SERVER-60576 On downgrade, remove the catalog flag that indicates whether a time-series bucket has mixed-schema data

4 files changed, 62 insertions, 25 deletions

diff --git a/src/mongo/db/catalog/coll_mod.cpp b/src/mongo/db/catalog/coll_mod.cpp
index dc02c016862..8ae88794bb7 100644
--- a/src/mongo/db/catalog/coll_mod.cpp
+++ b/src/mongo/db/catalog/coll_mod.cpp
@@ -639,8 +639,9 @@ Status _collModInternal(OperationContext* opCtx,
                 opCtx, coll.getWritableCollection(), desc);
         }
 
-        // TODO SERVER-60911: When kLatest is 5.3, only check when upgrading from kLastLTS (5.0).
-        // TODO SERVER-60912: When kLastLTS is 6.0, remove this FCV-gated upgrade code.
+        // TODO SERVER-60911: When kLatest is 5.3, only check when upgrading from or downgrading to
+        // kLastLTS (5.0).
+        // TODO SERVER-60912: When kLastLTS is 6.0, remove this FCV-gated upgrade/downgrade code.
         if (coll->getTimeseriesOptions() && !coll->getTimeseriesBucketsMayHaveMixedSchemaData() &&
             serverGlobalParams.featureCompatibility.isFCVUpgradingToOrAlreadyLatest()) {
             // While upgrading the FCV to 5.2+, collMod is called as part of the upgrade process to
@@ -649,6 +650,14 @@ Status _collModInternal(OperationContext* opCtx,
             // time-series collection existed in earlier server versions and may have mixed-schema
             // data.
             coll.getWritableCollection()->setTimeseriesBucketsMayHaveMixedSchemaData(opCtx, true);
+        } else if (coll->getTimeseriesBucketsMayHaveMixedSchemaData() &&
+                   serverGlobalParams.featureCompatibility
+                       .isFCVDowngradingOrAlreadyDowngradedFromLatest()) {
+            // While downgrading the FCV from 5.2, collMod is called as part of the downgrade
+            // process to remove the 'timeseriesBucketsMayHaveMixedSchemaData' catalog entry
+            // flag for time-series collections that have the flag.
+            coll.getWritableCollection()->setTimeseriesBucketsMayHaveMixedSchemaData(opCtx,
+                                                                                     boost::none);
         }
 
         // Only observe non-view collMods, as view operations are observed as operations on the
diff --git a/src/mongo/db/catalog/collection.h b/src/mongo/db/catalog/collection.h
index 051a1c46164..0ba948c2afe 100644
--- a/src/mongo/db/catalog/collection.h
+++ b/src/mongo/db/catalog/collection.h
@@ -536,7 +536,7 @@ public:
      * Sets the 'timeseriesBucketsMayHaveMixedSchemaData' catalog entry flag to 'setting' for this
      * collection.
      *
-     * Throws if FCV < 5.2 or if this is not a time-series collection.
+     * Throws if this is not a time-series collection.
      */
     virtual void setTimeseriesBucketsMayHaveMixedSchemaData(OperationContext* opCtx,
                                                             boost::optional<bool> setting) = 0;
diff --git a/src/mongo/db/catalog/collection_impl.cpp b/src/mongo/db/catalog/collection_impl.cpp
index 3c001c5af53..33426a8804b 100644
--- a/src/mongo/db/catalog/collection_impl.cpp
+++ b/src/mongo/db/catalog/collection_impl.cpp
@@ -1352,12 +1352,12 @@ void CollectionImpl::setTimeseriesBucketsMayHaveMixedSchemaData(OperationContext
                                                                 boost::optional<bool> setting) {
     uassert(6057500, "This is not a time-series collection", _metadata->options.timeseries);
 
-    // TODO SERVER-60911: When kLatest is 5.3, only check when upgrading from kLastLTS (5.0).
-    // TODO SERVER-60912: When kLastLTS is 6.0, remove this FCV-gated upgrade code.
-    uassert(
-        6057501,
-        "Cannot set the 'timeseriesBucketsMayHaveMixedSchemaData' catalog entry flag if FCV < 5.2",
-        serverGlobalParams.featureCompatibility.isFCVUpgradingToOrAlreadyLatest());
+    LOGV2_DEBUG(6057601,
+                1,
+                "Setting 'timeseriesBucketsMayHaveMixedSchemaData' catalog entry flag",
+                logAttrs(ns()),
+                logAttrs(uuid()),
+                "setting"_attr = setting);
 
