Wrap the code indexer (#9476)

* Wrap the code indexer

In order to prevent a data race in the code indexer it must be wrapped
with a holder otherwise it is possible to Search/Index on an
incompletely initialised indexer, and search will fail with a nil
pointer until the repository indexer is initialised.

Further a completely initialised repository indexer should not be closed
until Termination otherwise actions in Hammer/Shutdown phases could
block or be lost.

Finally, there is a complex dance of shutdown etiquette should the index
initialisation fail. This PR restores that.

* Always return err if closed whilst waiting

Co-authored-by: techknowlogick <matti@mdranta.net>

modules/indexer/code/queue.go

@@ -22,8 +22,6 @@ type repoIndexerOperation struct {
 var repoIndexerOperationQueue chan repoIndexerOperation
 
 func processRepoIndexerOperationQueue(indexer Indexer) {
-	defer indexer.Close()
-
 	repoIndexerOperationQueue = make(chan repoIndexerOperation, setting.Indexer.UpdateQueueLength)
 	for {
 		select {