1 files changed, 78 insertions, 0 deletions
diff --git a/jstests/multiVersion/unionWith_fcv.js b/jstests/multiVersion/unionWith_fcv.js
new file mode 100644
index 00000000000..35b9a3c15f2
--- /dev/null
+++ b/jstests/multiVersion/unionWith_fcv.js
@@ -0,0 +1,78 @@
+/**
+ * Test the behavior of the $unionWith aggregation stage against a standalone/sharded cluster during
+ * upgrade.
+ *
+ *  Checking UUID consistency uses cached connections, which are not valid across restarts or
+ *  stepdowns.
+ */
+TestData.skipCheckingUUIDsConsistentAcrossCluster = true;
+
+(function() {
+"use strict";
+
+load("jstests/multiVersion/libs/multi_cluster.js");  // For upgradeCluster.
+
+let conn = MongoRunner.runMongod({binVersion: "latest"});
+assert.neq(null, conn, "mongod was unable to start up");
+let testDB = conn.getDB(jsTestName());
+
+// Set the feature compatibility version to the last-stable version.
+assert.commandWorked(testDB.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}));
+
+// Seed the two involved collections.
+assert.commandWorked(testDB.collA.insert({fromA: 1}));
+assert.commandWorked(testDB.collB.insert({fromB: 1}));
+
+// Verify that we can still use $unionWith since the binary version is 4.4.
+const pipeline = [{$unionWith: "collB"}, {$project: {_id: 0}}];
+assert.sameMembers([{fromA: 1}, {fromB: 1}], testDB.collA.aggregate(pipeline).toArray());
+
+// Set the feature compatibility version to the latest version.
+assert.commandWorked(testDB.adminCommand({setFeatureCompatibilityVersion: latestFCV}));
+
+// Verify that we can still use $unionWith.
+assert.sameMembers([{fromA: 1}, {fromB: 1}], testDB.collA.aggregate(pipeline).toArray());
+
+MongoRunner.stopMongod(conn);
+
+// Start a sharded cluster in which all mongod and mongos processes are "last-stable" binVersion.
+let st = new ShardingTest({
+    shards: 2,
+    rs: {nodes: 2, binVersion: "last-stable"},
+    other: {mongosOptions: {binVersion: "last-stable"}}
+});
+
+testDB = st.s.getDB(jsTestName());
+assert.commandWorked(testDB.runCommand({create: "collA"}));
+st.ensurePrimaryShard(testDB.getName(), st.shard0.shardName);
+
+// Seed the two involved collections.
+assert.commandWorked(testDB.collA.insert({fromA: 1}));
+assert.commandWorked(testDB.collB.insert({fromB: 1}));
+
+// Aggregations with $unionWith should fail against older binary versions.
+assert.commandFailedWithCode(
+    testDB.runCommand({aggregate: "collA", pipeline: pipeline, cursor: {}}), 40324);
+
+// Upgrade the config servers and the shards to the "latest" binVersion.
+st.upgradeCluster("latest", {upgradeShards: true, upgradeConfigs: true, upgradeMongos: false});
+
+// Since mongos is still on 4.2, $unionWith should fail to parse.
+assert.commandFailedWithCode(
+    testDB.runCommand({aggregate: "collA", pipeline: pipeline, cursor: {}}), 40324);
+
+// Upgrade mongos to the "latest" binVersion but keep the old FCV.
+st.upgradeCluster("latest", {upgradeShards: false, upgradeConfigs: false, upgradeMongos: true});
+testDB = st.s.getDB(jsTestName());
+
+// Now an aggregation containing $unionWith should pass because all nodes are on binary version 4.4.
+assert.sameMembers([{fromA: 1}, {fromB: 1}], testDB.collA.aggregate(pipeline).toArray());
+
+// For completeness, set the FCV to the latest.
+assert.commandWorked(testDB.adminCommand({setFeatureCompatibilityVersion: latestFCV}));
+
+// Verify that $unionWith is allowed in a fully upgraded cluster.
+assert.sameMembers([{fromA: 1}, {fromB: 1}], testDB.collA.aggregate(pipeline).toArray());
+
+st.stop();
+}());