A transparent cache for datastore, backed by memcache.

filter/dscache/ds_txn_state.go

Index: filter/dscache/ds_txn_state.go
diff --git a/filter/dscache/ds_txn_state.go b/filter/dscache/ds_txn_state.go
new file mode 100644
index 0000000000000000000000000000000000000000..4eb505e233daf15d7a1563d7d9b733d0c73cc431
--- /dev/null
+++ b/filter/dscache/ds_txn_state.go
@@ -0,0 +1,86 @@
+// Copyright 2015 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+package dscache
+
+import (
+	"sync"
+
+	"github.com/luci/gae/service/datastore"
+	"github.com/luci/gae/service/memcache"
+	log "github.com/luci/luci-go/common/logging"
+)
+
+type dsTxnState struct {
+	sync.Mutex
+
+	toLock   []memcache.Item
+	toDelete map[string]struct{}
+}
+
+// reset sets the transaction state back to its 0 state. This is used so that
+// when a transaction retries the function, we don't accidentally leak state
+// from one function to the next.
+func (s *dsTxnState) reset() {
+	s.Lock()
+	defer s.Unlock()
+	// reduce capacity back to 0, but keep the allocated array. If the transaction
+	// body retries, it'll probably end up re-allocating the same amount of space
+	// anyway.
+	s.toLock = s.toLock[:0]
+	s.toDelete = make(map[string]struct{}, len(s.toDelete))
+}
+
+// apply is called right before the trasnaction is about to commit. It's job
+// is to lock all the to-be-changed memcache keys.
+func (s *dsTxnState) apply(sc *supportContext) error {
+	s.Lock()
+	defer s.Unlock()
+
+	// this is a hard failure. No mutation can occur if we're unable to set
+	// locks out. See "DANGER ZONE" in the docs.
+	err := sc.mc.SetMulti(s.toLock)
+	if err != nil {
+		(log.Fields{log.ErrorKey: err}).Errorf(
+			sc.c, "dscache: HARD FAILURE: dsTxnState.apply(): mc.SetMulti")
+	}
+	return err
+}
+
+// release is called right after a successful transaction completion. It's job
+// is to clear out all the locks, if possible (but if not, no worries,
+// they'll expire soon).
+func (s *dsTxnState) release(sc *supportContext) {
+	s.Lock()
+	defer s.Unlock()
+
+	delKeys := make([]string, 0, len(s.toDelete))
+	for k := range s.toDelete {
+		delKeys = append(delKeys, k)
+	}
+
+	err := sc.mc.DeleteMulti(delKeys)
+	if err != nil {
+		(log.Fields{log.ErrorKey: err}).Warningf(
+			sc.c, "dscache: txn.release: memcache.DeleteMulti")
+	}
+}
+
+func (s *dsTxnState) add(sc *supportContext, keys []datastore.Key) {
+	lockItems, lockKeys := sc.mkAllLockItems(keys)
+	if lockItems == nil {
+		return
+	}
+
+	s.Lock()
+	defer s.Unlock()
+
+	for i, li := range lockItems {
+		k := lockKeys[i]
+		if _, ok := s.toDelete[k]; !ok {
+			s.toLock = append(s.toLock, li)
+			s.toDelete[k] = struct{}{}
+		}
+	}
+}