Fix #897 and shuffle directory around (#1054)

* Fix #897 and shuffle directory around

* Update find-lint

* goimports

Co-authored-by: Neil Alexander <neilalexander@users.noreply.github.com>

internal/transactions/transactions.go

@@ -0,0 +1,95 @@
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package transactions
+
+import (
+	"sync"
+	"time"
+
+	"github.com/matrix-org/util"
+)
+
+// DefaultCleanupPeriod represents the default time duration after which cacheCleanService runs.
+const DefaultCleanupPeriod time.Duration = 30 * time.Minute
+
+type txnsMap map[CacheKey]*util.JSONResponse
+
+// CacheKey is the type for the key in a transactions cache.
+// This is needed because the spec requires transaction IDs to have a per-access token scope.
+type CacheKey struct {
+	AccessToken string
+	TxnID       string
+}
+
+// Cache represents a temporary store for response entries.
+// Entries are evicted after a certain period, defined by cleanupPeriod.
+// This works by keeping two maps of entries, and cycling the maps after the cleanupPeriod.
+type Cache struct {
+	sync.RWMutex
+	txnsMaps      [2]txnsMap
+	cleanupPeriod time.Duration
+}
+
+// New is a wrapper which calls NewWithCleanupPeriod with DefaultCleanupPeriod as argument.
+func New() *Cache {
+	return NewWithCleanupPeriod(DefaultCleanupPeriod)
+}
+
+// NewWithCleanupPeriod creates a new Cache object, starts cacheCleanService.
+// Takes cleanupPeriod as argument.
+// Returns a reference to newly created Cache.
+func NewWithCleanupPeriod(cleanupPeriod time.Duration) *Cache {
+	t := Cache{txnsMaps: [2]txnsMap{make(txnsMap), make(txnsMap)}}
+	t.cleanupPeriod = cleanupPeriod
+
+	// Start clean service as the Cache is created
+	go cacheCleanService(&t)
+	return &t
+}
+
+// FetchTransaction looks up an entry for the (accessToken, txnID) tuple in Cache.
+// Looks in both the txnMaps.
+// Returns (JSON response, true) if txnID is found, else the returned bool is false.
+func (t *Cache) FetchTransaction(accessToken, txnID string) (*util.JSONResponse, bool) {
+	t.RLock()
+	defer t.RUnlock()
+	for _, txns := range t.txnsMaps {
+		res, ok := txns[CacheKey{accessToken, txnID}]
+		if ok {
+			return res, true
+		}
+	}
+	return nil, false
+}
+
+// AddTransaction adds an entry for the (accessToken, txnID) tuple in Cache.
+// Adds to the front txnMap.
+func (t *Cache) AddTransaction(accessToken, txnID string, res *util.JSONResponse) {
+	t.Lock()
+	defer t.Unlock()
+
+	t.txnsMaps[0][CacheKey{accessToken, txnID}] = res
+}
+
+// cacheCleanService is responsible for cleaning up entries after cleanupPeriod.
+// It guarantees that an entry will be present in cache for at least cleanupPeriod & at most 2 * cleanupPeriod.
+// This cycles the txnMaps forward, i.e. back map is assigned the front and front is assigned an empty map.
+func cacheCleanService(t *Cache) {
+	ticker := time.NewTicker(t.cleanupPeriod).C
+	for range ticker {
+		t.Lock()
+		t.txnsMaps[1] = t.txnsMaps[0]
+		t.txnsMaps[0] = make(txnsMap)
+		t.Unlock()
+	}
+}