From f91d4498df94314ae797908bee77b0cc8e437e8e Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Tue, 30 Jun 2026 09:15:13 +0200
Subject: [PATCH 01/15] feat: extract bins

---
 internal/archive/archive_test.go |  2 ++
 internal/slicer/slicer.go        |  2 ++
 internal/tarball/extract.go      | 13 ++++++++---
 internal/tarball/extract_test.go | 17 +++++++++++++++
 internal/tarball/xz.go           | 15 +++++++++++++
 internal/testutil/pkgdata.go     | 37 ++++++++++++++++++++++++++++++++
 6 files changed, 83 insertions(+), 3 deletions(-)
 create mode 100644 internal/tarball/xz.go

diff --git a/internal/archive/archive_test.go b/internal/archive/archive_test.go
index 90e3f0047..fcabc2a25 100644
--- a/internal/archive/archive_test.go
+++ b/internal/archive/archive_test.go
@@ -17,6 +17,7 @@ import (
 	"strings"
 
 	"github.com/canonical/chisel/internal/archive"
+	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/archive/testarchive"
 	"github.com/canonical/chisel/internal/tarball"
 	"github.com/canonical/chisel/internal/testutil"
@@ -986,6 +987,7 @@ func (s *S) testOpenArchiveArch(c *C, test realArchiveTest, arch string) {
 	err = tarball.Extract(pkg, &tarball.ExtractOptions{
 		Package:   test.pkg,
 		TargetDir: extractDir,
+		OpenData:  deb.DataReader,
 		Extract: map[string][]tarball.ExtractInfo{
 			fmt.Sprintf("/usr/share/doc/%s/copyright", test.pkg): {
 				{Path: "/copyright"},
diff --git a/internal/slicer/slicer.go b/internal/slicer/slicer.go
index 6f4783b88..00ed3ee37 100644
--- a/internal/slicer/slicer.go
+++ b/internal/slicer/slicer.go
@@ -16,6 +16,7 @@ import (
 	"github.com/klauspost/compress/zstd"
 
 	"github.com/canonical/chisel/internal/archive"
+	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/fsutil"
 	"github.com/canonical/chisel/internal/manifestutil"
 	"github.com/canonical/chisel/internal/scripts"
@@ -243,6 +244,7 @@ func Run(options *RunOptions) error {
 			Package:   slice.Package,
 			Extract:   extract[slice.Package],
 			TargetDir: targetDir,
+			OpenData:  deb.DataReader,
 			Create:    create,
 		})
 		reader.Close()
diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index 40db2cbac..ce1fda5b2 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -12,7 +12,6 @@ import (
 	"strings"
 	"syscall"
 
-	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/fsutil"
 	"github.com/canonical/chisel/internal/strdist"
 )
@@ -21,6 +20,11 @@ type ExtractOptions struct {
 	Package   string
 	TargetDir string
 	Extract   map[string][]ExtractInfo
+	// OpenData opens the uncompressed tar data stream of the package. It
+	// abstracts over the package format (e.g. a deb archive opener or a plain
+	// tarball opener), allowing Extract to operate on any package whose data
+	// payload is a tar stream.
+	OpenData func(io.ReadSeeker) (io.ReadCloser, error)
 	// Create can optionally be set to control the creation of extracted entries.
 	// extractInfos is set to the matching entries in Extract, and is nil in cases where
 	// the created entry is implicit and unlisted (for example, parent directories).
@@ -35,6 +39,9 @@ type ExtractInfo struct {
 }
 
 func getValidOptions(options *ExtractOptions) (*ExtractOptions, error) {
+	if options.OpenData == nil {
+		return nil, fmt.Errorf("internal error: ExtractOptions.OpenData is unset")
+	}
 	for extractPath, extractInfos := range options.Extract {
 		isGlob := strings.ContainsAny(extractPath, "*?")
 		if isGlob {
@@ -83,7 +90,7 @@ func Extract(pkgReader io.ReadSeeker, options *ExtractOptions) (err error) {
 }
 
