pkg/cache: use a shared buffer to limit allocations

Previously, new buffers were allocated on each file we read in, which
was unnecessary and wasteful.

Signed-off-by: Steve Kuznetsov <skuznets@redhat.com>

pkg/cache/json.go

@@ -165,13 +165,14 @@ func (q *JSON) existingDigest() (string, error) {
 }
 
 func (q *JSON) computeDigest(fbcFsys fs.FS) (string, error) {
+	buf := make([]byte, 32*1024)
 	computedHasher := fnv.New64a()
-	if err := fsToTar(computedHasher, fbcFsys); err != nil {
+	if err := fsToTar(computedHasher, fbcFsys, buf); err != nil {
 		return "", err
 	}
 
 	if cacheFS, err := fs.Sub(os.DirFS(q.baseDir), jsonDir); err == nil {
-		if err := fsToTar(computedHasher, cacheFS); err != nil && !errors.Is(err, os.ErrNotExist) {
+		if err := fsToTar(computedHasher, cacheFS, buf); err != nil && !errors.Is(err, os.ErrNotExist) {
 			return "", fmt.Errorf("compute hash: %v", err)
 		}
 	}

pkg/cache/tar.go

@@ -13,7 +13,10 @@ import (
 // This function unsets user and group information in the tar archive so that readers
 // of archives produced by this function do not need to account for differences in
 // permissions between source and destination filesystems.
-func fsToTar(w io.Writer, fsys fs.FS) error {
+func fsToTar(w io.Writer, fsys fs.FS, buf []byte) error {
+	if buf == nil || len(buf) == 0 {
+		buf = make([]byte, 32*1024)
+	}
 	tw := tar.NewWriter(w)
 	if err := fs.WalkDir(fsys, ".", func(path string, d fs.DirEntry, err error) error {
 		if err != nil {
@@ -52,7 +55,7 @@ func fsToTar(w io.Writer, fsys fs.FS) error {
 			return fmt.Errorf("open file %q: %v", path, err)
 		}
 		defer f.Close()
-		if _, err := io.Copy(tw, f); err != nil {
+		if _, err := io.CopyBuffer(tw, f, buf); err != nil {
 			return fmt.Errorf("write tar data for %q: %v", path, err)
 		}
 		return nil