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.

2 files changed, 148 insertions, 41 deletions

diff --git a/tools/nixery/server/builder/builder.go b/tools/nixery/server/builder/builder.go
index c53b702e0537..e241d3d0b004 100644
--- a/tools/nixery/server/builder/builder.go
+++ b/tools/nixery/server/builder/builder.go
@@ -69,10 +69,10 @@ func ImageFromName(name string, tag string) Image {
 //
 // The later field is simply treated as opaque JSON and passed through.
 type BuildResult struct {
-	Error string   `json:"error"`
-	Pkgs  []string `json:"pkgs"`
+	Error    string          `json:"error"`
+	Pkgs     []string        `json:"pkgs"`
+	Manifest json.RawMessage `json:"manifest"`
 
-	Manifest       json.RawMessage `json:"manifest"`
 	LayerLocations map[string]struct {
 		Path string `json:"path"`
 		Md5  []byte `json:"md5"`
@@ -99,50 +99,57 @@ func convenienceNames(packages []string) []string {
 
 // Call out to Nix and request that an image be built. Nix will, upon success,
 // return a manifest for the container image.
-func BuildImage(ctx *context.Context, cfg *config.Config, image *Image, bucket *storage.BucketHandle) (*BuildResult, error) {
-	packages, err := json.Marshal(image.Packages)
-	if err != nil {
-		return nil, err
-	}
+func BuildImage(ctx *context.Context, cfg *config.Config, cache *BuildCache, image *Image, bucket *storage.BucketHandle) (*BuildResult, error) {
+	resultFile, cached := cache.manifestFromCache(image)
 
-	args := []string{
-		"--argstr", "name", image.Name,
-		"--argstr", "packages", string(packages),
-	}
+	if !cached {
+		packages, err := json.Marshal(image.Packages)
+		if err != nil {
+			return nil, err
+		}
 
-	if cfg.Pkgs != nil {
-		args = append(args, "--argstr", "pkgSource", cfg.Pkgs.Render(image.Tag))
-	}
-	cmd := exec.Command("nixery-build-image", args...)
+		args := []string{
+			"--argstr", "name", image.Name,
+			"--argstr", "packages", string(packages),
+		}
 
-	outpipe, err := cmd.StdoutPipe()
-	if err != nil {
-		return nil, err
-	}
+		if cfg.Pkgs != nil {
+			args = append(args, "--argstr", "pkgSource", cfg.Pkgs.Render(image.Tag))
+		}
+		cmd := exec.Command("nixery-build-image", args...)
 
-	errpipe, err := cmd.StderrPipe()
-	if err != nil {
-		return nil, err
-	}
+		outpipe, err := cmd.StdoutPipe()
+		if err != nil {
+			return nil, err
+		}
 
-	if err = cmd.Start(); err != nil {
-		log.Println("Error starting nix-build:", err)
-		return nil, err
-	}
-	log.Printf("Started Nix image build for '%s'", image.Name)
+		errpipe, err := cmd.StderrPipe()
+		if err != nil {
+			return nil, err
+		}
 
-	stdout, _ := ioutil.ReadAll(outpipe)
-	stderr, _ := ioutil.ReadAll(errpipe)
+		if err = cmd.Start(); err != nil {
+			log.Println("Error starting nix-build:", err)
+			return nil, err
+		}
+		log.Printf("Started Nix image build for '%s'", image.Name)
 
-	if err = cmd.Wait(); err != nil {
-		// TODO(tazjin): Propagate errors upwards in a usable format.
-		log.Printf("nix-build execution error: %s\nstdout: %s\nstderr: %s\n", err, stdout, stderr)
-		return nil, err
-	}
+		stdout, _ := ioutil.ReadAll(outpipe)
+		stderr, _ := ioutil.ReadAll(errpipe)
 
-	log.Println("Finished Nix image build")
+		if err = cmd.Wait(); err != nil {
+			// TODO(tazjin): Propagate errors upwards in a usable format.
+			log.Printf("nix-build execution error: %s\nstdout: %s\nstderr: %s\n", err, stdout, stderr)
+			return nil, err
+		}
+
+		log.Println("Finished Nix image build")
+
+		resultFile = strings.TrimSpace(string(stdout))
+		cache.cacheManifest(image, resultFile)
+	}
 
-	buildOutput, err := ioutil.ReadFile(strings.TrimSpace(string(stdout)))
+	buildOutput, err := ioutil.ReadFile(resultFile)
 	if err != nil {
 		return nil, err
 	}
@@ -151,15 +158,20 @@ func BuildImage(ctx *context.Context, cfg *config.Config, image *Image, bucket *
 	// contained layers to the bucket. Only the manifest itself is
 	// re-serialised to JSON and returned.
 	var result BuildResult
+
 	err = json.Unmarshal(buildOutput, &result)
 	if err != nil {
 		return nil, err
 	}
 
 	for layer, meta := range result.LayerLocations {
-		err = uploadLayer(ctx, bucket, layer, meta.Path, meta.Md5)
-		if err != nil {
-			return nil, err
+		if !cache.hasSeenLayer(layer) {
+			err = uploadLayer(ctx, bucket, layer, meta.Path, meta.Md5)
+			if err != nil {
+				return nil, err
+			}
+
+			cache.sawLayer(layer)
 		}
 	}
 
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()
+}