 func extractData(pkgReader io.ReadSeeker, options *ExtractOptions) error {
-	dataReader, err := deb.DataReader(pkgReader)
+	dataReader, err := options.OpenData(pkgReader)
 	if err != nil {
 		return err
 	}
@@ -300,7 +307,7 @@ type extractHardLinkOptions struct {
 // extractHardLinks iterates through the tarball a second time to extract the
 // hard links that were not extracted in the first pass.
 func extractHardLinks(pkgReader io.ReadSeeker, opts *extractHardLinkOptions) error {
-	dataReader, err := deb.DataReader(pkgReader)
+	dataReader, err := opts.OpenData(pkgReader)
 	if err != nil {
 		return err
 	}
diff --git a/internal/tarball/extract_test.go b/internal/tarball/extract_test.go
index fd5eb6147..f855d9bc4 100644
--- a/internal/tarball/extract_test.go
+++ b/internal/tarball/extract_test.go
@@ -10,6 +10,7 @@ import (
 
 	. "gopkg.in/check.v1"
 
+	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/fsutil"
 	"github.com/canonical/chisel/internal/tarball"
 	"github.com/canonical/chisel/internal/testutil"
@@ -496,6 +497,8 @@ func (s *S) TestExtract(c *C) {
 		options := test.options
 		options.Package = "test-package"
 		options.TargetDir = dir
+		// The test fixtures are .deb archives, so use the deb data opener.
+		options.OpenData = deb.DataReader
 		createdPaths := make(map[string]bool)
 		options.Create = func(_ []tarball.ExtractInfo, o *fsutil.CreateOptions) error {
 			relPath := filepath.Clean("/" + strings.TrimPrefix(o.Path, dir))
@@ -605,6 +608,8 @@ func (s *S) TestExtractCreateCallback(c *C) {
 		options := test.options
 		options.Package = "test-package"
 		options.TargetDir = dir
+		// The test fixtures are .deb archives, so use the deb data opener.
+		options.OpenData = deb.DataReader
 		createExtractInfos := map[string][]tarball.ExtractInfo{}
 		options.Create = func(extractInfos []tarball.ExtractInfo, o *fsutil.CreateOptions) error {
 			if extractInfos == nil {
@@ -628,3 +633,15 @@ func (s *S) TestExtractCreateCallback(c *C) {
 		c.Assert(createExtractInfos, DeepEquals, test.calls)
 	}
 }
+
+func (s *S) TestExtractMissingOpenData(c *C) {
+	options := tarball.ExtractOptions{
+		Package:   "test-package",
+		TargetDir: c.MkDir(),
+		Extract: map[string][]tarball.ExtractInfo{
+			"/dir/file": {{Path: "/dir/file"}},
+		},
+	}
+	err := tarball.Extract(bytes.NewReader(testutil.PackageData["test-package"]), &options)
+	c.Assert(err, ErrorMatches, `cannot extract from package "test-package": internal error: ExtractOptions.OpenData is unset`)
+}
diff --git a/internal/tarball/xz.go b/internal/tarball/xz.go
new file mode 100644
index 000000000..700215a09
--- /dev/null
+++ b/internal/tarball/xz.go
@@ -0,0 +1,15 @@
+package tarball
+
+import (
+	"io"
+
+	"github.com/ulikunitz/xz"
+)
+
+func XZDataReader(pkgReader io.ReadSeeker) (io.ReadCloser, error) {
+	xzReader, err := xz.NewReader(pkgReader)
+	if err != nil {
+		return nil, err
+	}
+	return io.NopCloser(xzReader), nil
+}
diff --git a/internal/testutil/pkgdata.go b/internal/testutil/pkgdata.go
index f901873d3..8fb8fb8b2 100644
--- a/internal/testutil/pkgdata.go
+++ b/internal/testutil/pkgdata.go
@@ -8,6 +8,7 @@ import (
 
 	"github.com/blakesmith/ar"
 	"github.com/klauspost/compress/zstd"
+	"github.com/ulikunitz/xz"
 )
 
 var PackageData = map[string][]byte{}
@@ -160,6 +161,42 @@ func MustMakeDeb(entries []TarEntry) []byte {
 	return data
 }
 
