channel: Introduce padding config

Nothing actually reads this yet, and we also want a client-global
default for padding.

crates/arti/src/arti-example-config.toml

@@ -208,7 +208,19 @@
 
 # Channels and their behaviour
 [channel]
-# (currently no settings: TODO)
+
+# Should we use reduced channel padding?  (This roughly halves the padding
+# cell frequency, and makes the padding unidrecteional, increasing the
+# traceability of the client connections.)
+# Or disable it entirely?
+#
+#padding = "normal"
+#   padding = "reduced"
+#   padding = "none"
+
+# Full manual control of the precise padding timing parameters is available
+# by setting `override_net_params.nf_ito_low` et al.
+# (See torpsec/padding-spec.txt section 3.4.)
 
 # Rules for how long circuits should survive, and how long pending
 # requests should wait for a circuit.

crates/tor-chanmgr/src/config.rs

@@ -5,7 +5,7 @@
 //! Most types in this module are re-exported by `arti-client`.
 
 use tor_config::impl_standard_builder;
-use tor_config::ConfigBuildError;
+use tor_config::{ConfigBuildError, PaddingLevel};
 
 use derive_builder::Builder;
 use serde::{Deserialize, Serialize};
@@ -17,10 +17,10 @@ use serde::{Deserialize, Serialize};
 #[derive(Debug, Clone, Builder, Eq, PartialEq)]
 #[builder(build_fn(error = "ConfigBuildError"))]
 #[builder(derive(Debug, Serialize, Deserialize))]
-#[non_exhaustive] // TODO needed until there are fields
-#[builder_struct_attr(non_exhaustive)] // TODO needed until there are fields
 pub struct ChannelConfig {
-    // TODO
+    /// Control of channel padding
+    #[builder(default)]
+    pub(crate) padding: PaddingLevel,
 }
 impl_standard_builder! { ChannelConfig }
 
@@ -38,8 +38,8 @@ mod test {
 
     #[test]
     fn channel_config() {
-        let _config = ChannelConfig::default();
+        let config = ChannelConfig::default();
 
-        // TODO
+        assert_eq!(PaddingLevel::Normal, config.padding);
     }
 }