internal/pkg: return reader for files

This improves efficiency for cache hits.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/pkg.go

@@ -87,10 +87,11 @@ func (c *CureContext) Cure(a Artifact) (
 	return c.cache.Cure(a)
 }
 
-// LoadData tries to load [File] from [Cache], and if that fails, obtains it via
-// [File.Data] instead. Notably, it does not cure [File].
-func (c *CureContext) LoadData(f File) (data []byte, err error) {
-	return c.cache.loadData(f)
+// OpenFile tries to load [File] from [Cache], and if that fails, obtains it via
+// [File.Data] instead. Notably, it does not cure [File]. If err is nil, the
+// caller is responsible for closing the resulting [io.ReadCloser].
+func (c *CureContext) OpenFile(f File) (r io.ReadCloser, err error) {
+	return c.cache.openFile(f)
 }
 
 // An Artifact is a read-only reference to a piece of data that may be created
@@ -555,9 +556,8 @@ func (c *Cache) finaliseIdent(
 	close(done)
 }
 
-// loadData provides [CureContext.LoadData] for [Artifact.Cure].
-func (c *Cache) loadData(f File) (data []byte, err error) {
-	var r *os.File
+// openFile provides [CureContext.OpenFile] for [Artifact.Cure].
+func (c *Cache) openFile(f File) (r io.ReadCloser, err error) {
 	if kc, ok := f.(KnownChecksum); ok {
 		c.checksumMu.RLock()
 		r, err = os.Open(c.base.Append(
@@ -578,13 +578,11 @@ func (c *Cache) loadData(f File) (data []byte, err error) {
 		if !errors.Is(err, os.ErrNotExist) {
 			return
 		}
-		return f.Data()
-	}
-
-	data, err = io.ReadAll(r)
-	closeErr := r.Close()
-	if err == nil {
-		err = closeErr
+		var data []byte
+		if data, err = f.Data(); err != nil {
+			return
+		}
+		r = io.NopCloser(bytes.NewReader(data))
 	}
 	return
 }