SERVER-47846 fix use-after-free coding error in collMod hidden indexes

(cherry picked from commit 951a6c5f089fdb1c5e2cae1641a6ca20d8c2662c)

1 files changed, 3 insertions, 1 deletions

diff --git a/src/mongo/db/catalog/coll_mod.cpp b/src/mongo/db/catalog/coll_mod.cpp
index a0a5cc325e1..46151cdec52 100644
--- a/src/mongo/db/catalog/coll_mod.cpp
+++ b/src/mongo/db/catalog/coll_mod.cpp
@@ -348,6 +348,8 @@ Status _collModInternal(OperationContext* opCtx,
     auto viewOn = cmrNew.viewOn;
     auto indexExpireAfterSeconds = cmrNew.indexExpireAfterSeconds;
     auto indexHidden = cmrNew.indexHidden;
+    // WriteConflictExceptions thrown in the writeConflictRetry loop below can cause cmrNew.idx to
+    // become invalid, so save a copy to use in the loop until we can refresh it.
     auto idx = cmrNew.idx;
 
     if (indexHidden) {
@@ -441,7 +443,7 @@ Status _collModInternal(OperationContext* opCtx,
                                                           : Seconds(oldExpireSecs.safeNumberLong()),
                                  !indexHidden ? boost::optional<bool>() : newHidden.booleanSafe(),
                                  !indexHidden ? boost::optional<bool>() : oldHidden.booleanSafe(),
-                                 cmrNew.idx->indexName()};
+                                 idx->indexName()};
 
             // Notify the index catalog that the definition of this index changed. This will
             // invalidate the local idx pointer. On rollback of this WUOW, the idx pointer in