internal/pkg: tar optional file

This allows tar to take a single-file directory Artifact as input.

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

internal/pkg/tar.go

@@ -28,15 +28,17 @@ const (
 // A tarArtifact is an [Artifact] unpacking a tarball backed by a [File].
 type tarArtifact struct {
 	// Caller-supplied backing tarball.
-	f File
+	f Artifact
 	// Compression on top of the tarball.
 	compression uint64
 }
 
-// NewTar returns a new [Artifact] backed by the supplied [File] and
-// compression method.
-func NewTar(f File, compression uint64) Artifact {
-	return &tarArtifact{f: f, compression: compression}
+// NewTar returns a new [Artifact] backed by the supplied [Artifact] and
+// compression method. If f implements [File], its data might be used directly,
+// eliminating the roundtrip to vfs. If f is a directory, it must contain a
+// single regular file.
+func NewTar(a Artifact, compression uint64) Artifact {
+	return &tarArtifact{a, compression}
 }
 
 // NewHTTPGetTar is abbreviation for NewHTTPGet passed to NewTar.
@@ -76,26 +78,46 @@ func (a *tarArtifact) Cure(c *CureContext) (err error) {
 	temp := c.GetTempDir()
 	var tr io.ReadCloser
 
-	{
-
-		if tr, err = c.OpenFile(a.f); err != nil {
+	if file, ok := a.f.(File); ok {
+		if tr, err = c.OpenFile(file); err != nil {
+			return
+		}
+	} else {
+		var pathname *check.Absolute
+		if pathname, _, err = c.Cure(a.f); err != nil {
+			return
+		}
+
+		var entries []os.DirEntry
+		if entries, err = os.ReadDir(pathname.String()); err != nil {
+			return
+		}
+
+		if len(entries) != 1 || !entries[0].Type().IsRegular() {
+			return errors.New(
+				"input directory does not contain a single regular file",
+			)
+		} else {
+			pathname = pathname.Append(entries[0].Name())
+		}
+
+		if tr, err = os.Open(pathname.String()); err != nil {
 			return
 		}
-		defer func(f io.ReadCloser) {
-			closeErr := f.Close()
-			if err == nil {
-				err = closeErr
-			}
-		}(tr)
-		tr = io.NopCloser(tr)
 	}
 
-	defer func() {
+	defer func(f io.ReadCloser) {
 		closeErr := tr.Close()
 		if err == nil {
 			err = closeErr
 		}
-	}()
+
+		closeErr = f.Close()
+		if err == nil {
+			err = closeErr
+		}
+	}(tr)
+	tr = io.NopCloser(tr)
 
 	switch a.compression {
 	case TarUncompressed: