src: Allow for empty set variable definition

Allow for empty set definition in variables if they are merged to
non-empty set definition:

 define BASE_ALLOWED_INCOMING_TCP_PORTS = {22, 80, 443}
 define EXTRA_ALLOWED_INCOMING_TCP_PORTS = {}

 table inet filter {
    chain input {
        type filter hook input priority 0; policy drop;
        tcp dport {$BASE_ALLOWED_INCOMING_TCP_PORTS, $EXTRA_ALLOWED_INCOMING_TCP_PORTS} ct state new counter accept
    }
 }

However, disallow this:

 define EXTRA_ALLOWED_INCOMING_TCP_PORTS = {}

 table inet filter {
    chain input {
        type filter hook input priority 0; policy drop;
        tcp dport {$EXTRA_ALLOWED_INCOMING_TCP_PORTS} ct state new counter accept
    }
 }

 # nft -f x.nft
 /tmp/x.nft:6:18-52: Error: Set is empty
        tcp dport {$EXTRA_ALLOWED_INCOMING_TCP_PORTS} ct state new counter accept
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

1 files changed, 17 insertions, 0 deletions

diff --git a/tests/shell/testcases/sets/0050set_define_1 b/tests/shell/testcases/sets/0050set_define_1
new file mode 100755
index 00000000..c12de177
--- /dev/null
+++ b/tests/shell/testcases/sets/0050set_define_1
@@ -0,0 +1,17 @@
+#!/bin/bash
+
+set -e
+
+EXPECTED="define BASE_ALLOWED_INCOMING_TCP_PORTS = {}
+
+table inet filter {
+	chain input {
+		type filter hook input priority 0; policy drop;
+		tcp dport {\$BASE_ALLOWED_INCOMING_TCP_PORTS} ct state new counter accept
+	}
+}
+"
+
+$NFT -f - <<< "$EXPECTED" &> /dev/null || exit 0
+echo "E: Accepted empty set" 1>&2
+exit 1