consensus: Check block times against the local clock

Part of #477.

zebra-consensus/src/verify.rs

@@ -88,16 +88,35 @@ where
     fn call(&mut self, block: Arc<Block>) -> Self::Future {
         // TODO(jlusby): Error = Report, handle errors from state_service.
         // TODO(teor):
-        //   - handle chain reorgs, adjust state_service "unique block height" conditions
-        //   - handle block validation errors (including errors in the block's transactions)
+        //   - handle chain reorgs
+        //   - adjust state_service "unique block height" conditions
+        //   - move expensive checks into the async block
 
+        // Create an AddBlock Future, but don't run it yet.
+        //
         // `state_service.call` is OK here because we already called
         // `state_service.poll_ready` in our `poll_ready`.
-        let add_block = self
-            .state_service
-            .call(zebra_state::Request::AddBlock { block });
+        //
+        // `tower::Buffer` expects exactly one `call` for each
+        // `poll_ready`. So we unconditionally create the AddBlock
+        // Future using `state_service.call`. If verification fails,
+        // we return an error, and implicitly cancel the future.
+        let add_block = self.state_service.call(zebra_state::Request::AddBlock {
+            block: block.clone(),
+        });
 
         async move {
+            // If verification fails, return an error result.
+            // The AddBlock Future is implicitly cancelled by the
+            // error return in `?`.
+
+            // Since errors cause an early exit, try to do the
+            // quick checks first.
+            block::node_time_check(block)?;
+
+            // Verification was successful.
+            // Add the block to the state by awaiting the AddBlock
+            // Future, and return its result.
             match add_block.await? {
                 zebra_state::Response::Added { hash } => Ok(hash),
                 _ => Err("adding block to zebra-state failed".into()),