fix: reset sync_head to header_head on initial transition to HeaderSync (#1531)

* fix for bug #1524, reset sync_head to header_head on initial transition to HeaderSync.
* using @antiochp fix solution in #1539

chain/src/chain.rs

@@ -931,6 +931,14 @@ impl Chain {
 			.block_exists(&h)
 			.map_err(|e| ErrorKind::StoreErr(e, "chain block exists".to_owned()).into())
 	}
+
+	/// reset sync_head to header_head
+	pub fn init_sync_head(&self, header_head: &Tip) -> Result<(), Error> {
+		let batch = self.store.batch()?;
+		batch.init_sync_head(header_head)?;
+		batch.commit()?;
+		Ok(())
+	}
 }
 
 fn setup_head(

chain/src/pipe.rs

@@ -215,9 +215,15 @@ pub fn sync_block_header(
 	validate_header(&bh, sync_ctx)?;
 	add_block_header(bh, batch)?;
 
-	// now update the header_head (if new header with most work) and the sync_head
-	// (always)
+	// Update header_head (but only if this header increases our total known work).
+	// i.e. Only if this header is now the head of the current "most work" chain.
 	update_header_head(bh, header_ctx, batch)?;
+
+	// Update sync_head regardless of total work.
+	// We may be syncing a long fork that will *eventually* increase the work
+	// and become the "most work" chain.
+	// header_head and sync_head will diverge in this situation until we switch to
+	// a single "most work" chain.
 	update_sync_head(bh, sync_ctx, batch)
 }