     _writeMetadata(opCtx, [&](BSONCollectionCatalogEntry::MetaData& md) {
         md.timeseriesBucketsMayHaveMixedSchemaData = setting;
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 061faf02f6f..77d9800f8d0 100644
--- a/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
+++ b/src/mongo/db/commands/set_feature_compatibility_version_command.cpp
@@ -489,16 +489,26 @@ private:
 
         _cancelTenantMigrations(opCtx);
 
-        // Secondary indexes on time-series measurements are only supported in 5.2 and up. If the
-        // user tries to downgrade the cluster to an earlier version, they must first remove all
-        // incompatible secondary indexes on time-series measurements.
-        if (requestedVersion < multiversion::FeatureCompatibilityVersion::kVersion_5_2) {
+        {
+            // Take the global lock in S mode to create a barrier for operations taking the global
+            // IX or X locks. This ensures that either
+            //   - The global IX/X locked operation will start after the FCV change, see the
+            //     downgrading to the last-lts or last-continuous FCV and act accordingly.
+            //   - The global IX/X locked operation began prior to the FCV change, is acting on that
+            //     assumption and will finish before downgrade procedures begin right after this.
+            Lock::GlobalLock lk(opCtx, MODE_S);
+        }
+
+        // TODO SERVER-60911: When kLatest is 5.3, only check when downgrading to kLastLTS (5.0).
+        // TODO SERVER-60912: When kLastLTS is 6.0, remove this FCV-gated downgrade code.
+        if (serverGlobalParams.featureCompatibility
+                .isFCVDowngradingOrAlreadyDowngradedFromLatest()) {
             for (const auto& dbName : DatabaseHolder::get(opCtx)->getNames()) {
-                Lock::DBLock dbLock(opCtx, dbName, MODE_IS);
+                Lock::DBLock dbLock(opCtx, dbName, MODE_IX);
                 catalog::forEachCollectionFromDb(
                     opCtx,
                     dbName,
-                    MODE_IS,
+                    MODE_X,
                     [&](const CollectionPtr& collection) {
                         invariant(collection->getTimeseriesOptions());
 
@@ -508,6 +518,10 @@ private:
 
                         while (indexIt->more()) {
                             auto indexEntry = indexIt->next();
+                            // Secondary indexes on time-series measurements are only supported
+                            // in 5.2 and up. If the user tries to downgrade the cluster to an
+                            // earlier version, they must first remove all incompatible secondary
+                            // indexes on time-series measurements.
                             uassert(ErrorCodes::CannotDowngrade,
                                     str::stream()
                                         << "Cannot downgrade the cluster when there are secondary "
@@ -540,6 +554,30 @@ private:
                             }
                         }
 
+                        if (!collection->getTimeseriesBucketsMayHaveMixedSchemaData()) {
+                            // The catalog entry flag has already been removed. This can happen if
+                            // the downgrade process was interrupted and is being run again. The
+                            // downgrade process cannot be aborted at this point.
+                            return true;
+                        }
+
+                        BSONObjBuilder unusedBuilder;
+                        Status status = collMod(opCtx,
+                                                collection->ns(),
+                                                BSON("collMod" << collection->ns().coll()),
+                                                &unusedBuilder);
+
+                        if (!status.isOK()) {
+                            LOGV2_FATAL(
+                                6057600,
+                                "Failed to remove catalog entry during downgrade",
+                                "error"_attr = status,
+                                "timeseriesBucketsMayHaveMixedSchemaData"_attr =
+                                    collection->getTimeseriesBucketsMayHaveMixedSchemaData(),
+                                logAttrs(collection->ns()),
+                                logAttrs(collection->uuid()));
+                        }
+
                         return true;
                     },
                     [&](const CollectionPtr& collection) {
@@ -548,16 +586,6 @@ private:
             }
         }
 
-        {
-            // Take the global lock in S mode to create a barrier for operations taking the global
-            // IX or X locks. This ensures that either
-            //   - The global IX/X locked operation will start after the FCV change, see the
-            //     downgrading to the last-lts or last-continuous FCV and act accordingly.
-            //   - The global IX/X locked operation began prior to the FCV change, is acting on that
-            //     assumption and will finish before downgrade procedures begin right after this.
-            Lock::GlobalLock lk(opCtx, MODE_S);
-        }
-
         uassert(ErrorCodes::Error(549181),
                 "Failing upgrade due to 'failDowngrading' failpoint set",
                 !failDowngrading.shouldFail());