FIX: missing messages in UDP server

related to: Oldes/Rebol-issues#1803

src/core/p-net.c

@@ -217,6 +217,7 @@ enum Transport_Types {
 		//Print("(max read length %d)", sock->length);
 		result = OS_DO_DEVICE(sock, RDC_READ); // recv can happen immediately
 		if (result < 0) Trap_Port(RE_READ_ERROR, port, sock->error);
+		VAL_TAIL(arg) += sock->actual;
 		break;
 
 	case A_WRITE:

src/tests/test-udp-client.r3

@@ -0,0 +1,40 @@
+Rebol [
+	Title: "Test UDP client"
+	File: %test-udp-client.r3
+	Note: https://github.com/Oldes/Rebol-issues/issues/1803
+]
+
+; start test UDP server...
+launch probe to-real-file %test-udp-server.r3
+
+; wait some time for server to boot...
+wait 0:0:0.1
+
+stdout: system/ports/output
+
+; open port for output...
+port: open udp://127.0.0.1:1189
+port/awake: func[event][
+	print ["[UDP Client] event:" event/type]
+	;; console output is buffered and so messages could come out of order;
+	;; to prevent it, just use the `flush`! 
+	flush stdout
+	true
+]
+; wait for port to be opened (required on Windows!)...
+wait port
+
+write-udp:  func[msg [string!]][
+	print ["[UDP Client] sending:" as-yellow mold msg]
+	flush stdout ;= flush console buffer!
+	write port msg
+]
+write-udp "Hello"
+write-udp "Rebol"
+wait 1 ;= pretending some work here...
+write-udp "I'm done!"
+write-udp "quit"
+
+either system/options/script [
+	  ask as-red "CLIENT DONE"
+][	print as-red "CLIENT DONE" wait 0.5]

src/tests/test-udp-server.r3

@@ -10,18 +10,23 @@ udp-server: try/except [open udp://:1189][
 	print as-purple "Failed to listen on UDP port 1189!"
 	quit
 ]
+stdout: system/ports/output
 
 udp-server/awake: func [event] [
-	print ["UDP event:" as-yellow event/type]
+	print ["[UDP Server] event:" event/type]
+	;; console output is buffered and so messages could come out of order;
+	;; to prevent it, just use the `flush`! 
+	flush stdout
 	true
 ]
 
 forever [
 	wait read udp-server
 	str: to string! udp-server/data
-	print ["UDP input:" as-green mold str]
+	print ["[UDP Server] received:" as-green mold str]
+	flush stdout ;= flush console buffer!
 	if str = "quit" [close udp-server break]
 	clear udp-server/data
 ]
-print as-red "SERVER DONE"
-wait 0:0:1
+ 
+print as-red "SERVER DONE"