+// compressBytesXz compresses the input using XZ, the compression format used by
+// store (bin) packages.
+func compressBytesXz(input []byte) ([]byte, error) {
+	var buf bytes.Buffer
+	writer, err := xz.NewWriter(&buf)
+	if err != nil {
+		return nil, err
+	}
+	if _, err = writer.Write(input); err != nil {
+		return nil, err
+	}
+	if err = writer.Close(); err != nil {
+		return nil, err
+	}
+	return buf.Bytes(), nil
+}
+
+// MakeBin builds a store (bin) package from the given tar entries: an
+// XZ-compressed plain tarball. Unlike MakeDeb, there is no ar container.
+func MakeBin(entries []TarEntry) ([]byte, error) {
+	tarData, err := makeTar(entries)
+	if err != nil {
+		return nil, err
+	}
+	return compressBytesXz(tarData)
+}
+
+// MustMakeBin is the panicking variant of MakeBin.
+func MustMakeBin(entries []TarEntry) []byte {
+	data, err := MakeBin(entries)
+	if err != nil {
+		panic(err)
+	}
+	return data
+}
+
 // Reg is a shortcut for creating a regular file TarEntry structure (with
 // tar.Typeflag set tar.TypeReg). Reg stands for "REGular file".
 func Reg(mode int64, path, content string) TarEntry {

From 166344644b84dbb1058028913eac09dbda4c45a4 Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Wed, 29 Jul 2026 11:54:44 +0200
Subject: [PATCH 02/15] refactor: rework approach

---
 .../cmd_debug_check_release_archives.go       |  2 +-
 internal/archive/archive_test.go              |  5 +-
 internal/deb/extract.go                       |  7 ++-
 internal/slicer/slicer.go                     |  3 +-
 internal/tarball/extract.go                   | 58 ++++++++++++++-----
 internal/tarball/extract_test.go              | 16 +++--
 internal/tarball/xz.go                        | 15 -----
 7 files changed, 57 insertions(+), 49 deletions(-)
 delete mode 100644 internal/tarball/xz.go

diff --git a/cmd/chisel/cmd_debug_check_release_archives.go b/cmd/chisel/cmd_debug_check_release_archives.go
index c9c6270e4..29a8189a8 100644
--- a/cmd/chisel/cmd_debug_check_release_archives.go
+++ b/cmd/chisel/cmd_debug_check_release_archives.go
@@ -150,7 +150,7 @@ func computePathObservations(release *setup.Release, archives map[string]archive
 			if err != nil {
 				return nil, err
 			}
-			dataReader, err := deb.DataReader(pkgReader)
+			dataReader, err := deb.OpenTar(pkgReader)
 			if err != nil {
 				return nil, err
 			}
diff --git a/internal/archive/archive_test.go b/internal/archive/archive_test.go
index fcabc2a25..efde6658c 100644
--- a/internal/archive/archive_test.go
+++ b/internal/archive/archive_test.go
@@ -17,8 +17,8 @@ import (
 	"strings"
 
 	"github.com/canonical/chisel/internal/archive"
-	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/archive/testarchive"
+	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/tarball"
 	"github.com/canonical/chisel/internal/testutil"
 )
@@ -984,10 +984,9 @@ func (s *S) testOpenArchiveArch(c *C, test realArchiveTest, arch string) {
 	c.Assert(info.Name, DeepEquals, test.pkg)
 	c.Assert(info.Arch, DeepEquals, arch)
 
-	err = tarball.Extract(pkg, &tarball.ExtractOptions{
+	err = tarball.Extract(pkg, deb.OpenTar, &tarball.ExtractOptions{
 		Package:   test.pkg,
 		TargetDir: extractDir,
-		OpenData:  deb.DataReader,
 		Extract: map[string][]tarball.ExtractInfo{
 			fmt.Sprintf("/usr/share/doc/%s/copyright", test.pkg): {
 				{Path: "/copyright"},
diff --git a/internal/deb/extract.go b/internal/deb/extract.go
index bf543c2d8..49c0ba13b 100644
--- a/internal/deb/extract.go
+++ b/internal/deb/extract.go
@@ -10,9 +10,10 @@ import (
 	"github.com/ulikunitz/xz"
 )
 
