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/wrapped.go

@@ -0,0 +1,94 @@
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package code
+
+import (
+	"fmt"
+	"sync"
+)
+
+var (
+	indexer = newWrappedIndexer()
+)
+
+// ErrWrappedIndexerClosed is the error returned if the indexer was closed before it was ready
+var ErrWrappedIndexerClosed = fmt.Errorf("Indexer closed before ready")
+
+type wrappedIndexer struct {
+	internal Indexer
+	lock     sync.RWMutex
+	cond     *sync.Cond
+	closed   bool
+}
+
+func newWrappedIndexer() *wrappedIndexer {
+	w := &wrappedIndexer{}
+	w.cond = sync.NewCond(w.lock.RLocker())
+	return w
+}
+
+func (w *wrappedIndexer) set(indexer Indexer) {
+	w.lock.Lock()
+	defer w.lock.Unlock()
+	if w.closed {
+		// Too late!
+		indexer.Close()
+	}
+	w.internal = indexer
+	w.cond.Broadcast()
+}
+
+func (w *wrappedIndexer) get() (Indexer, error) {
+	w.lock.RLock()
+	defer w.lock.RUnlock()
+	if w.internal == nil {
+		if w.closed {
+			return nil, ErrWrappedIndexerClosed
+		}
+		w.cond.Wait()
+		if w.closed {
+			return nil, ErrWrappedIndexerClosed
+		}
+	}
+	return w.internal, nil
+}
+
+func (w *wrappedIndexer) Index(repoID int64) error {
+	indexer, err := w.get()
+	if err != nil {
+		return err
+	}
+	return indexer.Index(repoID)
+}
+
+func (w *wrappedIndexer) Delete(repoID int64) error {
+	indexer, err := w.get()
+	if err != nil {
+		return err
+	}
+	return indexer.Delete(repoID)
+}
+
+func (w *wrappedIndexer) Search(repoIDs []int64, keyword string, page, pageSize int) (int64, []*SearchResult, error) {
+	indexer, err := w.get()
+	if err != nil {
+		return 0, nil, err
+	}
+	return indexer.Search(repoIDs, keyword, page, pageSize)
+
+}
+
+func (w *wrappedIndexer) Close() {
+	w.lock.Lock()
+	defer w.lock.Unlock()
+	if w.closed {
+		return
+	}
+	w.closed = true
+	w.cond.Broadcast()
+	if w.internal != nil {
+		w.internal.Close()
+	}
+}