Move all contextual validation code into its own function

This change has two benefits:
* reduces conflicts with the sled refactor and any replacement
* allows the function to be called independently for testing

zebra-state/src/service.rs

@@ -185,37 +185,16 @@ impl StateService {
         }
     }
 
-    /// Check that `block` is contextually valid based on the committed finalized
-    /// and non-finalized state.
+    /// Check that `block` is contextually valid for the configured network,
+    /// based on the committed finalized and non-finalized state.
     fn check_contextual_validity(&mut self, block: &Block) -> Result<(), ValidateContextError> {
-        let height = block
-            .coinbase_height()
-            .expect("semantically valid blocks have a coinbase height");
-        let hash = block.hash();
+        check::block_is_contextually_valid(
+            block,
+            self.network,
+            self.sled.finalized_tip_height(),
+            self.chain(block.header.previous_block_hash),
+        )?;
 
-        let span = tracing::info_span!("StateService::check_contextual_validity",
-                                             ?height, network = ?self.network, ?hash);
-        let _entered = span.enter();
-
-        let finalized_tip_height = self.sled.finalized_tip_height().expect(
-            "finalized state must contain at least one block to use the non-finalized state",
-        );
-        check::block_is_not_orphaned(finalized_tip_height, block)?;
-
-        let mut relevant_chain = self.chain(block.header.previous_block_hash);
-        let parent_block = relevant_chain
-            .next()
-            .expect("state must contain parent block to do contextual validation");
-        let parent_height = parent_block
-            .coinbase_height()
-            .expect("valid blocks have a coinbase height");
-        let parent_hash = parent_block.hash();
-        check::height_one_more_than_parent_height(parent_height, block)?;
-        // should be impossible by design, so no handleable error is thrown
-        assert_eq!(parent_hash, block.header.previous_block_hash);
-
-        // TODO: validate difficulty adjustment
-        // TODO: other contextual validation design and implelentation
         Ok(())
     }
 

zebra-state/src/service/check.rs

@@ -1,9 +1,64 @@
 //! Consensus critical contextual checks
 
-use zebra_chain::block::{self, Block};
+use zebra_chain::{
+    block::{self, Block},
+    parameters::Network,
+};
 
 use crate::ValidateContextError;
 
+use super::check;
+
+/// Check that `block` is contextually valid for `network`, based on the
+/// `finalized_tip_height` and `relevant_chain`.
+///
+/// The relevant chain is an iterator over the ancestors of `block`, starting
+/// with its parent block.
+pub(crate) fn block_is_contextually_valid<C>(
+    block: &Block,
+    network: Network,
+    finalized_tip_height: Option<block::Height>,
+    relevant_chain: C,
+) -> Result<(), ValidateContextError>
+where
+    C: IntoIterator,
+    C::Item: AsRef<Block>,
+{
+    let height = block
+        .coinbase_height()
+        .expect("semantically valid blocks have a coinbase height");
+    let hash = block.hash();
+
+    let span = tracing::info_span!(
+        "StateService::check_contextual_validity",
+        ?height,
+        ?network,
+        ?hash
+    );
+    let _entered = span.enter();
+
+    let finalized_tip_height = finalized_tip_height
+        .expect("finalized state must contain at least one block to use the non-finalized state");
+    check::block_is_not_orphaned(finalized_tip_height, block)?;
+
+    let mut relevant_chain = relevant_chain.into_iter();
+    let parent_block = relevant_chain
+        .next()
+        .expect("state must contain parent block to do contextual validation");
+    let parent_block = parent_block.as_ref();
+    let parent_height = parent_block
+        .coinbase_height()
+        .expect("valid blocks have a coinbase height");
+    let parent_hash = parent_block.hash();
+    check::height_one_more_than_parent_height(parent_height, block)?;
+    // should be impossible by design, so no handleable error is thrown
+    assert_eq!(parent_hash, block.header.previous_block_hash);
+
+    // TODO: validate difficulty adjustment
+    // TODO: other contextual validation design and implelentation
+    Ok(())
+}
+
 /// Returns `ValidateContextError::OrphanedBlock` if the height of the given
 /// block is less than or equal to the finalized tip height.
 pub(super) fn block_is_not_orphaned(