consensus: Add a block time check function

Add a function that checks the block header miner times against the
node's local clock.

(We'll use this function in the next commit.)

Part of #477.

Cargo.lock

@@ -2327,6 +2327,7 @@ version = "0.1.0"
 name = "zebra-consensus"
 version = "0.1.0"
 dependencies = [
+ "chrono",
  "color-eyre",
  "futures-util",
  "spandoc",

zebra-consensus/Cargo.toml

@@ -10,6 +10,7 @@ edition = "2018"
 [dependencies]
 zebra-chain = { path = "../zebra-chain" }
 zebra-state = { path = "../zebra-state" }
+chrono = "0.4.11"
 futures-util = "0.3.5"
 tower = "0.3.1"
 

zebra-consensus/src/verify.rs

@@ -31,6 +31,43 @@ struct BlockVerifier<S> {
 // TODO(jlusby): Error = Report ?
 type Error = Box<dyn error::Error + Send + Sync + 'static>;
 
+/// Block validity checks
+mod block {
+    use super::Error;
+
+    use chrono::{Duration, Utc};
+    use std::sync::Arc;
+
+    use zebra_chain::block::Block;
+
+    /// Check if the block header time is less than or equal to
+    /// 2 hours in the future, according to the node's local clock.
+    ///
+    /// This is a non-deterministic rule, as clocks vary over time, and
+    /// between different nodes.
+    ///
+    /// "In addition, a full validator MUST NOT accept blocks with nTime
+    /// more than two hours in the future according to its clock. This
+    /// is not strictly a consensus rule because it is nondeterministic,
+    /// and clock time varies between nodes. Also note that a block that
+    /// is rejected by this rule at a given point in time may later be
+    /// accepted."[S 7.5][7.5]
+    ///
+    /// [7.5]: https://zips.z.cash/protocol/protocol.pdf#blockheader
+    pub(super) fn node_time_check(block: Arc<Block>) -> Result<(), Error> {
+        let now = Utc::now();
+        let two_hours_in_the_future = now
+            .checked_add_signed(Duration::hours(2))
+            .ok_or("overflow when calculating 2 hours in the future")?;
+
+        if block.header.time <= two_hours_in_the_future {
+            Ok(())
+        } else {
+            Err("block header time is more than 2 hours in the future".into())
+        }
+    }
+}
+
 /// The BlockVerifier service implementation.
 ///
 /// After verification, blocks are added to the underlying state service.