From f1af163bcde78896b83ee5115eb8545e086b5c49 Mon Sep 17 00:00:00 2001
From: Matt Boros <matt.boros@mongodb.com>
Date: Fri, 6 Jan 2023 21:55:11 +0000
Subject: remove fcv startup check, add downgrade check

---
 jstests/multiVersion/partial_index_fcv.js          | 38 ++++++++++++++++++++++
 src/mongo/db/catalog/index_catalog_impl.cpp        |  9 ++---
 .../set_feature_compatibility_version_command.cpp  | 37 +++++++++++++++++++++
 3 files changed, 80 insertions(+), 4 deletions(-)
 create mode 100644 jstests/multiVersion/partial_index_fcv.js

diff --git a/jstests/multiVersion/partial_index_fcv.js b/jstests/multiVersion/partial_index_fcv.js
new file mode 100644
index 00000000000..b2cd76d0cd4
--- /dev/null
+++ b/jstests/multiVersion/partial_index_fcv.js
@@ -0,0 +1,38 @@
+/**
+ *
+ */
+
+(function() {
+// 'use strict';
+
+const dbpath = MongoRunner.dataPath + 'partial_index_fcv';
+resetDbpath(dbpath);
+
+let conn = MongoRunner.runMongod({dbpath: dbpath, binVersion: '6.0', noCleanData: true});
+
+db = conn.getDB('test');
+coll = db['partial_index_fcv'];
+assert.commandWorked(coll.createIndex(
+    {a: 1, b: 1}, {partialFilterExpression: {$or: [{a: {$lt: 20}}, {b: {$lt: 10}}]}}))
+
+coll.insert({a: 1, b: 1})
+coll.insert({a: 5, b: 6})
+coll.insert({a: 1, b: 20})
+coll.insert({a: 30, b: 1})
+coll.insert({a: 30, b: 20})
+
+assert.commandFailedWithCode(db.adminCommand({setFeatureCompatibilityVersion: '5.0'}),
+                             ErrorCodes.CannotDowngrade);
+coll.dropIndexes();
+assert.commandWorked(db.adminCommand({setFeatureCompatibilityVersion: '5.0'}));
+
+MongoRunner.stopMongod(conn);
+
+conn = MongoRunner.runMongod({dbpath: dbpath, binVersion: '5.0', noCleanData: true});
+
+db = conn.getDB('test');
+coll = db['partial_index_fcv'];
+assert.eq(coll.aggregate().toArray().length, 5);
+
+MongoRunner.stopMongod(conn);
+})();
\ No newline at end of file
diff --git a/src/mongo/db/catalog/index_catalog_impl.cpp b/src/mongo/db/catalog/index_catalog_impl.cpp
index 6be0b560bf9..f99fcb3e4a2 100644
--- a/src/mongo/db/catalog/index_catalog_impl.cpp
+++ b/src/mongo/db/catalog/index_catalog_impl.cpp
@@ -890,10 +890,11 @@ Status IndexCatalogImpl::_isSpecOk(OperationContext* opCtx,
         }
         const std::unique_ptr<MatchExpression> filterExpr = std::move(statusWithMatcher.getValue());
 
-        Status status =
-            _checkValidFilterExpressions(filterExpr.get(),
-                                         feature_flags::gTimeseriesMetricIndexes.isEnabled(
-                                             serverGlobalParams.featureCompatibility));
+        Status status = _checkValidFilterExpressions(
+            filterExpr.get(),
+            serverGlobalParams.featureCompatibility.isVersionInitialized() &&
+                feature_flags::gTimeseriesMetricIndexes.isEnabled(
+                    serverGlobalParams.featureCompatibility));
         if (!status.isOK()) {
             return status;
         }
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 0c8f5a42c99..15c10444a72 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -84,6 +84,7 @@
 #include "mongo/db/session_catalog.h"
 #include "mongo/db/session_catalog_mongod.h"
 #include "mongo/db/session_txn_record_gen.h"
+#include "mongo/db/storage/storage_parameters_gen.h"
 #include "mongo/db/timeseries/timeseries_index_schema_conversion_functions.h"
 #include "mongo/db/vector_clock.h"
 #include "mongo/idl/cluster_server_parameter_gen.h"
@@ -831,6 +832,42 @@ private:
             }
         }
 
+        if (!feature_flags::gTimeseriesMetricIndexes.isEnabledOnVersion(requestedVersion)) {
+            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, /*includeUnfinishedIndexes=*/true);
+
+                        while (indexIt->more()) {
+                            auto indexEntry = indexIt->next();
+
+                            if (auto filter = indexEntry->getFilterExpression()) {
+                                auto status = IndexCatalogImpl::checkValidFilterExpressions(
+                                    filter,
+                                    /*timeseriesMetricIndexesFeatureFlagEnabled*/ false);
+                                uassert(ErrorCodes::CannotDowngrade,
+                                        str::stream()
+                                            << "Cannot downgrade the cluster when there are "
+                                               "secondary indexes with partial filter expressions "
+                                               "that contain $in/$or/$geoWithin or an $and that is "
+                                               "not top level. Drop all indexes containing these "
+                                               "partial filter elements before downgrading. First "
+                                               "detected incompatible index name: '"
+                                            << indexEntry->descriptor()->indexName()
+                                            << "' on collection: '" << collection->ns().coll()
+                                            << "'",
+                                        status.isOK());
+                            }
+                        }
+                        return true;
+                    });
+            }
+        }
+
         if (serverGlobalParams.featureCompatibility
                 .isFCVDowngradingOrAlreadyDowngradedFromLatest()) {
             for (const auto& tenantDbName : DatabaseHolder::get(opCtx)->getNames()) {
-- 
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