Ensure adding a broken tar doesn't silently fail

Signed-off-by: Aidan Hobson Sayers <aidanhs@cantab.net>

archive/archive.go

@@ -77,6 +77,11 @@ var (
 	defaultArchiver   = &Archiver{Untar: Untar, UIDMaps: nil, GIDMaps: nil}
 )
 
+const (
+	// HeaderSize is the size in bytes of a tar header
+	HeaderSize = 512
+)
+
 const (
 	// Uncompressed represents the uncompressed.
 	Uncompressed Compression = iota
@@ -88,7 +93,8 @@ const (
 	Xz
 )
 
-// IsArchive checks if it is a archive by the header.
+// IsArchive checks for the magic bytes of a tar or any supported compression
+// algorithm.
 func IsArchive(header []byte) bool {
 	compression := DetectCompression(header)
 	if compression != Uncompressed {
@@ -99,6 +105,23 @@ func IsArchive(header []byte) bool {
 	return err == nil
 }
 
+// IsArchivePath checks if the (possibly compressed) file at the given path
+// starts with a tar file header.
+func IsArchivePath(path string) bool {
+	file, err := os.Open(path)
+	if err != nil {
+		return false
+	}
+	defer file.Close()
+	rdr, err := DecompressStream(file)
+	if err != nil {
+		return false
+	}
+	r := tar.NewReader(rdr)
+	_, err = r.Next()
+	return err == nil
+}
+
 // DetectCompression detects the compression algorithm of the source.
 func DetectCompression(source []byte) Compression {
 	for compression, m := range map[Compression][]byte{
@@ -800,10 +823,7 @@ func (archiver *Archiver) UntarPath(src, dst string) error {
 			GIDMaps: archiver.GIDMaps,
 		}
 	}
-	if err := archiver.Untar(archive, dst, options); err != nil {
+	return archiver.Untar(archive, dst, options)
-		return err
-	}
-	return nil
 }
 
 // UntarPath is a convenience function which looks for an archive

archive/archive_test.go

@@ -49,6 +49,45 @@ func TestIsArchive7zip(t *testing.T) {
 	}
 }
 
+func TestIsArchivePathDir(t *testing.T) {
+	cmd := exec.Command("/bin/sh", "-c", "mkdir -p /tmp/archivedir")
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Fail to create an archive file for test : %s.", output)
+	}
+	if IsArchivePath("/tmp/archivedir") {
+		t.Fatalf("Incorrectly recognised directory as an archive")
+	}
+}
+
+func TestIsArchivePathInvalidFile(t *testing.T) {
+	cmd := exec.Command("/bin/sh", "-c", "dd if=/dev/zero bs=1K count=1 of=/tmp/archive && gzip --stdout /tmp/archive > /tmp/archive.gz")
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Fail to create an archive file for test : %s.", output)
+	}
+	if IsArchivePath("/tmp/archive") {
+		t.Fatalf("Incorrectly recognised invalid tar path as archive")
+	}
+	if IsArchivePath("/tmp/archive.gz") {
+		t.Fatalf("Incorrectly recognised invalid compressed tar path as archive")
+	}
+}
+
+func TestIsArchivePathTar(t *testing.T) {
+	cmd := exec.Command("/bin/sh", "-c", "touch /tmp/archivedata && tar -cf /tmp/archive /tmp/archivedata && gzip --stdout /tmp/archive > /tmp/archive.gz")
+	output, err := cmd.CombinedOutput()
+	if err != nil {
+		t.Fatalf("Fail to create an archive file for test : %s.", output)
+	}
+	if !IsArchivePath("/tmp/archive") {
+		t.Fatalf("Did not recognise valid tar path as archive")
+	}
+	if !IsArchivePath("/tmp/archive.gz") {
+		t.Fatalf("Did not recognise valid compressed tar path as archive")
+	}
+}
+
 func TestDecompressStreamGzip(t *testing.T) {
 	cmd := exec.Command("/bin/sh", "-c", "touch /tmp/archive && gzip -f /tmp/archive")
 	output, err := cmd.CombinedOutput()