mongodb/jstests/replsets/primary_casts_vote_on_stepdown.js

/**
 * In a 2 (or 3) node replica set, a new candidate should be able to overtake a current primary with
 * a single round of election votes. This is enabled by the ability of a current primary to both
 * step down *and* cast its vote for a new primary in a single step, in response to a vote request
 * from a higher term than its own. This test verifies that an old primary is able to do this
 * successfully.
 */
(function() {
"use strict";

let name = "primary_casts_vote_on_stepdown";
let replTest = new ReplSetTest({name: name, nodes: 2});

let nodes = replTest.startSet();
replTest.initiate();

// Make sure node 0 is initially primary, and then step up node 1 and make sure it is able to
// become primary in one election, gathering the vote of node 0, who will be forced to step
// down in the act of granting its vote to node 1.
jsTestLog("Make sure node 0 (" + nodes[0] + ") is primary.");
replTest.waitForState(nodes[0], ReplSetTest.State.PRIMARY);
let res = assert.commandWorked(nodes[0].adminCommand("replSetGetStatus"));
let firstPrimaryTerm = res.term;

jsTestLog("Stepping up node 1 (" + nodes[1] + ").");
replTest.stepUp(nodes[1]);
replTest.waitForState(nodes[1], ReplSetTest.State.PRIMARY);
// The election should have happened in a single attempt, so the term of the new primary should
// be exactly 1 greater than the old primary.
res = assert.commandWorked(nodes[1].adminCommand("replSetGetStatus"));
assert.eq(firstPrimaryTerm + 1, res.term);

replTest.stopSet();
})();