try_header_head in the syncer loop with a short timeout (#3165)

to prevent it getting locked up after txhashset download and validation

servers/src/grin/sync/syncer.rs

@@ -182,7 +182,18 @@ impl SyncRunner {
 			// if syncing is needed
 			let head = unwrap_or_restart_loop!(self.chain.head());
 			let tail = self.chain.tail().unwrap_or_else(|_| head.clone());
-			let header_head = unwrap_or_restart_loop!(self.chain.header_head());
+
+			// We still do not fully understand what is blocking this but if this blocks here after
+			// we download and validate the txhashet we do not reliably proceed to block_sync,
+			// potentially blocking for an extended period of time (> 10 mins).
+			// Does not appear to be deadlock as it does resolve itself eventually.
+			// So as a workaround we try_header_head with a relatively short timeout and simply
+			// retry the syncer loop.
+			let maybe_header_head =
+				unwrap_or_restart_loop!(self.chain.try_header_head(time::Duration::from_secs(1)));
+			let header_head = unwrap_or_restart_loop!(
+				maybe_header_head.ok_or("failed to obtain lock for try_header_head")
+			);
 
 			// run each sync stage, each of them deciding whether they're needed
 			// except for state sync that only runs if body sync return true (means txhashset is needed)