SERVER-45408 Enable serialization of config "term" field

SERVER-45092 Remove “term” field of config document on downgrade

2 files changed, 74 insertions, 2 deletions

diff --git a/jstests/multiVersion/remove_config_term_on_replset_downgrade.js b/jstests/multiVersion/remove_config_term_on_replset_downgrade.js
new file mode 100644
index 00000000000..4315fdcc506
--- /dev/null
+++ b/jstests/multiVersion/remove_config_term_on_replset_downgrade.js
@@ -0,0 +1,73 @@
+/**
+ * Test the downgrade of a replica set from latest version to last-stable version succeeds and
+ * removes the "term" field of config document.
+ */
+
+(function() {
+'use strict';
+
+load('jstests/multiVersion/libs/multi_rs.js');
+load('jstests/libs/test_background_ops.js');
+
+let newVersion = "latest";
+let oldVersion = "last-stable";
+
+let nodes = {
+    n1: {binVersion: newVersion},
+    n2: {binVersion: newVersion},
+    n3: {binVersion: newVersion}
+};
+
+let rst = new ReplSetTest({nodes: nodes});
+rst.startSet();
+rst.initiate();
+
+let primary = rst.getPrimary();
+
+// The default FCV is latestFCV for non-shard replica sets.
+let primaryAdminDB = rst.getPrimary().getDB("admin");
+checkFCV(primaryAdminDB, latestFCV);
+
+// Reconfig in FCV 4.4.
+let originalConfig = rst.getReplSetConfigFromNode();
+originalConfig.version++;
+reconfig(rst, originalConfig);
+rst.awaitNodesAgreeOnConfigVersion();
+
+// Check that the term field exists in the config document on all nodes.
+rst.nodes.forEach(function(node) {
+    jsTestLog("Checking the config term on node " + tojson(node.host) + " before downgrade.");
+    let config = node.getDB("local").getCollection("system.replset").findOne();
+    assert(config.hasOwnProperty("term"));
+});
+
+// Remember the config from the primary before the downgrade.
+let configInNewVersion = rst.getReplSetConfigFromNode();
+
+jsTest.log("Downgrading FCV to 4.2");
+assert.commandWorked(primary.adminCommand({setFeatureCompatibilityVersion: lastStableFCV}));
+rst.awaitReplication();
+// Check that the term field doesn't exist in the config document on all nodes.
+rst.nodes.forEach(function(node) {
+    jsTestLog("Checking the config term on node " + tojson(node.host) + " after FCV downgrade.");
+    let config = node.getDB("local").getCollection("system.replset").findOne();
+    assert(!config.hasOwnProperty("term"), tojson(config));
+    config.term = configInNewVersion.term;
+    assert.docEq(configInNewVersion, config);
+});
+
+jsTest.log("Downgrading replica set..");
+rst.upgradeSet({binVersion: oldVersion});
+jsTest.log("Downgrade complete.");
+
+// Check that the term field doesn't exist in the config document on all nodes.
+rst.nodes.forEach(function(node) {
+    reconnect(node);
+    jsTestLog("Checking the config term on node " + tojson(node.host) + " after binary downgrade.");
+    let config = node.getDB("local").getCollection("system.replset").findOne();
+    assert(!config.hasOwnProperty("term"), tojson(config));
+    config.term = configInNewVersion.term;
+    assert.docEq(configInNewVersion, config);
+});
+rst.stopSet();
+})();
diff --git a/jstests/replsets/reconfig_ignores_term_field.js b/jstests/replsets/reconfig_ignores_term_field.js
index bfe3d5b9d20..2cc48ac950d 100644
--- a/jstests/replsets/reconfig_ignores_term_field.js
+++ b/jstests/replsets/reconfig_ignores_term_field.js
@@ -25,7 +25,6 @@ assert.commandWorked(primary.getDB("admin").runCommand({replSetReconfig: config}
 replTest.awaitReplication();
 
 config = primary.getDB("local").system.replset.findOne();
-// TODO SERVER-45408: uncomment once we enable serialization of the term field.
-// assert.eq(config.term, 1);
+assert.eq(config.term, 1);
 replTest.stopSet();
 }());
\ No newline at end of file