chain: ensure eventType from ZMQ response is human-readable

In this commit, we fix a small issue where it's possible that we read a
malformed message from the ZMQ connection to bitcoind due to it shutting
down. To fix this, we ensure that the event type is human readable
before attempting to log it.

chain/bitcoind_conn.go

@@ -221,6 +221,14 @@ func (c *BitcoindConn) blockEventHandler(conn *gozmq.Conn) {
 			}
 			c.rescanClientsMtx.Unlock()
 		default:
+			// It's possible that the message wasn't fully read if
+			// bitcoind shuts down, which will produce an unreadable
+			// event type. To prevent from logging it, we'll make
+			// sure it conforms to the ASCII standard.
+			if !isASCII(eventType) {
+				continue
+			}
+
 			log.Warnf("Received unexpected event type from "+
 				"rawblock subscription: %v", eventType)
 		}
@@ -364,3 +372,14 @@ func (c *BitcoindConn) RemoveClient(id uint64) {
 
 	delete(c.rescanClients, id)
 }
+
+// isASCII is a helper method that checks whether all bytes in `data` would be
+// printable ASCII characters if interpreted as a string.
+func isASCII(s string) bool {
+	for _, c := range s {
+		if c < 32 || c > 126 {
+			return false
+		}
+	}
+	return true
+}