SERVER-40136 enable key generator on setting FCV to 3.6

2 files changed, 85 insertions, 0 deletions

diff --git a/jstests/multiVersion/causal_consistency_downgrade_upgrade_rs.js b/jstests/multiVersion/causal_consistency_downgrade_upgrade_rs.js
new file mode 100644
index 00000000000..9ca4662f73e
--- /dev/null
+++ b/jstests/multiVersion/causal_consistency_downgrade_upgrade_rs.js
@@ -0,0 +1,81 @@
+/**
+ * Test the downgrade of a standalone replica set from latest to last-stable version succeeds,
+ * verifying behavior related to causal consistency at each stage.
+ */
+(function() {
+    "use strict";
+
+    load("jstests/multiVersion/libs/multi_rs.js");
+    load("jstests/multiVersion/libs/causal_consistency_helpers.js");
+
+    var newVersion = "latest";
+    var oldVersion = "last-stable";
+
+    var name = "causal_consistency_rs_downgrade";
+    var rst = new ReplSetTest(
+        {name: name, nodes: 3, waitForKeys: true, nodeOptions: {binVersion: newVersion}});
+    rst.startSet();
+    var replSetConfig = rst.getReplSetConfig();
+    // Hard-code catchup timeout to be compatible with 3.4
+    replSetConfig.settings = {catchUpTimeoutMillis: 2000};
+    rst.initiate(replSetConfig);
+
+    var primary = rst.getPrimary();
+    primary.getDB("test").runCommand({insert: "foo", documents: [{_id: 1, x: 1}]});
+    rst.awaitReplication();
+
+    // Nodes can accept afterClusterTime reads.
+    assert.soonNoExcept(() => {
+        assertContainsLogicalAndOperationTime(primary.getDB("test").runCommand({isMaster: 1}),
+                                              {initialized: true, signed: false});
+        return true;
+    });
+
+    rst.getSecondaries().forEach(function(secondary) {
+        assert.soonNoExcept(() => {
+            assertContainsLogicalAndOperationTime(secondary.getDB("test").runCommand({isMaster: 1}),
+                                                  {initialized: true, signed: false});
+            return true;
+        });
+    });
+
+    jsTest.log("Setting FCV to 3.4 ...");
+    assert.commandWorked(primary.adminCommand({setFeatureCompatibilityVersion: "3.4"}));
+    rst.awaitReplication();
+
+    // The system keys collection should have been dropped.
+    assertHasNoKeys(rst.getPrimary());
+
+    assertDoesNotContainLogicalOrOperationTime(
+        rst.getPrimary().getDB("test").runCommand({isMaster: 1}));
+
+    rst.getSecondaries().forEach(function(secondary) {
+        assertDoesNotContainLogicalOrOperationTime(
+            secondary.getDB("test").runCommand({isMaster: 1}));
+    });
+
+    jsTest.log("Setting FCV to 3.6 ...");
+
+    assert.commandWorked(rst.getPrimary().adminCommand({setFeatureCompatibilityVersion: "3.6"}));
+    rst.awaitReplication();
+
+    rst.getPrimary().getDB("test").runCommand({insert: "foo", documents: [{_id: 2, x: 1}]});
+    rst.awaitReplication();
+
+    assert.soonNoExcept(() => {
+        assertContainsLogicalAndOperationTime(
+            rst.getPrimary().getDB("test").runCommand({isMaster: 1}),
+            {initialized: true, signed: false});
+        return true;
+    });
+
+    rst.getSecondaries().forEach(function(secondary) {
+        assert.soonNoExcept(() => {
+            assertContainsLogicalAndOperationTime(secondary.getDB("test").runCommand({isMaster: 1}),
+                                                  {initialized: true, signed: false});
+            return true;
+        });
+    });
+
+    rst.stopSet();
+})();
diff --git a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
index 07e12dee765..bae0c8e7d66 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -194,6 +194,10 @@ public:
                             BSON(FeatureCompatibilityVersion::kCommandName << requestedVersion)))));
             }
 
+            if (LogicalTimeValidator::get(opCtx)) {
+                LogicalTimeValidator::get(opCtx)->enableKeyGenerator(opCtx, true);
+            }
+
             if (ShardingState::get(opCtx)->enabled()) {
                 // Ensure we try reading the keys for signing clusterTime immediately on upgrade.
                 // Remove after 3.4 -> 3.6 upgrade.