1 files changed, 95 insertions, 0 deletions
diff --git a/tools/nixery/server/builder/cache.go b/tools/nixery/server/builder/cache.go
new file mode 100644
index 000000000000..0014789afff5
--- /dev/null
+++ b/tools/nixery/server/builder/cache.go
@@ -0,0 +1,95 @@
+// Copyright 2019 Google LLC
+//
+// 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
+//
+//     https://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 builder
+
+import (
+	"sync"
+	"time"
+)
+
+// recencyThreshold is the amount of time that a manifest build will be cached
+// for. When using the channel mechanism for retrieving nixpkgs, Nix will
+// occasionally re-fetch the channel so things can in fact change while the
+// instance is running.
+const recencyThreshold = time.Duration(6) * time.Hour
+
+type manifestEntry struct {
+	built time.Time
+	path  string
+}
+
+type void struct{}
+
+type BuildCache struct {
+	mmtx   sync.RWMutex
+	mcache map[string]manifestEntry
+
+	lmtx   sync.RWMutex
+	lcache map[string]void
+}
+
+func NewCache() BuildCache {
+	return BuildCache{
+		mcache: make(map[string]manifestEntry),
+		lcache: make(map[string]void),
+	}
+}
+
+// Has this layer hash already been seen by this Nixery instance? If
+// yes, we can skip upload checking and such because it has already
+// been done.
+func (c *BuildCache) hasSeenLayer(hash string) bool {
+	c.lmtx.RLock()
+	defer c.lmtx.RUnlock()
+	_, seen := c.lcache[hash]
+	return seen
+}
+
+// Layer has now been seen and should be stored.
+func (c *BuildCache) sawLayer(hash string) {
+	c.lmtx.Lock()
+	defer c.lmtx.Unlock()
+	c.lcache[hash] = void{}
+}
+
+// Has this manifest been built already? If yes, we can reuse the
+// result given that the build happened recently enough.
+func (c *BuildCache) manifestFromCache(image *Image) (string, bool) {
+	c.mmtx.RLock()
+
+	entry, ok := c.mcache[image.Name+image.Tag]
+	c.mmtx.RUnlock()
+
+	if !ok {
+		return "", false
+	}
+
+	if time.Since(entry.built) > recencyThreshold {
+		return "", false
+	}
+
+	return entry.path, true
+}
+
+// Adds the result of a manifest build to the cache.
+func (c *BuildCache) cacheManifest(image *Image, path string) {
+	entry := manifestEntry{
+		built: time.Now(),
+		path:  path,
+	}
+
+	c.mmtx.Lock()
+	c.mcache[image.Name+image.Tag] = entry
+	c.mmtx.Unlock()
+}