/**
 * Tests that a stale mongos does not return a stale shardVersion error to the client for explain
 * find sent using the legacy query mode (it retries on the stale shardVersion error internally).
 */
(function() {
"use strict";

const dbName = "test";
const collName = "foo";
const ns = dbName + "." + collName;

const st = new ShardingTest({mongos: 2, shards: 1, verbose: 2});

let staleMongos = st.s0;
let freshMongos = st.s1;

jsTest.log("Make the stale mongos load a cache entry for db " + dbName + " once");
assert.commandWorked(staleMongos.getDB(dbName).getCollection(collName).insert({_id: 1}));

jsTest.log("Call shardCollection on " + ns + " from the fresh mongos");
assert.commandWorked(freshMongos.adminCommand({enableSharding: dbName}));
assert.commandWorked(freshMongos.adminCommand({shardCollection: ns, key: {"_id": 1}}));

jsTest.log("Ensure the shard knows " + ns + " is sharded");
const clusterVersionInfo = st.getClusterVersionInfo();
if (clusterVersionInfo.isMixedVersion) {
    assert.commandWorked(
        st.shard0.adminCommand({_flushRoutingTableCacheUpdates: ns, syncFromConfig: true}));
} else {
    // _flushRoutingTableCacheUpdatesWithWriteConcern did not exist in older versions of the server.
    // Confirm that flushing the routing table with write concern does not change the existing
    // behavior of this test.
    assert.commandWorked(st.shard0.adminCommand({
        _flushRoutingTableCacheUpdatesWithWriteConcern: ns,
        syncFromConfig: true,
        writeConcern: {w: "majority"}
    }));
}

jsTest.log("Run explain find on " + ns + " from the stale mongos");
staleMongos.getDB(dbName).getMongo().forceReadMode("legacy");
staleMongos.getDB(dbName).getCollection(collName).find({$query: {}, $explain: true}).next();

st.stop();
})();