Commits on Aug 14, 2024

1. Fix KV create race after delete/purge …

   This change fixes an issue when using a JetStream K/V store where a user
   is creating, deleting, and re-creating keys. If the last entry for a key
   is a `Operation::Delete` or `Operation::Purge`, the initial
   `self.update()` returns an error, causing the second part of the method
   to be exercised.
   
   Prior to this change, if the entry was deleted or purged a `kv.put()`
   call is used which ignores the revision of that last entry. A single
   writer to the K/V store would succeed (as no other writers would write
   first) so no problem. However, if 2 writers attempt to create a key,
   then a second writer *could* call the `kv.put()` before the first writer
   calls `kv.put()`. This means that *both* writers get an `Ok(revision)`
   and can assume that they won the creation of the key.
   
   When using a "distributed lock" pattern (that is many writers race to
   create a key and the first successful writer wins), this above scenario
   results in potentially more than one writer who believes they have
   uniquely acquired the distributed lock.
   
   This change replaces the `kv.put()` call to a `kv.update()` call and
   provides the `revision` from the deleted/purged entry to ensure that no
   other writer has beaten the caller to this update. This change closes
   the race period between concurrent writers to between the first update
   and the second update call with some optimistic write concurrency to
   detect another writer.
   
   It appears as though this strategy is in effect in the Go client code
   [kv.Create] implementation.
   
   [kv.Create]: https://github.com/nats-io/nats.go/blob/278f9f188bca4d7bdee283a0e98ab66b82530c60/jetstream/kv.go#L944-L963
   
   Co-authored-by: John Keiser <[email protected]>
   Signed-off-by: Fletcher Nichol <[email protected]>
   Signed-off-by: Fletcher Nichol <[email protected]>

   

   fnichol and John Keiser committed Aug 14, 2024
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