fix(nixery): Avoid race when the same image is fetched in parallel r/4924

Remove a race condition which appears when uploadHashLayer is called
with the same key from multiple threads simultaneously. This can
easily happen when the same image path is requested by multiple
clients at the same time. When it does, a 500 status is returned and
the following error message is logged:

{
  "context": {
    "filePath": "github.com/google/nixery/builder/builder.go",
    "lineNumber": 440,
    "functionName": "github.com/google/nixery/builder.uploadHashLayer"
  },
  "error": "rename /var/lib/nixery/staging/<hash> /var/lib/nixery/layers/<hash>: no such file or directory",
  "eventTime": "...",
  "layer": "<hash>",
  "message": "failed to move layer from staging",
  ...
}

To solve this issue, introduce a mutex keyed on the uploaded hash and
move all layer caching into uploadHashLayer. This could additionally
provide a small performance benefit when an already built image is
requested and NIXERY_PKGS_PATH is set, since symlink layers and config
layers are now also cached.

Change-Id: I50788a7ec7940cb5e5760f244692e361019a9bb7
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6695
Reviewed-by: tazjin <tazjin@tvl.su>
Tested-by: BuildkiteCI

1 files changed, 50 insertions, 42 deletions

diff --git a/tools/nixery/builder/builder.go b/tools/nixery/builder/builder.go
index 901373b57e99..7f0bd7fffdb9 100644
--- a/tools/nixery/builder/builder.go
+++ b/tools/nixery/builder/builder.go
@@ -26,6 +26,7 @@ import (
 	"github.com/google/nixery/layers"
 	"github.com/google/nixery/manifest"
 	"github.com/google/nixery/storage"
+	"github.com/im7mortal/kmutex"
 	log "github.com/sirupsen/logrus"
 )
 
@@ -37,10 +38,11 @@ const LayerBudget int = 94
 // State holds the runtime state that is carried around in Nixery and
 // passed to builder functions.
 type State struct {
-	Storage storage.Backend
-	Cache   *LocalCache
-	Cfg     config.Config
-	Pop     layers.Popularity
+	Storage     storage.Backend
+	Cache       *LocalCache
+	Cfg         config.Config
+	Pop         layers.Popularity
+	UploadMutex *kmutex.Kmutex
 }
 
 // Architecture represents the possible CPU architectures for which
@@ -292,30 +294,19 @@ func prepareLayers(ctx context.Context, s *State, image *Image, result *ImageRes
 	// Missing layers are built and uploaded to the storage
 	// bucket.
 	for _, l := range grouped {
-		if entry, cached := layerFromCache(ctx, s, l.Hash()); cached {
-			entries = append(entries, *entry)
-		} else {
-			lh := l.Hash()
-
-			// While packing store paths, the SHA sum of
-			// the uncompressed layer is computed and
-			// written to `tarhash`.
-			//
-			// TODO(tazjin): Refactor this to make the
-			// flow of data cleaner.
-			var tarhash string
-			lw := func(w io.Writer) error {
-				var err error
-				tarhash, err = packStorePaths(&l, w)
-				return err
-			}
-
-			entry, err := uploadHashLayer(ctx, s, lh, lw)
+		lh := l.Hash()
+
+		// While packing store paths, the SHA sum of
+		// the uncompressed layer is computed and
+		// written to `tarhash`.
+		//
+		// TODO(tazjin): Refactor this to make the
+		// flow of data cleaner.
+		lw := func(w io.Writer) (string, error) {
+			tarhash, err := packStorePaths(&l, w)
 			if err != nil {
-				return nil, err
+				return "", err
 			}
-			entry.MergeRating = l.MergeRating
-			entry.TarHash = tarhash
 
 			var pkgs []string
 			for _, p := range l.Contents {
@@ -328,15 +319,21 @@ func prepareLayers(ctx context.Context, s *State, image *Image, result *ImageRes
 				"tarhash":  tarhash,
 			}).Info("created image layer")
 
-			go cacheLayer(ctx, s, l.Hash(), *entry)
-			entries = append(entries, *entry)
+			return tarhash, err
+		}
+
+		entry, err := uploadHashLayer(ctx, s, lh, l.MergeRating, lw)
+		if err != nil {
+			return nil, err
 		}
+
+		entries = append(entries, *entry)
 	}
 