-// DataReader takes a Reader for the ar file belonging to a Debian package and
-// returns a Reader to the inner tarball.
-func DataReader(pkgReader io.ReadSeeker) (io.ReadCloser, error) {
+// OpenTar takes a Reader for the ar file belonging to a Debian package and
+// returns a Reader to the uncompressed inner tarball. It implements
+// tarball.OpenTarFunc.
+func OpenTar(pkgReader io.ReadSeeker) (io.ReadCloser, error) {
 	arReader := ar.NewReader(pkgReader)
 	var dataReader io.ReadCloser
 	for dataReader == nil {
diff --git a/internal/slicer/slicer.go b/internal/slicer/slicer.go
index 00ed3ee37..8fcc1c0e2 100644
--- a/internal/slicer/slicer.go
+++ b/internal/slicer/slicer.go
@@ -240,11 +240,10 @@ func Run(options *RunOptions) error {
 		if reader == nil {
 			continue
 		}
-		err := tarball.Extract(reader, &tarball.ExtractOptions{
+		err := tarball.Extract(reader, deb.OpenTar, &tarball.ExtractOptions{
 			Package:   slice.Package,
 			Extract:   extract[slice.Package],
 			TargetDir: targetDir,
-			OpenData:  deb.DataReader,
 			Create:    create,
 		})
 		reader.Close()
diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index ce1fda5b2..bc78534ac 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -12,19 +12,41 @@ import (
 	"strings"
 	"syscall"
 
+	"github.com/ulikunitz/xz"
+
 	"github.com/canonical/chisel/internal/fsutil"
 	"github.com/canonical/chisel/internal/strdist"
 )
 
+// OpenTarFunc opens the uncompressed tar stream carried by a package, hiding
+// the package format from Extract. An implementation may unwrap a container
+// before decompressing (see deb.OpenTar) or decompress the package itself (see
+// OpenXZ).
+//
+// Implementations must satisfy the following contract:
+//
+//   - They must read pkgReader from its current offset, which is always the
+//     start of the package.
+//   - They must be stateless. Extract may call the same function more than once
+//     for the same package, rewinding pkgReader to the start beforehand, and
+//     each call must yield the complete tar stream again.
+//   - The caller closes the returned reader.
+type OpenTarFunc func(pkgReader io.ReadSeeker) (io.ReadCloser, error)
+
+// OpenXZ opens a package which is a plain XZ-compressed tarball, such as a
+// store (bin) package. It implements OpenTarFunc.
+func OpenXZ(pkgReader io.ReadSeeker) (io.ReadCloser, error) {
+	xzReader, err := xz.NewReader(pkgReader)
+	if err != nil {
+		return nil, err
+	}
+	return io.NopCloser(xzReader), nil
+}
+
 type ExtractOptions struct {
 	Package   string
 	TargetDir string
 	Extract   map[string][]ExtractInfo
-	// OpenData opens the uncompressed tar data stream of the package. It
-	// abstracts over the package format (e.g. a deb archive opener or a plain
-	// tarball opener), allowing Extract to operate on any package whose data
-	// payload is a tar stream.
-	OpenData func(io.ReadSeeker) (io.ReadCloser, error)
 	// Create can optionally be set to control the creation of extracted entries.
 	// extractInfos is set to the matching entries in Extract, and is nil in cases where
 	// the created entry is implicit and unlisted (for example, parent directories).
@@ -39,9 +61,6 @@ type ExtractInfo struct {
 }
 
 func getValidOptions(options *ExtractOptions) (*ExtractOptions, error) {
-	if options.OpenData == nil {
-		return nil, fmt.Errorf("internal error: ExtractOptions.OpenData is unset")
-	}
 	for extractPath, extractInfos := range options.Extract {
 		isGlob := strings.ContainsAny(extractPath, "*?")
 		if isGlob {
@@ -65,7 +84,10 @@ func getValidOptions(options *ExtractOptions) (*ExtractOptions, error) {
 	return options, nil
 }
 
-func Extract(pkgReader io.ReadSeeker, options *ExtractOptions) (err error) {
+// Extract extracts from pkgReader the entries listed in options.Extract.
+// openTar opens the tar stream carried by the package and must not be nil; see
+// OpenTarFunc for the contract it must satisfy.
+func Extract(pkgReader io.ReadSeeker, openTar OpenTarFunc, options *ExtractOptions) (err error) {
 	defer func() {
 		if err != nil {
 			err = fmt.Errorf("cannot extract from package %q: %w", options.Package, err)
@@ -74,6 +96,10 @@ func Extract(pkgReader io.ReadSeeker, options *ExtractOptions) (err error) {
 
 	logf("Extracting files from package %q...", options.Package)
 
