Ensure links and targets are in the same tree

Without this we see weirdly vague errors like:

link etc/terminfo/v/vt200 /var/cache/vinyl/vin/packages/ncurses-terminfo-base/6.3.0/etc/terminfo/v/vt220: no such file or directory

which can be avoided by prefixing hard link names with the dest directory. Luckily for us, our cpio installation strategy for alpine APKs magically repoints hardlinks (at least; if I delete the cached package source, files are still readable somehow).

os.go

@@ -209,13 +209,22 @@ func decompressLoop(tr *tar.Reader, dest string) (err error) {
 			f.Close()
 
 		case tar.TypeLink:
-			err = os.Link(header.Linkname, target)
+			// remove target if it exists.
+			//
+			// ignoring the error is fine here; if there's an error
+			// we'll see it when we try to link anyway /shrug
+			os.Remove(target)
+
+			err = os.Link(filepath.Join(dest, header.Linkname), target)
 			if err != nil {
 				return
 			}
 
 		case tar.TypeSymlink:
-			err = os.Symlink(header.Linkname, target)
+			// see comment for the tar.TypeLink case above;
+			os.Remove(target)
+
+			err = os.Symlink(filepath.Join(dest, header.Linkname), target)
 			if err != nil {
 				return
 			}

os_test.go

@@ -51,19 +51,20 @@ func TestUntar(t *testing.T) {
 }
 
 func TestUntar_WithLinks(t *testing.T) {
-	dir := "testdata"
-
-	err := untar("testdata/complex.tar.bz2", dir)
+	dir, err := ioutil.TempDir("", "")
 	if err != nil {
 		t.Fatalf("unexpected error: %+v", err)
 	}
 
-	defer os.RemoveAll("testdata/testdata")
+	err = untar("testdata/complex.tar.bz2", dir)
+	if err != nil {
+		t.Fatalf("unexpected error: %+v", err)
+	}
 
 	t.Run("checksum", func(t *testing.T) {
 		expect := "9fbefb949709fc7086ab4be43544d08406f0ededa0733f6683424e774a6cb799"
 
-		sum, err := checksum(filepath.Join(dir, "testdata", "raw"))
+		sum, err := checksum(filepath.Join(dir, "raw"))
 		if err != nil {
 			t.Fatalf("unexpected error: %+v", err)
 		}
@@ -74,14 +75,14 @@ func TestUntar_WithLinks(t *testing.T) {
 	})
 
 	t.Run("hard links", func(t *testing.T) {
-		_, err := os.Stat(filepath.Join(dir, "testdata", "hardlink"))
+		_, err := os.Stat(filepath.Join(dir, "hardlink"))
 		if err != nil {
 			t.Fatalf("unexpected error: %+v", err)
 		}
 	})
 
 	t.Run("symlinks", func(t *testing.T) {
-		_, err := os.Stat(filepath.Join(dir, "testdata", "symlink"))
+		_, err := os.Stat(filepath.Join(dir, "symlink"))
 		if err != nil {
 			t.Fatalf("unexpected error: %+v", err)
 		}