InfluxDB creates duplicated legacy mappings on `/dbrps`

[hinst]

Steps to reproduce

1. Run two API requests in parallel to create dbrp mapping
   2. All arguments are the same: database, retention_policy, orgID, bucketID
   3. POST http://192.168.0.4:8086/api/v2/dbrps

Expected behavior

Only one of the requests succeeds, while the other gets rejected

Actual behavior

Both requests succeed. 🐞 Two equal dbrp mappings are created, leading to an invalid state

Environment info

• System info: Linux-5.11.0-25-generic x86_64
• InfluxDB version: 2.0.7

Config

All default

Additional info for bug reproduction

This script can be used to reproduce the bug 100% of time: https://gist.github.com/hinst/5943f75d002eaa2a7398fe2181c81cff

1. Run the script
2. Check the number of mappings using command
   3. influx v1 dbrp list -o HOME
   4. -> there will be two mappings

If the same requests are run in a sequence, and not in parallel, then the second request to create the legacy mapping will fail with an error: such mapping already exists

[williamhbaker]

I repro'd this using a similar script in go. Seems to be a race condition between checking if the dbrp is unique and creating it. Most likely the logic around here needs to be changed so that the uniqueness check happens while under the lock of the s.store.Update method.