internal/pkg: implement tar artifact

This is useful for unpacking tarballs downloaded from the internet.

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

internal/pkg/pkg_test.go

@@ -1,6 +1,7 @@
 package pkg_test
 
 import (
+	"archive/tar"
 	"bytes"
 	"crypto/sha512"
 	"encoding/binary"
@@ -40,7 +41,7 @@ func TestIdent(t *testing.T) {
 		want   pkg.ID
 	}{
 		{"tar", pkg.KindTar, []byte{
-			1, 0, 0, 0, 0, 0, 0, 0,
+			pkg.TarGzip, 0, 0, 0, 0, 0, 0, 0,
 		}, []pkg.Artifact{
 			stubArtifact{pkg.KindHTTP, pkg.ID{}},
 		}, pkg.MustDecode("HnySzeLQvSBZuTUcvfmLEX_OmH4yJWWH788NxuLuv7kVn8_uPM6Ks4rqFWM2NZJY")},
@@ -74,6 +75,7 @@ func checkWithCache(t *testing.T, testCases []cacheTestCase) {
 
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {
+			t.Helper()
 			t.Parallel()
 
 			base := check.MustAbs(t.TempDir())
@@ -307,7 +309,7 @@ func TestCache(t *testing.T) {
 
 		{"directory", nil, func(t *testing.T, base *check.Absolute, c *pkg.Cache) {
 			id := pkg.KindTar.Ident(
-				binary.LittleEndian.AppendUint64(nil, 1),
+				binary.LittleEndian.AppendUint64(nil, pkg.TarGzip),
 				stubArtifact{pkg.KindHTTP, testdataChecksum},
 			)
 			makeSample := func(work *check.Absolute) error {
@@ -371,7 +373,7 @@ func TestCache(t *testing.T) {
 
 			// check exist
 			id0 := pkg.KindTar.Ident(
-				binary.LittleEndian.AppendUint64(nil, 1),
+				binary.LittleEndian.AppendUint64(nil, pkg.TarGzip),
 				stubArtifact{pkg.KindHTTP, pkg.ID{}},
 			)
 			wantPathname0 := base.Append(
@@ -478,6 +480,10 @@ func TestErrors(t *testing.T) {
 		{"ResponseStatusError", pkg.ResponseStatusError(
 			http.StatusNotAcceptable,
 		), "the requested URL returned non-OK status: Not Acceptable"},
+
+		{"DisallowedTypeflagError", pkg.DisallowedTypeflagError(
+			tar.TypeChar,
+		), "disallowed typeflag '3'"},
 	}
 	for _, tc := range testCases {
 		t.Run(tc.name, func(t *testing.T) {