+	if openTar == nil {
+		return fmt.Errorf("internal error: no tar opener provided")
+	}
+
 	validOpts, err := getValidOptions(options)
 	if err != nil {
 		return err
@@ -86,11 +112,11 @@ func Extract(pkgReader io.ReadSeeker, options *ExtractOptions) (err error) {
 		return err
 	}
 
-	return extractData(pkgReader, validOpts)
+	return extractData(pkgReader, openTar, validOpts)
 }
 
-func extractData(pkgReader io.ReadSeeker, options *ExtractOptions) error {
-	dataReader, err := options.OpenData(pkgReader)
+func extractData(pkgReader io.ReadSeeker, openTar OpenTarFunc, options *ExtractOptions) error {
+	dataReader, err := openTar(pkgReader)
 	if err != nil {
 		return err
 	}
@@ -172,7 +198,7 @@ func extractData(pkgReader io.ReadSeeker, options *ExtractOptions) error {
 		}
 
 		var contentCache []byte
-		var contentIsCached = len(targetPaths) > 1 && !sourceIsDir
+		contentIsCached := len(targetPaths) > 1 && !sourceIsDir
 		if contentIsCached {
 			// Read and cache the content so it may be reused.
 			// As an alternative, to avoid having an entire file in
@@ -272,7 +298,7 @@ func extractData(pkgReader io.ReadSeeker, options *ExtractOptions) error {
 		if err != nil {
 			return err
 		}
-		err = extractHardLinks(pkgReader, extractHardLinkOptions)
+		err = extractHardLinks(pkgReader, openTar, extractHardLinkOptions)
 		if err != nil {
 			return err
 		}
@@ -306,8 +332,8 @@ type extractHardLinkOptions struct {
 
 // extractHardLinks iterates through the tarball a second time to extract the
 // hard links that were not extracted in the first pass.
-func extractHardLinks(pkgReader io.ReadSeeker, opts *extractHardLinkOptions) error {
-	dataReader, err := opts.OpenData(pkgReader)
+func extractHardLinks(pkgReader io.ReadSeeker, openTar OpenTarFunc, opts *extractHardLinkOptions) error {
+	dataReader, err := openTar(pkgReader)
 	if err != nil {
 		return err
 	}
diff --git a/internal/tarball/extract_test.go b/internal/tarball/extract_test.go
index f855d9bc4..a2246bbec 100644
--- a/internal/tarball/extract_test.go
+++ b/internal/tarball/extract_test.go
@@ -497,8 +497,6 @@ func (s *S) TestExtract(c *C) {
 		options := test.options
 		options.Package = "test-package"
 		options.TargetDir = dir
-		// The test fixtures are .deb archives, so use the deb data opener.
-		options.OpenData = deb.DataReader
 		createdPaths := make(map[string]bool)
 		options.Create = func(_ []tarball.ExtractInfo, o *fsutil.CreateOptions) error {
 			relPath := filepath.Clean("/" + strings.TrimPrefix(o.Path, dir))
@@ -514,7 +512,8 @@ func (s *S) TestExtract(c *C) {
 			test.hackopt(c, &options)
 		}
 
-		err := tarball.Extract(bytes.NewReader(test.pkgdata), &options)
+		// The test fixtures are .deb archives, so use the deb tar opener.
+		err := tarball.Extract(bytes.NewReader(test.pkgdata), deb.OpenTar, &options)
 		if test.error != "" {
 			c.Assert(err, ErrorMatches, test.error)
 			continue
@@ -608,8 +607,6 @@ func (s *S) TestExtractCreateCallback(c *C) {
 		options := test.options
 		options.Package = "test-package"
 		options.TargetDir = dir
-		// The test fixtures are .deb archives, so use the deb data opener.
-		options.OpenData = deb.DataReader
 		createExtractInfos := map[string][]tarball.ExtractInfo{}
 		options.Create = func(extractInfos []tarball.ExtractInfo, o *fsutil.CreateOptions) error {
 			if extractInfos == nil {
@@ -627,14 +624,15 @@ func (s *S) TestExtractCreateCallback(c *C) {
 			return nil
 		}
 
