refactor(server): Cache manifest entries for layer builds

MD5 hash checking is no longer performed by Nixery (it does not seem
to be necessary), hence the layer cache now only keeps the SHA256 hash
and size in the form of the manifest entry.

This makes it possible to restructure the builder code to perform
cache-fetching and cache-populating for layers in the same place.

tools/nixery/server/builder/cache.go

@@ -21,12 +21,9 @@ import (
 	"io/ioutil"
 	"log"
 	"sync"
-)
 
-type Build struct {
-	SHA256 string `json:"sha256"`
-	MD5    string `json:"md5"`
-}
+	"github.com/google/nixery/manifest"
+)
 
 // LocalCache implements the structure used for local caching of
 // manifests and layer uploads.
@@ -37,13 +34,13 @@ type LocalCache struct {
 
 	// Layer cache
 	lmtx   sync.RWMutex
-	lcache map[string]Build
+	lcache map[string]manifest.Entry
 }
 
 func NewCache() LocalCache {
 	return LocalCache{
 		mcache: make(map[string]string),
-		lcache: make(map[string]Build),
+		lcache: make(map[string]manifest.Entry),
 	}
 }
 
@@ -68,19 +65,19 @@ func (c *LocalCache) localCacheManifest(key, path string) {
 	c.mmtx.Unlock()
 }
 
-// Retrieve a cached build from the local cache.
-func (c *LocalCache) buildFromLocalCache(key string) (*Build, bool) {
+// Retrieve a layer build from the local cache.
+func (c *LocalCache) layerFromLocalCache(key string) (*manifest.Entry, bool) {
 	c.lmtx.RLock()
-	b, ok := c.lcache[key]
+	e, ok := c.lcache[key]
 	c.lmtx.RUnlock()
 
-	return &b, ok
+	return &e, ok
 }
 
-// Add a build result to the local cache.
-func (c *LocalCache) localCacheBuild(key string, b Build) {
+// Add a layer build result to the local cache.
+func (c *LocalCache) localCacheLayer(key string, e manifest.Entry) {
 	c.lmtx.Lock()
-	c.lcache[key] = b
+	c.lcache[key] = e
 	c.lmtx.Unlock()
 }
 
@@ -141,12 +138,11 @@ func cacheManifest(ctx context.Context, s *State, key string, m json.RawMessage)
 	log.Printf("Cached manifest sha1:%s (%v bytes written)\n", key, size)
 }
 
-// Retrieve a build from the cache, first checking the local cache
-// followed by the bucket cache.
-func buildFromCache(ctx context.Context, s *State, key string) (*Build, bool) {
-	build, cached := s.Cache.buildFromLocalCache(key)
-	if cached {
-		return build, true
+// Retrieve a layer build from the cache, first checking the local
+// cache followed by the bucket cache.
+func layerFromCache(ctx context.Context, s *State, key string) (*manifest.Entry, bool) {
+	if entry, cached := s.Cache.layerFromLocalCache(key); cached {
+		return entry, true
 	}
 
 	obj := s.Bucket.Object("builds/" + key)
@@ -157,7 +153,7 @@ func buildFromCache(ctx context.Context, s *State, key string) (*Build, bool) {
 
 	r, err := obj.NewReader(ctx)
 	if err != nil {
-		log.Printf("Failed to retrieve build '%s' from cache: %s\n", key, err)
+		log.Printf("Failed to retrieve layer build '%s' from cache: %s\n", key, err)
 		return nil, false
 	}
 	defer r.Close()
@@ -165,27 +161,27 @@ func buildFromCache(ctx context.Context, s *State, key string) (*Build, bool) {
 	jb := bytes.NewBuffer([]byte{})
 	_, err = io.Copy(jb, r)
 	if err != nil {
-		log.Printf("Failed to read build '%s' from cache: %s\n", key, err)
+		log.Printf("Failed to read layer build '%s' from cache: %s\n", key, err)
 		return nil, false
 	}
 
-	var b Build
-	err = json.Unmarshal(jb.Bytes(), &build)
+	var entry manifest.Entry
+	err = json.Unmarshal(jb.Bytes(), &entry)
 	if err != nil {
-		log.Printf("Failed to unmarshal build '%s' from cache: %s\n", key, err)
+		log.Printf("Failed to unmarshal layer build '%s' from cache: %s\n", key, err)
 		return nil, false
 	}
 
-	go s.Cache.localCacheBuild(key, b)
-	return &b, true
+	go s.Cache.localCacheLayer(key, entry)
+	return &entry, true
 }
 
-func cacheBuild(ctx context.Context, s *State, key string, build Build) {
-	go s.Cache.localCacheBuild(key, build)
+func cacheLayer(ctx context.Context, s *State, key string, entry manifest.Entry) {
+	s.Cache.localCacheLayer(key, entry)
 
 	obj := s.Bucket.Object("builds/" + key)
 
-	j, _ := json.Marshal(&build)
+	j, _ := json.Marshal(&entry)
 
 	w := obj.NewWriter(ctx)
 

tools/nixery/server/manifest/manifest.go

@@ -25,7 +25,7 @@ const (
 )
 
 type Entry struct {
-	MediaType string `json:"mediaType"`
+	MediaType string `json:"mediaType,omitempty"`
 	Size      int64  `json:"size"`
 	Digest    string `json:"digest"`
 }