guestchain: clear candidates changed flag only if new epoch to start (#…

…386)

The Candidates struct has a changed flag indicating whether the set of
candidates has changed (in such a way that it would affect the
validators set). This is used to determine whether a new epoch should be
started or not. Once next epoch is scheduled in the block, the flag
should be cleared.

Alas, the code clears the flag each time block is generated even if a
new epoch isn’t about to start. As a consequence, if set of candidates
changes in the middle of an epoch, the change will not be reflected once
the new epoch is ready to start.

Fix this by clearing the flag only if next_epoch is populated in
produced block.

common/guestchain/src/manager.rs

@@ -198,13 +198,13 @@ impl<PK: crate::PubKey> ChainManager<PK> {
         host_height: crate::HostHeight,
         host_timestamp: NonZeroU64,
         state_root: CryptoHash,
-    ) -> Result<bool, GenerateError> {
+    ) -> Result<(), GenerateError> {
         let next_epoch = self.validate_generate_next(
             host_height,
             host_timestamp,
             &state_root,
         )?;
-        let epoch_ends = self.header.next_epoch_commitment.is_some();
+        let has_next_epoch = next_epoch.is_some();
         let next_block = self.header.generate_next(
             host_height,
             host_timestamp,
@@ -227,11 +227,21 @@ impl<PK: crate::PubKey> ChainManager<PK> {
             signers: Set::new(),
             signing_stake: 0,
         });
-        self.candidates.clear_changed_flag();
 
-        Ok(epoch_ends)
+        if has_next_epoch {
+            self.candidates.clear_changed_flag();
+        }
+
+        Ok(())
     }
 
+    /// Verifies whether new block can be generated.
+    ///
+    /// Like [`generate_next`] returns an error if the new block cannot be
+    /// generated.  If it can, returns an `Ok` value.
+    ///
+    /// If the new block should contain a next epoch commitment, returns `Some`
+    /// new epoch.  Otherwise returns `None`.
     pub fn validate_generate_next(
         &self,
         host_height: crate::HostHeight,