 	// Symlink layer (built in the first Nix build) needs to be
 	// included here manually:
 	slkey := result.SymlinkLayer.TarHash
-	entry, err := uploadHashLayer(ctx, s, slkey, func(w io.Writer) error {
+	entry, err := uploadHashLayer(ctx, s, slkey, 0, func(w io.Writer) (string, error) {
 		f, err := os.Open(result.SymlinkLayer.Path)
 		if err != nil {
 			log.WithError(err).WithFields(log.Fields{
@@ -345,7 +342,7 @@ func prepareLayers(ctx context.Context, s *State, image *Image, result *ImageRes
 				"layer": slkey,
 			}).Error("failed to open symlink layer")
 
-			return err
+			return "", err
 		}
 		defer f.Close()
 
@@ -358,18 +355,16 @@ func prepareLayers(ctx context.Context, s *State, image *Image, result *ImageRes
 				"layer": slkey,
 			}).Error("failed to upload symlink layer")
 
-			return err
+			return "", err
 		}
 
-		return gz.Close()
+		return "sha256:" + slkey, gz.Close()
 	})
 
 	if err != nil {
 		return nil, err
 	}
 
-	entry.TarHash = "sha256:" + result.SymlinkLayer.TarHash
-	go cacheLayer(ctx, s, slkey, *entry)
 	entries = append(entries, *entry)
 
 	return entries, nil
@@ -380,7 +375,7 @@ func prepareLayers(ctx context.Context, s *State, image *Image, result *ImageRes
 //
 // This type exists to avoid duplication between the handling of
 // symlink layers and store path layers.
-type layerWriter func(w io.Writer) error
+type layerWriter func(w io.Writer) (string, error)
 
 // byteCounter is a special io.Writer that counts all bytes written to
 // it and does nothing else.
@@ -408,8 +403,16 @@ func (b *byteCounter) Write(p []byte) (n int, err error) {
 //
 // The return value is the layer's SHA256 hash, which is used in the
 // image manifest.
-func uploadHashLayer(ctx context.Context, s *State, key string, lw layerWriter) (*manifest.Entry, error) {
+func uploadHashLayer(ctx context.Context, s *State, key string, mrating uint64, lw layerWriter) (*manifest.Entry, error) {
+	s.UploadMutex.Lock(key)
+	defer s.UploadMutex.Unlock(key)
+
+	if entry, cached := layerFromCache(ctx, s, key); cached {
+		return entry, nil
+	}
+
 	path := "staging/" + key
+	var tarhash string
 	sha256sum, size, err := s.Storage.Persist(ctx, path, manifest.LayerType, func(sw io.Writer) (string, int64, error) {
 		// Sets up a "multiwriter" that simultaneously runs both hash
 		// algorithms and uploads to the storage backend.
@@ -417,7 +420,8 @@ func uploadHashLayer(ctx context.Context, s *State, key string, lw layerWriter)
 		counter := &byteCounter{}
 		multi := io.MultiWriter(sw, shasum, counter)
 
-		err := lw(multi)
+		var err error
+		tarhash, err = lw(multi)
 		sha256sum := fmt.Sprintf("%x", shasum.Sum([]byte{}))
 
 		return sha256sum, counter.count, err
@@ -449,10 +453,14 @@ func uploadHashLayer(ctx context.Context, s *State, key string, lw layerWriter)
 	}).Info("created and persisted layer")
 
 	entry := manifest.Entry{
-		Digest: "sha256:" + sha256sum,
-		Size:   size,
+		Digest:      "sha256:" + sha256sum,
+		Size:        size,
+		TarHash:     tarhash,
+		MergeRating: mrating,
 	}
 
+	cacheLayer(ctx, s, key, entry)
+
 	return &entry, nil
 }
 
@@ -493,13 +501,13 @@ func BuildImage(ctx context.Context, s *State, image *Image) (*BuildResult, erro
 	}
 	m, c := manifest.Manifest(image.Arch.imageArch, layers, cmd)
 
-	lw := func(w io.Writer) error {
+	lw := func(w io.Writer) (string, error) {
 		r := bytes.NewReader(c.Config)
 		_, err := io.Copy(w, r)
-		return err
+		return "", err
 	}
 
-	if _, err = uploadHashLayer(ctx, s, c.SHA256, lw); err != nil {
+	if _, err = uploadHashLayer(ctx, s, c.SHA256, 0, lw); err != nil {
 		log.WithError(err).WithFields(log.Fields{
 			"image": image.Name,
 			"tag":   image.Tag,