SERVER-63501 Disallow downgrade when the index `disallowNewDuplicateKeys` field existsr6.0.0-alpha

2 files changed, 96 insertions, 0 deletions

diff --git a/jstests/noPassthrough/index_disallowNewDuplicateKeys_downgrade.js b/jstests/noPassthrough/index_disallowNewDuplicateKeys_downgrade.js
new file mode 100644
index 00000000000..4c733772788
--- /dev/null
+++ b/jstests/noPassthrough/index_disallowNewDuplicateKeys_downgrade.js
@@ -0,0 +1,66 @@
+/**
+ * Tests that the cluster cannot be downgraded when there are indexes with the
+ * 'disallowNewDuplicateKeys' field present.
+ *
+ * TODO SERVER-63563: Update this test once kLastContinuous is 5.3.
+ * TODO SERVER-63564: 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_disallowNewDuplicateKeys_downgrade";
+const coll = db.getCollection(collName);
+assert.commandWorked(db.createCollection(coll.getName()));
+
+function checkIndexForDowngrade(withFCV, fixIndex, isCompatible) {
+    assert.commandWorked(coll.createIndex({a: 1}, {disallowNewDuplicateKeys: true}));
+    assert.commandWorked(coll.createIndex({b: 1}, {disallowNewDuplicateKeys: true}));
+
+    if (fixIndex) {
+        // Resolves the incompatibility before the downgrade.
+        assert.commandWorked(coll.dropIndex({a: 1}));
+        assert.commandWorked(db.runCommand(
+            {collMod: collName, index: {keyPattern: {b: 1}, disallowNewDuplicateKeys: false}}));
+    } 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 'disallowNewDuplicateKeys' field.
+checkIndexForDowngrade(lastContinuousFCV, true, true);
+
+// Successfully downgrades to 5.0 after removing the 'disallowNewDuplicateKeys' field.
+checkIndexForDowngrade(lastLTSFCV, true, true);
+
+MongoRunner.stopMongod(conn);
+}());
\ No newline at end of file
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 f07008508d8..5ceb71ed310 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -719,6 +719,36 @@ private:
                     });
             }
 
+            // TODO SERVER-63563: Only check on last-lts when FCV 5.3 becomes last-continuous.
+            // TODO SERVER-63564: Remove once FCV 6.0 becomes last-lts.
+            for (const auto& tenantDbName : DatabaseHolder::get(opCtx)->getNames()) {
+                const auto& dbName = tenantDbName.dbName();
+                Lock::DBLock dbLock(opCtx, dbName, MODE_IX);
+                catalog::forEachCollectionFromDb(
+                    opCtx, tenantDbName, MODE_X, [&](const CollectionPtr& collection) {
+                        auto indexCatalog = collection->getIndexCatalog();
+                        auto indexIt = indexCatalog->getIndexIterator(
+                            opCtx, true /* includeUnfinishedIndexes */);
+                        while (indexIt->more()) {
+                            auto indexEntry = indexIt->next();
+                            uassert(
+                                ErrorCodes::CannotDowngrade,
+                                fmt::format(
+                                    "Cannot downgrade the cluster when there are indexes that have "
+                                    "the 'disallowNewDuplicateKeys' field. Use listIndexes to find "
+                                    "them and drop "
+                                    "the indexes or use collMod to manually set it to false to "
+                                    "remove the field "
+                                    "before downgrading. First detected incompatible index name: "
+                                    "'{}' on collection: '{}'",
+                                    indexEntry->descriptor()->indexName(),
+                                    collection->ns().toString()),
+                                !indexEntry->descriptor()->disallowNewDuplicateKeys());
+                        }
+                        return true;
+                    });
+            }
+
             // Drop the pre-images collection if 'changeStreamPreAndPostImages' feature flag is not
             // enabled on the downgrade version.
             // TODO SERVER-61770: Remove once FCV 6.0 becomes last-lts.