feature: Add a "Genesis" network upgrade

We can use this network upgrade to implement different consensus rules
and chain context handling for genesis blocks.

Part of the chain state design in #682.

zebra-consensus/src/parameters/network_upgrade.rs

@@ -8,12 +8,18 @@ use std::ops::Bound::*;
 use zebra_chain::types::BlockHeight;
 use zebra_chain::{Network, Network::*};
 
-/// A Zcash network protocol upgrade.
-//
-// TODO: are new network upgrades a breaking change, or should we make this
-//       enum non-exhaustive?
+/// A Zcash network upgrade.
+///
+/// Network upgrades can change the Zcash network protocol or consensus rules in
+/// incompatible ways.
 #[derive(Copy, Clone, Debug, Eq, Hash, PartialEq)]
 pub enum NetworkUpgrade {
+    /// The Zcash protocol for a Genesis block.
+    ///
+    /// Zcash genesis blocks use a different set of consensus rules from
+    /// other BeforeOverwinter blocks, so we treat them like a separate network
+    /// upgrade.
+    Genesis,
     /// The Zcash protocol before the Overwinter upgrade.
     ///
     /// We avoid using `Sprout`, because the specification says that Sprout
@@ -36,7 +42,8 @@ pub enum NetworkUpgrade {
 /// This is actually a bijective map, but it is const, so we use a vector, and
 /// do the uniqueness check in the unit tests.
 pub(crate) const MAINNET_ACTIVATION_HEIGHTS: &[(BlockHeight, NetworkUpgrade)] = &[
-    (BlockHeight(0), BeforeOverwinter),
+    (BlockHeight(0), Genesis),
+    (BlockHeight(1), BeforeOverwinter),
     (BlockHeight(347_500), Overwinter),
     (BlockHeight(419_200), Sapling),
     (BlockHeight(653_600), Blossom),
@@ -49,7 +56,8 @@ pub(crate) const MAINNET_ACTIVATION_HEIGHTS: &[(BlockHeight, NetworkUpgrade)] =
 /// This is actually a bijective map, but it is const, so we use a vector, and
 /// do the uniqueness check in the unit tests.
 pub(crate) const TESTNET_ACTIVATION_HEIGHTS: &[(BlockHeight, NetworkUpgrade)] = &[
-    (BlockHeight(0), BeforeOverwinter),
+    (BlockHeight(0), Genesis),
+    (BlockHeight(1), BeforeOverwinter),
     (BlockHeight(207_500), Overwinter),
     (BlockHeight(280_000), Sapling),
     (BlockHeight(584_000), Blossom),
@@ -68,8 +76,8 @@ pub struct ConsensusBranchId(u32);
 /// Branch ids are the same for mainnet and testnet. If there is a testnet
 /// rollback after a bug, the branch id changes.
 ///
-/// Branch ids were introduced in the Overwinter upgrade, so there is no
-/// BeforeOverwinter branch id.
+/// Branch ids were introduced in the Overwinter upgrade, so there are no
+/// Genesis or BeforeOverwinter branch ids.
 ///
 /// This is actually a bijective map, but it is const, so we use a vector, and
 /// do the uniqueness check in the unit tests.

zebra-consensus/src/parameters/tests.rs

@@ -39,21 +39,32 @@ fn activation_extremes_testnet() {
 /// Test the activation_list, activation_height, current, and next functions
 /// for `network` with extreme values.
 fn activation_extremes(network: Network) {
-    // The first two upgrades are BeforeOverwinter and Overwinter
+    // The first three upgrades are Genesis, BeforeOverwinter, and Overwinter
     assert_eq!(
         NetworkUpgrade::activation_list(network).get(&BlockHeight(0)),
+        Some(&Genesis)
+    );
+    assert_eq!(Genesis.activation_height(network), Some(BlockHeight(0)));
+    assert_eq!(NetworkUpgrade::current(network, BlockHeight(0)), Genesis);
+    assert_eq!(
+        NetworkUpgrade::next(network, BlockHeight(0)),
+        Some(BeforeOverwinter)
+    );
+
+    assert_eq!(
+        NetworkUpgrade::activation_list(network).get(&BlockHeight(1)),
         Some(&BeforeOverwinter)
     );
     assert_eq!(
         BeforeOverwinter.activation_height(network),
-        Some(BlockHeight(0))
+        Some(BlockHeight(1))
     );
     assert_eq!(
-        NetworkUpgrade::current(network, BlockHeight(0)),
+        NetworkUpgrade::current(network, BlockHeight(1)),
         BeforeOverwinter
     );
     assert_eq!(
-        NetworkUpgrade::next(network, BlockHeight(0)),
+        NetworkUpgrade::next(network, BlockHeight(1)),
         Some(Overwinter)
     );
 
@@ -61,12 +72,9 @@ fn activation_extremes(network: Network) {
     // (even if we suspect that won't be true)
     assert_ne!(
         NetworkUpgrade::activation_list(network).get(&BlockHeight::MAX),
-        Some(&BeforeOverwinter)
-    );
-    assert_ne!(
-        NetworkUpgrade::current(network, BlockHeight::MAX),
-        BeforeOverwinter
+        Some(&Genesis)
     );
+    assert_ne!(NetworkUpgrade::current(network, BlockHeight::MAX), Genesis);
     assert_eq!(NetworkUpgrade::next(network, BlockHeight::MAX), None);
 }
 

zebra-network/src/protocol/external/types.rs

@@ -43,7 +43,7 @@ impl Version {
         // TODO: Should we reject earlier protocol versions during our initial
         //       sync? zcashd accepts 170_002 or later during its initial sync.
         Version(match (network, network_upgrade) {
-            (_, BeforeOverwinter) => 170_002,
+            (_, Genesis) | (_, BeforeOverwinter) => 170_002,
             (Testnet, Overwinter) => 170_003,
             (Mainnet, Overwinter) => 170_005,
             (_, Sapling) => 170_007,