feat(builder): Implement build cache for manifests & layers

Implements a cache that keeps track of:

a) Manifests that have already been built (for up to 6 hours)
b) Layers that have already been seen (and uploaded to GCS)

This significantly speeds up response times for images that are full
or partial matches with previous images served by an instance.

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 0000000000..0014789aff
--- /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()
+}