SERVER-47642 Ensure no active snapshot before acquiring collection X lock

3 files changed, 17 insertions, 5 deletions

diff --git a/src/mongo/db/catalog/drop_indexes.cpp b/src/mongo/db/catalog/drop_indexes.cpp
index 98c2025918d..93bc44b3a0f 100644
--- a/src/mongo/db/catalog/drop_indexes.cpp
+++ b/src/mongo/db/catalog/drop_indexes.cpp
@@ -397,14 +397,14 @@ Status dropIndexes(OperationContext* opCtx,
             hangAfterAbortingIndexes.pauseWhileSet();
         }
 
-        // Take an exclusive lock on the collection now to be able to perform index catalog writes
-        // when removing ready indexes from disk.
-        collection.emplace(opCtx, dbAndUUID, MODE_X);
-
         // Abandon the snapshot as the index catalog will compare the in-memory state to the disk
         // state, which may have changed when we released the lock temporarily.
         opCtx->recoveryUnit()->abandonSnapshot();
 
+        // Take an exclusive lock on the collection now to be able to perform index catalog writes
+        // when removing ready indexes from disk.
+        collection.emplace(opCtx, dbAndUUID, MODE_X);
+
         db = collection->getDb();
         if (!*collection) {
             return Status(ErrorCodes::NamespaceNotFound,
diff --git a/src/mongo/db/catalog_raii.cpp b/src/mongo/db/catalog_raii.cpp
index d0a91958cc3..0f29d258bcd 100644
--- a/src/mongo/db/catalog_raii.cpp
+++ b/src/mongo/db/catalog_raii.cpp
@@ -79,12 +79,23 @@ AutoGetCollection::AutoGetCollection(OperationContext* opCtx,
               !nsOrUUID.dbname().empty() ? nsOrUUID.dbname() : nsOrUUID.nss()->db(),
               isSharedLockMode(modeColl) ? MODE_IS : MODE_IX,
               deadline) {
-    if (auto& nss = nsOrUUID.nss()) {
+    auto& nss = nsOrUUID.nss();
+    if (nss) {
         uassert(ErrorCodes::InvalidNamespace,
                 str::stream() << "Namespace " << *nss << " is not a valid collection name",
                 nss->isValid());
     }
 
+    // Out of an abundance of caution, force operations to acquire new snapshots after
+    // acquiring exclusive collection locks. Operations that hold MODE_X locks make an
+    // assumption that all writes are visible in their snapshot and no new writes will commit.
+    // This may not be the case if an operation already has a snapshot open before acquiring an
+    // exclusive lock.
+    if (modeColl == MODE_X) {
+        invariant(!opCtx->recoveryUnit()->inActiveTxn(),
+                  str::stream() << "Snapshot opened before acquiring X lock for " << *nss);
+    }
+
     _collLock.emplace(opCtx, nsOrUUID, modeColl, deadline);
     _resolvedNss = CollectionCatalog::get(opCtx).resolveNamespaceStringOrUUID(opCtx, nsOrUUID);
 
diff --git a/src/mongo/dbtests/storage_timestamp_tests.cpp b/src/mongo/dbtests/storage_timestamp_tests.cpp
index ab385218c86..05d87f448da 100644
--- a/src/mongo/dbtests/storage_timestamp_tests.cpp
+++ b/src/mongo/dbtests/storage_timestamp_tests.cpp
@@ -237,6 +237,7 @@ public:
     void reset(NamespaceString nss) const {
         ::mongo::writeConflictRetry(_opCtx, "deleteAll", nss.ns(), [&] {
             _opCtx->recoveryUnit()->setTimestampReadSource(RecoveryUnit::ReadSource::kNoTimestamp);
+            _opCtx->recoveryUnit()->abandonSnapshot();
             AutoGetCollection collRaii(_opCtx, nss, LockMode::MODE_X);
 
             if (collRaii) {