Problem:

Customer saw this in their logs when using GridFS

Thu Jan 13 12:34:19 [conn14216] insert punchbowl-production.system.indexes exception 12588 cannot add index with a background operation in progress 0ms
Thu Jan 13 12:34:21 [conn14009] Caught Assertion in insert , continuing
Thu Jan 13 12:34:21 [conn14009] insert punchbowl-production.system.indexes exception 12588 cannot add index with a background operation in progress 0ms

Reproduce:
1. create a number of files in GridFS
2. drop the index FILES_ID
3. start a loop that inserts new files into GridFS
4. create the FILES_ID index with

5. error logged

Solution:
Make GridFS code periodically run ensureIndex, rather than on most calls.