Fix: Support optional UIDL argument in POP3 server (#552)

* fix: use single-line response when UIDL has an argument

The test changes included here don't necessarily deal with the fact that
the response used to be multi-line: the failure wouldn't occur during
the `c.Uidl()` calls, but rather on the next one as the client would
still have data from the server to receive, causing a parsing error like
so:

    pop3_test.go:103: strconv.Atoi: parsing "unique-id": invalid syntax

The server now correctly replies with a single line response when an
argument is passed, as required by [the spec][1]

[1]: https://www.rfc-editor.org/rfc/rfc1939.html#page-12

* fix: UIDL accepts at most one argument

internal/pop3/pop3_test.go

@@ -84,6 +84,43 @@ func TestPOP3(t *testing.T) {
 		}
 	}
 
+	t.Log("Checking UIDL with multiple arguments")
+
+	_, err = c.Cmd("UIDL", false, 1, 2, 3)
+	if err == nil {
+		t.Error("UIDL with multiple arguments should return an error")
+		return
+	}
+
+	t.Log("Checking UIDL without a message id")
+
+	messageIDs, err := c.Uidl(0)
+	if err != nil {
+		t.Error(err.Error())
+		return
+	}
+
+	if len(messageIDs) != 50 {
+		assertEqual(t, len(messageIDs), 50, "incorrect UIDL message count")
+	}
+
+	t.Log("Checking UIDL with a message ID")
+
+	messageIDs, err = c.Uidl(50)
+	if err != nil {
+		t.Error(err.Error())
+		return
+	}
+
+	assertEqual(t, len(messageIDs), 1, "incorrect UIDL message count")
+
+	t.Log("Checking UIDL with an invalid message ID")
+
+	if _, err := c.Uidl(51); err == nil {
+		t.Errorf("UIDL 51 should return an error")
+		return
+	}
+
 	t.Log("Deleting 25 messages")
 
 	for i := 1; i <= 25; i++ {

internal/pop3/server.go

@@ -239,11 +239,31 @@ func handleTransactionCommand(conn net.Conn, cmd string, args []string, messages
 			sendResponse(conn, ".")
 		}
 	case "UIDL":
-		sendResponse(conn, "+OK unique-id listing follows")
-		for row, m := range messages {
-			sendResponse(conn, fmt.Sprintf("%d %s", row+1, m.ID))
+		if len(args) > 1 {
+			sendResponse(conn, "-ERR UIDL takes at most one argument")
+		} else if len(args) == 1 {
+			nr, err := strconv.Atoi(args[0])
+			if err != nil {
+				sendResponse(conn, "-ERR no such message")
+				return
+			}
+
+			if nr < 1 || nr > len(messages) {
+				sendResponse(conn, "-ERR no such message")
+				return
+			}
+
+			m := messages[nr-1]
+
+			sendResponse(conn, fmt.Sprintf("+OK %d %s", nr, m.ID))
+		} else {
+			sendResponse(conn, "+OK unique-id listing follows")
+			for row, m := range messages {
+				sendResponse(conn, fmt.Sprintf("%d %s", row+1, m.ID))
+			}
+			sendResponse(conn, ".")
 		}
-		sendResponse(conn, ".")
+
 	case "RETR":
 		if len(args) != 1 {
 			sendResponse(conn, "-ERR no such message")