Transit: fix race in the key update api (#28839)

- The key update API would release the lock a little too early
   after it persisted the update so the reference could be updated
   when it was preparing the response to the caller across updates
   and/or key rotations
 - The storage updates were okay, just the response back to the caller
   of the update might see a mixture of different updates

builtin/logical/transit/path_keys.go

@@ -260,9 +260,10 @@ func (b *backend) pathPolicyWrite(ctx context.Context, req *logical.Request, d *
 	if p == nil {
 		return nil, fmt.Errorf("error generating key: returned policy was nil")
 	}
-	if b.System().CachingDisabled() {
+	if !b.System().CachingDisabled() {
-		p.Unlock()
+		p.Lock(true)
 	}
+	defer p.Unlock()
 
 	resp, err := b.formatKeyPolicy(p, nil)
 	if err != nil {

changelog/28839.txt

@@ -0,0 +1,3 @@
+```release-note:bug
+secrets/transit: Fix a race in which responses from the key update api could contain results from another subsequent update
+```