fix(disk.Store): make File.Size() work after Close() is called

While Kraken code itself never calls Size() after Close() is called,
we use a library to expose the Docker Registry V2 API and we sometimes
pass disk.File to that library, after which it may call Size() after
Close(). This breaks the whole image push flow, as PATCH requests when
uploading a blob need to know how far they've gotten with pushing, so
they call Size(), after which they pass the resulting offset to the next
PATCH request, which uses it to know which bytes to send to the server.
Currently, Size() returns 0 if Close() is called beforehand, even if 0
is not the right offset, causing errors for the push flow.

Author: Anton-Kalpakchiev

lib/store/disk/file.go

@@ -3,20 +3,28 @@ package disk
 import (
 	"os"
 
+	"github.com/docker/distribution/registry/storage/driver"
 	storelib "github.com/uber/kraken/lib/store"
+	"github.com/uber/kraken/utils/log"
 )
 
-func newFile(f *os.File) *File {
+func newFile(f *os.File, path string) *File {
 	return &File{
-		fd: f,
+		fd:   f,
+		path: path,
 	}
 }
 
-var _ storelib.FileReadWriter = &File{}
+var (
+	_ storelib.FileReadWriter = &File{}
+	_ driver.FileWriter       = &File{}
+)
 
 // File represends an open file descriptor to a blob in [Store].
 type File struct {
 	fd *os.File
+	// May become stale after a while, but that's ok, as clients don't call Size() after MarkComplete/RenameKey.
+	path string
 }
 
 func (f *File) Read(p []byte) (n int, err error)               { return f.fd.Read(p) }
@@ -28,8 +36,13 @@ func (f *File) Close() error                                   { return f.fd.Clo
 
 // Size returns the number of bytes the file contains.
 func (f *File) Size() int64 {
-	info, err := f.fd.Stat()
+	// Size is sometimes called after Close() is called, meaning that fd.Stat() will return ErrClosed.
+	info, err := os.Stat(f.path)
 	if err != nil {
+		log.Default().With(
+			"path", f.path,
+			"error", err,
+		).Error("disk.File Stat failed as os.Stat failed")
 		return 0
 	}
 	return info.Size()

lib/store/disk/store.go

@@ -120,7 +120,7 @@ func (s *store) Open(key string, scope storelib.BlobScope) (*File, error) {
 	if err != nil {
 		return nil, fmt.Errorf("open: %w", err)
 	}
-	return newFile(f), nil
+	return newFile(f, path), nil
 }
 
 func (s *store) Has(key string, scope storelib.BlobScope) (inStore bool, inScope bool) {
@@ -195,7 +195,7 @@ func (s *store) Create(key string, size uint64) (*File, error) {
 	}
 
 	s.emitUsageMetrics()
-	return newFile(f), nil
+	return newFile(f, blobPath), nil
 }
 
 func (s *store) persistBlobSize(key string, size uint64) error {