Each of the fields in objectConfig.ObjectLockConfiguration.Rule.DefaultRetention.Days is of pointer type, so again possibility of nil pointer dereference.

here, it won't be an issue as caller for this func is checking the nil pointer reference.

if bucketImmutableExpiryTimeInDays != nil {
....
}

bucketImmutableExpiryTimeInDays is deduced from GetBucketImmutabilityTime(), which in turn returns objectConfig.ObjectLockConfiguration.Rule.DefaultRetention.Days. Simply checking bucketImmutableExpiryTimeInDays will not help, since you could have Rule an non-nil, but DefaultRetention could be nil, and throw an exception. That's the reason the entire path should be checked here.

ok, got your point. I will make the change.

How is bucketImmutableExpiryTimeInDays being used here? The snapshot gets added to the snapList without setting snap.ImmutabilityExpiryTime, which doesn't seem right. Can you please confirm?

yes, you are correct. Actually I added this to handle the case when previous api call failed to get bucketImmutableExpiryTimeInDays.
Another way to handle this is to add a retry mechanism in first api call, and remove this else handling part , wdyt ?

Yes, retry mechanism seems more intuitive here.

Can you please add some tests specific to handling immutable backups for S3 buckets? Like garbage collection for instance.