _h.complete_action("receiver receive")
+    _h.assert_eq[USize](0, data.size())
+    _h.complete(true)
+
+class \nodoc\ _TestUDPZeroByteDatagramSender is UDPNotify
+  let _h: TestHelper
+  let _ip: NetAddress
+
+  new create(h: TestHelper, ip: NetAddress) =>
+    _h = h
+    _ip = ip
+
+  fun ref not_listening(sock: UDPSocket ref) =>
+    _h.fail_action("sender listen")
+
+  fun ref listening(sock: UDPSocket ref) =>
+    _h.complete_action("sender listen")
+    sock.write("", _ip)
+
+  fun ref received(
+    sock: UDPSocket ref,
+    data: Array[U8] iso,
+    from: NetAddress)
+  =>
+    _h.fail("sender received unexpected datagram")
+
+class \nodoc\ iso _TestUDPZeroByteDatagram is UnitTest
+  """
+  Test receiving an empty UDP datagram.
+  """
+  fun name(): String => "net/UDPZeroByteDatagram"
+  fun label(): String => "unreliable-appveyor-osx"
+  fun exclusion_group(): String => "network"
+
+  fun ref apply(h: TestHelper) =>
+    h.expect_action("receiver listen")
+    h.expect_action("sender listen")
+    h.expect_action("receiver receive")
+
+    h.dispose_when_done(
+      UDPSocket(UDPAuth(h.env.root),
+        recover _TestUDPZeroByteDatagramReceiver(h) end))
+
+    h.long_test(TimeoutValue())
+
 class \nodoc\ _TestTCP is TCPListenNotify
   """
   Run a typical TCP test consisting of a single TCPListener that accepts a

packages/net/udp_socket.pony

@@ -290,9 +290,11 @@ actor UDPSocket is AsioEventNotify
 
   fun ref _pending_reads() =>
     """
-    Read while data is available, guessing the next packet length as we go. If
-    we read 4 kb of data, send ourself a resume message and stop reading, to
-    avoid starving other actors.
+    Read while data is available, guessing the next packet length as we go.
+    After roughly 4 KB worth of read work, send ourself a resume message and
+    stop reading, to avoid starving other actors. Zero-byte datagrams count as
+    1 byte against this budget so a stream of empty datagrams cannot keep this
+    loop running forever.
     """
     ifdef not windows then
       try
@@ -306,15 +308,18 @@ actor UDPSocket is AsioEventNotify
             @pony_os_recvfrom(_event, data.cpointer(), data.space(),
               from) ?
 
-          if len == 0 then
+          if len == USize.max_value() then
             _readable = false
             return
           end
 
           data.truncate(len)
           _notify.received(this, consume data, consume from)
 
-          sum = sum + len
+          // .max(1) charges 1 byte against the budget for a zero-byte
+          // datagram, preventing this loop from starving other actors on a
+          // stream of empty datagrams.
+          sum = sum + len.max(1)
 
           if sum > (1 << 12) then
             _read_again()

src/libponyrt/lang/socket.c

@@ -1119,10 +1119,8 @@ PONY_API size_t pony_os_recvfrom(asio_event_t* ev, char* buf, size_t len,
   if(recvd < 0)
   {
     if(errno == EWOULDBLOCK || errno == EAGAIN)
-      return 0;
+      return (size_t)-1;
 
-    pony_error();
-  } else if(recvd == 0) {
     pony_error();
   }