smite: fuzz the simple taproot channel funding flow

Cargo.toml

@@ -29,6 +29,7 @@ rand = { version = "0.10", default-features = false }
 simple_logger = { version = "5", default-features = false }
 postcard = { version = "1.1", default-features = false, features = ["alloc"] }
 bitcoin = "0.32"
+musig2 = { version = "0.4", default-features = false, features = ["secp256k1"] }
 serde = { version = "1", features = ["derive"] }
 thiserror = "2"
 serde_json = "1"

smite-ir/src/generators/funding_created.rs

@@ -26,6 +26,7 @@ impl Generator for FundingCreatedGenerator {
         let funding_satoshis = builder.pick_variable(VariableType::Amount, rng);
         let feerate_per_kw = builder.pick_variable(VariableType::FeeratePerKw, rng);
         let temporary_channel_id = builder.pick_variable(VariableType::ChannelId, rng);
+        let channel_type = builder.pick_variable(VariableType::Features, rng);
 
         // Create the BOLT 3 funding transaction.
         let funding_transaction = builder.append(
@@ -35,6 +36,7 @@ impl Generator for FundingCreatedGenerator {
                 acceptor_funding_pubkey,
                 funding_satoshis,
                 feerate_per_kw,
+                channel_type,
             ],
         );
 

smite-ir/src/generators/funding_flow.rs

@@ -42,7 +42,6 @@ impl Generator for FundingFlowGenerator {
         let feerate_per_kw = builder.pick_variable(VariableType::FeeratePerKw, rng);
         let to_self_delay = builder.pick_variable(VariableType::U16, rng);
         let max_accepted_htlcs = builder.pick_variable(VariableType::U16, rng);
-        let channel_flags = builder.pick_variable(VariableType::U8, rng);
         let shutdown_script_variant = ShutdownScriptVariant::random(rng);
         let upfront_shutdown_script =
             builder.append(Operation::LoadShutdownScript(shutdown_script_variant), &[]);
@@ -51,6 +50,16 @@ impl Generator for FundingFlowGenerator {
             .expect("ChannelTypeVariant::ALL is non-empty");
         let channel_type = builder.append(Operation::LoadChannelType(variant), &[]);
 
+        // Taproot channels cannot be announced, so a random `channel_flags`
+        // would leave the happy path unreachable for a third of the channel
+        // types. Mutators can still flip the bit afterwards to exercise how
+        // targets reject a public taproot channel.
+        let channel_flags = if variant.requires_unannounced_channel() {
+            builder.append(Operation::LoadU8(0), &[])
+        } else {
+            builder.pick_variable(VariableType::U8, rng)
+        };
+
         // Build and send open_channel.
         let open_channel_msg = builder.append(
             Operation::BuildOpenChannel,
@@ -94,6 +103,9 @@ impl Generator for FundingFlowGenerator {
                 acceptor_funding_pubkey,
                 funding_satoshis,
                 feerate_per_kw,
+                // Must match the type sent in open_channel, or the funding
+                // output will not be the one the target negotiated.
+                channel_type,
             ],
         );