SERVER-63089 Disallow downgrade when the index comment field exists

1 files changed, 67 insertions, 0 deletions

diff --git a/jstests/noPassthrough/index_comment_downgrade.js b/jstests/noPassthrough/index_comment_downgrade.js
new file mode 100644
index 00000000000..2f70d31089a
--- /dev/null
+++ b/jstests/noPassthrough/index_comment_downgrade.js
@@ -0,0 +1,67 @@
+/**
+ * Tests that the cluster cannot be downgraded when there are indexes with the 'comment' field
+ * present.
+ *
+ * TODO SERVER-63171: Update this test once kLastContinuous is 5.3.
+ * TODO SERVER-63172: Remove this test once kLastLTS is 6.0.
+ *
+ * @tags: [requires_fcv_53]
+ */
+(function() {
+"use strict";
+
+const conn = MongoRunner.runMongod();
+const db = conn.getDB("test");
+
+const collModIndexUniqueEnabled = assert
+                                      .commandWorked(db.getMongo().adminCommand(
+                                          {getParameter: 1, featureFlagCollModIndexUnique: 1}))
+                                      .featureFlagCollModIndexUnique.value;
+
+if (!collModIndexUniqueEnabled) {
+    jsTestLog('Skipping test because the collMod unique index feature flag is disabled.');
+    MongoRunner.stopMongod(conn);
+    return;
+}
+
+const collName = "index_comment_downgrade";
+const coll = db.getCollection(collName);
+assert.commandWorked(db.createCollection(coll.getName()));
+
+function checkIndexForDowngrade(withFCV, fixIndex, isCompatible) {
+    assert.commandWorked(coll.createIndex({a: 1}, {comment: {"index_to_be_dropped": 1}}));
+    // TODO SERVER-62971: Add the case to use collMod to remove the 'comment' field.
+    // assert.commandWorked(coll.createIndex({b: 1}, {comment: {"index_to_be_collMod": 1}}));
+
+    if (fixIndex) {
+        // Resolves the incompatibility before the downgrade.
+        assert.commandWorked(coll.dropIndex({a: 1}));
+        // assert.commandWorked(db.runCommand({collMod: collName, index: {keyPattern: {b: 1},
+        // comment: new Object()}}));
+    } else if (!isCompatible) {
+        assert.commandFailedWithCode(db.adminCommand({setFeatureCompatibilityVersion: withFCV}),
+                                     ErrorCodes.CannotDowngrade);
+        assert.commandWorked(coll.dropIndexes("*"));
+    }
+
+    // Downgrades to the version 'withFCV' and reset to 'latestFCV'.
+    assert.commandWorked(db.adminCommand({setFeatureCompatibilityVersion: withFCV}));
+    assert.commandWorked(db.adminCommand({setFeatureCompatibilityVersion: latestFCV}));
+
+    assert.commandWorked(coll.dropIndexes("*"));
+}
+
+// Fails to downgrade to 5.2.
+checkIndexForDowngrade(lastContinuousFCV, false, false);
+
+// Fails to downgrade to 5.0.
+checkIndexForDowngrade(lastLTSFCV, false, false);
+
+// Successfully downgrades to 5.2 after removing the 'comment' field.
+checkIndexForDowngrade(lastContinuousFCV, true, true);
+
+// Successfully downgrades to 5.0 after removing the 'comment' field.
+checkIndexForDowngrade(lastLTSFCV, true, true);
+
+MongoRunner.stopMongod(conn);
+}());