- Makes the `activation_heights` config field optional by adding a #[serde(default)]

- Panics if a non-zero activation height is provided for the `Genesis` network upgrade
- Always sets the `Genesis` and `BeforeOverwinter` network upgrade activation heights to 0 and 1, `BeforeOverwinter` could be overwritten by a later network upgrade
- Makes the `activation_heights` field on `Parameters` private, adds/uses an accessor method instead, and adds a builder struct and `build()` method

zebra-chain/src/parameters/network/testnet.rs

@@ -6,6 +6,54 @@ use crate::{
     parameters::{network_upgrade::TESTNET_ACTIVATION_HEIGHTS, NetworkUpgrade},
 };
 
+/// Builder for the [`Parameters`] struct.
+#[derive(Clone, Debug, Eq, PartialEq, Hash, Serialize, Deserialize)]
+pub struct ParametersBuilder {
+    /// The network upgrade activation heights for this network, see [`Parameters::activation_heights`] for more details.
+    activation_heights: BTreeMap<Height, NetworkUpgrade>,
+}
+
+impl Default for ParametersBuilder {
+    fn default() -> Self {
+        Self {
+            // # Correctness
+            //
+            // `Genesis` network upgrade activation height must always be 0
+            // TODO: Find out if `BeforeOverwinter` must always be active at height 1
+            activation_heights: [
+                (Height(0), NetworkUpgrade::Genesis),
+                (Height(1), NetworkUpgrade::BeforeOverwinter),
+            ]
+            .into_iter()
+            .collect(),
+        }
+    }
+}
+
+impl ParametersBuilder {
+    /// Extends network upgrade activation heights with the provided activation heights.
+    pub fn activation_heights(
+        mut self,
+        new_activation_heights: Vec<(Height, NetworkUpgrade)>,
+    ) -> Self {
+        let new_activation_heights: BTreeMap<_, _> =
+            new_activation_heights.iter().cloned().collect();
+
+        // # Correctness
+        //
+        // Height(0) must be reserved for the `NetworkUpgrade::Genesis`.
+        self.activation_heights
+            .extend(new_activation_heights.range(Height(1)..));
+        self
+    }
+
+    /// Converts the builder to a [`Parameters`] struct
+    pub fn finish(self) -> Parameters {
+        let Self { activation_heights } = self;
+        Parameters { activation_heights }
+    }
+}
+
 /// Network consensus parameters for test networks such as Regtest and the default Testnet.
 #[derive(Clone, Debug, Eq, PartialEq, Hash, Serialize, Deserialize)]
 pub struct Parameters {
@@ -14,21 +62,31 @@ pub struct Parameters {
     /// Note: This value is ignored by `Network::activation_list()` when `zebra-chain` is
     ///       compiled with the `zebra-test` feature flag AND the `TEST_FAKE_ACTIVATION_HEIGHTS`
     ///       environment variable is set.
-    pub activation_heights: BTreeMap<Height, NetworkUpgrade>,
+    activation_heights: BTreeMap<Height, NetworkUpgrade>,
 }
 
 impl Default for Parameters {
     /// Returns an instance of the default public testnet [`Parameters`].
     fn default() -> Self {
-        Self {
-            activation_heights: TESTNET_ACTIVATION_HEIGHTS.iter().cloned().collect(),
-        }
+        Self::build()
+            .activation_heights(TESTNET_ACTIVATION_HEIGHTS.to_vec())
+            .finish()
     }
 }
 
 impl Parameters {
+    /// Creates a new [`ParametersBuilder`].
+    pub fn build() -> ParametersBuilder {
+        ParametersBuilder::default()
+    }
+
     /// Returns true if the instance of [`Parameters`] represents the default public Testnet.
     pub fn is_default_testnet(&self) -> bool {
         self == &Self::default()
     }
+
+    /// Returns a reference to the network upgrade activation heights
+    pub fn activation_heights(&self) -> &BTreeMap<Height, NetworkUpgrade> {
+        &self.activation_heights
+    }
 }

zebra-chain/src/parameters/network_upgrade.rs

@@ -264,7 +264,7 @@ impl Network {
             }
 
             Mainnet => MAINNET_ACTIVATION_HEIGHTS.iter().cloned().collect(),
-            Testnet(params) => params.activation_heights.clone(),
+            Testnet(params) => params.activation_heights().clone(),
         }
     }
 }

zebra-network/src/config.rs

@@ -625,9 +625,9 @@ impl<'de> Deserialize<'de> for Config {
     where
         D: Deserializer<'de>,
     {
-        /// Network consensus parameters for test networks such as Regtest and the default Testnet.
-        #[derive(Clone, Debug, Eq, PartialEq, Hash, Serialize, Deserialize)]
-        pub struct DTestnetParameters {
+        #[derive(Deserialize)]
+        struct DTestnetParameters {
+            #[serde(default)]
             pub(super) activation_heights: Vec<(u32, NetworkUpgrade)>,
         }
 
@@ -686,6 +686,11 @@ impl<'de> Deserialize<'de> for Config {
             let activation_heights = activation_heights
                 .into_iter()
                 .map(|(height, network_upgrade)| {
+                    assert!(
+                        network_upgrade != NetworkUpgrade::Genesis || height == 0,
+                        "Genesis network upgrade activation height is not configurable"
+                    );
+
                     (
                         height.try_into().expect("activation height must be valid"),
                         network_upgrade,
@@ -693,7 +698,11 @@ impl<'de> Deserialize<'de> for Config {
                 })
                 .collect();
 
-            Network::new_configured_testnet(testnet::Parameters { activation_heights })
+            let testnet_parameters = testnet::Parameters::build()
+                .activation_heights(activation_heights)
+                .finish();
+
+            Network::new_configured_testnet(testnet_parameters)
         } else {
             // Convert to default `Network` for a `NetworkKind` if there are no testnet parameters.
             match network_kind {

zebrad/tests/common/configs/v1.7.0.toml

@@ -62,7 +62,6 @@ peerset_initial_target_size = 25
 
 [network.testnet_parameters]
 activation_heights = [
-    [0, "Genesis"],
     [1, "BeforeOverwinter"],
     [207_500, "Overwinter"],
     [280_000, "Sapling"],