rpc: extract rpcQueryTimeout method

This helps keep the logic in blockingQuery more focused. In the future we
may have a separate struct for RPC queries which may allow us to move this
off of Server.

agent/consul/rpc.go

@@ -938,19 +938,8 @@ func (s *Server) blockingQuery(queryOpts structs.QueryOptionsCompat, queryMeta s
 		return err
 	}
 
-	queryTimeout := queryOpts.GetMaxQueryTime()
+	timeout := s.rpcQueryTimeout(queryOpts.GetMaxQueryTime())
-	// Restrict the max query time, and ensure there is always one.
+	ctx, cancel := context.WithTimeout(ctx, timeout)
-	if queryTimeout > s.config.MaxQueryTime {
-		queryTimeout = s.config.MaxQueryTime
-	} else if queryTimeout <= 0 {
-		queryTimeout = s.config.DefaultQueryTime
-	}
-
-	// Apply a small amount of jitter to the request.
-	queryTimeout += lib.RandomStagger(queryTimeout / structs.JitterFraction)
-
-	// wrap the base context with a deadline
-	ctx, cancel := context.WithTimeout(ctx, queryTimeout)
 	defer cancel()
 
 	// instrument blockingQueries
@@ -1068,6 +1057,22 @@ func (s *Server) consistentRead() error {
 	return structs.ErrNotReadyForConsistentReads
 }
 
+// rpcQueryTimeout calculates the timeout for the query, ensures it is
+// constrained to the configured limit, and adds jitter to prevent multiple
+// blocking queries from all timing out at the same time.
+func (s *Server) rpcQueryTimeout(queryTimeout time.Duration) time.Duration {
+	// Restrict the max query time, and ensure there is always one.
+	if queryTimeout > s.config.MaxQueryTime {
+		queryTimeout = s.config.MaxQueryTime
+	} else if queryTimeout <= 0 {
+		queryTimeout = s.config.DefaultQueryTime
+	}
+
+	// Apply a small amount of jitter to the request.
+	queryTimeout += lib.RandomStagger(queryTimeout / structs.JitterFraction)
+	return queryTimeout
+}
+
 // maskResultsFilteredByACLs blanks out the ResultsFilteredByACLs flag if the
 // request is unauthenticated, to limit information leaking.
 //