Intial sync can temporarily reset the fcv value to uninitialized and sets the new value afterwards. This can cause call sites trying to inspect the fcv value to hit this invariant. We need to audit feature flag usage and determine if the feature flag check can be run during initial sync:

If it can never be called when initial sync is running, do nothing. Note that this can be tricky to prove as we once thought the catalog cache loader can never be run while initial sync is happening but it can.

If it might get run during initial sync, this could be the case if the feature is run during initial sync itself, if the feature is in a background thread that runs during initial sync, or if the feature is run in a command that is allowed during initial sync, such as hello, serverStatus, etc. In this case, use one of these options:

• Use isEnabledUseLastLTSFCVWhenUninitialized. It checks against the last LTS FCV version if the FCV version is unset, but note that this could result in the feature not being turned on even though the FCV will be set to latest once initial sync is complete.
• Use isEnabledUseLatestFCVWhenUninitialized. This instead checks against the latest FCV version if the FCV version is unset, but note that this could result in the feature being turned on even though the FCV has not been upgraded yet and will be set to lastLTS once initial sync is complete.
• Write your own special logic to avoid the invariant (for example, waiting for the FCV to become initialized before checking isEnabled, or uasserting instead of invariant-ing)

See this section of the README

List of feature flags from "git grep -ni featureflag src/ | grep "isEnabled(" | grep -v _test"

gFeatureFlagBucketUnpackWithSort
gFeatureFlagBulkWriteCommand
gfeatureFlagCappedCollectionsRelaxedSize
gFeatureFlagCatalogShard
gFeatureFlagChangeStreamsFurtherEnrichedEvents
gFeatureFlagColumnstoreIndexes
gFeatureFlagCommonQueryFramework
gFeatureFlagCompoundWildcardIndexes
gFeatureFlagConfigurableX509ClusterAuthn
gFeatureFlagAuditConfigClusterParameter
gFeatureFlagFLE2CompactForProtocolV2
gFeatureFlagFLE2Range
gFeatureFlagGlobalIndexes
gFeatureFlagHealthMonitoring
gFeatureFlagInternalWritesAreReplicatedTransactionally
gFeatureFlagServerlessChangeStreams
gFeatureFlagTelemetry
gFeatureFlagRequireTenantID
gFeatureFlagResharding
gFeatureFlagShardedTimeSeries
gFeatureFlagUpdateDocumentShardKeyUsingTransactionApi
gFeatureFlagUpdateOneWithoutShardKey
gFeatureFlagUserRoles
gGlobalIndexesShardingCatalog