node: Explicit default for `AddressConfig`

The intent to drop outgoing connections is modeled as `Option::None`
which is brittle and easy to miss.

Extend `enum AddressConfig` with a variant that is more explicit.

crates/radicle-cli/examples/rad-config.md

@@ -247,14 +247,7 @@ $ rad config schema
         },
         "onion": {
           "description": "Onion address config.",
-          "anyOf": [
-            {
-              "$ref": "#/$defs/AddressConfig"
-            },
-            {
-              "type": "null"
-            }
-          ]
+          "$ref": "#/$defs/AddressConfig"
         },
         "network": {
           "description": "Peer-to-peer network.",
@@ -410,6 +403,19 @@ $ rad config schema
           "required": [
             "mode"
           ]
+        },
+        {
+          "description": "Drop connections to this address type.",
+          "type": "object",
+          "properties": {
+            "mode": {
+              "type": "string",
+              "const": "drop"
+            }
+          },
+          "required": [
+            "mode"
+          ]
         }
       ]
     },

crates/radicle-node/src/wire.rs

@@ -1096,10 +1096,10 @@ pub fn dial<G: Ecdh<Pk = NodeId>>(
         (HostName::Tor(onion), proxy) => match config.onion {
             // In onion proxy mode, simply use the configured proxy address.
             // This takes precedence over any global proxy.
-            Some(AddressConfig::Proxy { address }) => address.into(),
+            AddressConfig::Proxy { address } => address.into(),
             // In "forward" mode, if a global proxy is set, we use that, otherwise
             // we treat `.onion` addresses as regular DNS names.
-            Some(AddressConfig::Forward) => {
+            AddressConfig::Forward => {
                 if let Some(proxy) = proxy {
                     proxy.into()
                 } else {
@@ -1107,7 +1107,7 @@ pub fn dial<G: Ecdh<Pk = NodeId>>(
                 }
             }
             // If onion address support isn't configured, refuse to connect.
-            None => {
+            AddressConfig::Drop => {
                 return Err(io::Error::new(
                     io::ErrorKind::Unsupported,
                     "no configuration found for .onion addresses",

crates/radicle-protocol/src/service.rs

@@ -30,6 +30,8 @@ use radicle::node;
 use radicle::node::address;
 use radicle::node::address::Store as _;
 use radicle::node::address::{AddressBook, AddressType, KnownAddress};
+#[cfg(feature = "tor")]
+use radicle::node::config::AddressConfig;
 use radicle::node::config::{PeerConfig, RateLimit};
 use radicle::node::device::Device;
 use radicle::node::refs::Store as _;
@@ -2656,7 +2658,7 @@ where
         match AddressType::from(address) {
             // Only consider onion addresses if configured.
             #[cfg(feature = "tor")]
-            AddressType::Onion => self.config.onion.is_some(),
+            AddressType::Onion => self.config.onion != AddressConfig::Drop,
             AddressType::Dns | AddressType::Ipv4 | AddressType::Ipv6 => true,
         }
     }

crates/radicle/src/node/config.rs

@@ -355,7 +355,7 @@ pub enum Relay {
 }
 
 /// Proxy configuration.
-#[derive(Debug, Clone, Serialize, Deserialize)]
+#[derive(Debug, Copy, Clone, Default, PartialEq, Serialize, Deserialize)]
 #[serde(rename_all = "camelCase", tag = "mode")]
 #[cfg_attr(feature = "schemars", derive(schemars::JsonSchema))]
 #[cfg(feature = "tor")]
@@ -368,6 +368,9 @@ pub enum AddressConfig {
     /// Forward address to the next layer. Either this is the global proxy,
     /// or the operating system, via DNS.
     Forward,
+    /// Drop connections to this address type.
+    #[default]
+    Drop,
 }
 
 /// Default seeding policy. Applies when no repository policies for the given repo are found.
@@ -546,8 +549,12 @@ pub struct Config {
     pub proxy: Option<net::SocketAddr>,
     /// Onion address config.
     #[cfg(feature = "tor")]
-    #[serde(default, skip_serializing_if = "Option::is_none")]
-    pub onion: Option<AddressConfig>,
+    #[serde(
+        default,
+        skip_serializing_if = "crate::serde_ext::is_default",
+        deserialize_with = "crate::serde_ext::null_to_default"
+    )]
+    pub onion: AddressConfig,
     /// Peer-to-peer network.
     #[serde(default)]
     pub network: Network,
@@ -603,7 +610,7 @@ impl Config {
             network: Network::default(),
             proxy: None,
             #[cfg(feature = "tor")]
-            onion: None,
+            onion: AddressConfig::Drop,
             relay: Relay::default(),
             limits: Limits::default(),
             workers: Workers::default(),
@@ -944,4 +951,27 @@ mod test {
             crate::storage::refs::FeatureLevel::Parent
         );
     }
+
+    #[cfg(feature = "tor")]
+    #[test]
+    fn onion_absent() {
+        let actual: super::Config = serde_json::from_value(json!({
+            "alias": "radicle",
+        }))
+        .unwrap();
+        assert_eq!(super::AddressConfig::Drop, actual.onion);
+    }
+
+    #[cfg(feature = "tor")]
+    #[test]
+    fn onion_null() {
+        // Backwards compatibility: Prior versions allowed to set `onion` to `null`,
+        // which should be treated the same as the default, i.e. `Drop`.
+        let actual: super::Config = serde_json::from_value(json!({
+            "alias": "radicle",
+            "onion": null,
+        }))
+        .unwrap();
+        assert_eq!(super::AddressConfig::Drop, actual.onion);
+    }
 }