-		err := tarball.Extract(bytes.NewReader(test.pkgdata), &options)
+		// The test fixtures are .deb archives, so use the deb tar opener.
+		err := tarball.Extract(bytes.NewReader(test.pkgdata), deb.OpenTar, &options)
 		c.Assert(err, IsNil)
 
 		c.Assert(createExtractInfos, DeepEquals, test.calls)
 	}
 }
 
-func (s *S) TestExtractMissingOpenData(c *C) {
+func (s *S) TestExtractMissingOpenTar(c *C) {
 	options := tarball.ExtractOptions{
 		Package:   "test-package",
 		TargetDir: c.MkDir(),
@@ -642,6 +640,6 @@ func (s *S) TestExtractMissingOpenData(c *C) {
 			"/dir/file": {{Path: "/dir/file"}},
 		},
 	}
-	err := tarball.Extract(bytes.NewReader(testutil.PackageData["test-package"]), &options)
-	c.Assert(err, ErrorMatches, `cannot extract from package "test-package": internal error: ExtractOptions.OpenData is unset`)
+	err := tarball.Extract(bytes.NewReader(testutil.PackageData["test-package"]), nil, &options)
+	c.Assert(err, ErrorMatches, `cannot extract from package "test-package": internal error: no tar opener provided`)
 }
diff --git a/internal/tarball/xz.go b/internal/tarball/xz.go
deleted file mode 100644
index 700215a09..000000000
--- a/internal/tarball/xz.go
+++ /dev/null
@@ -1,15 +0,0 @@
-package tarball
-
-import (
-	"io"
-
-	"github.com/ulikunitz/xz"
-)
-
-func XZDataReader(pkgReader io.ReadSeeker) (io.ReadCloser, error) {
-	xzReader, err := xz.NewReader(pkgReader)
-	if err != nil {
-		return nil, err
-	}
-	return io.NopCloser(xzReader), nil
-}

From 3435cee29c65c8eda6477bc253dd4f2a1845d625 Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Wed, 29 Jul 2026 13:23:15 +0200
Subject: [PATCH 03/15] refactor: refining

---
 internal/deb/extract.go      |  5 ++---
 internal/tarball/extract.go  | 20 ++++++-------------
 internal/testutil/pkgdata.go | 37 ------------------------------------
 3 files changed, 8 insertions(+), 54 deletions(-)

diff --git a/internal/deb/extract.go b/internal/deb/extract.go
index 49c0ba13b..d25f1f883 100644
--- a/internal/deb/extract.go
+++ b/internal/deb/extract.go
@@ -11,9 +11,8 @@ import (
 )
 
 // OpenTar takes a Reader for the ar file belonging to a Debian package and
-// returns a Reader to the uncompressed inner tarball. It implements
-// tarball.OpenTarFunc.
-func OpenTar(pkgReader io.ReadSeeker) (io.ReadCloser, error) {
+// returns a Reader to the uncompressed inner tarball.
+func OpenTar(pkgReader io.Reader) (io.ReadCloser, error) {
 	arReader := ar.NewReader(pkgReader)
 	var dataReader io.ReadCloser
 	for dataReader == nil {
diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index bc78534ac..26c8ec6a9 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -23,19 +23,14 @@ import (
 // before decompressing (see deb.OpenTar) or decompress the package itself (see
 // OpenXZ).
 //
-// Implementations must satisfy the following contract:
-//
-//   - They must read pkgReader from its current offset, which is always the
-//     start of the package.
-//   - They must be stateless. Extract may call the same function more than once
-//     for the same package, rewinding pkgReader to the start beforehand, and
-//     each call must yield the complete tar stream again.
-//   - The caller closes the returned reader.
-type OpenTarFunc func(pkgReader io.ReadSeeker) (io.ReadCloser, error)
+// Extract reads pkgReader from the start of the package, closes the returned
+// reader, and may open the same package more than once, rewinding pkgReader
+// beforehand.
+type OpenTarFunc func(pkgReader io.Reader) (io.ReadCloser, error)
 
 // OpenXZ opens a package which is a plain XZ-compressed tarball, such as a
-// store (bin) package. It implements OpenTarFunc.
-func OpenXZ(pkgReader io.ReadSeeker) (io.ReadCloser, error) {
+// store (bin) package.
+func OpenXZ(pkgReader io.Reader) (io.ReadCloser, error) {
 	xzReader, err := xz.NewReader(pkgReader)
 	if err != nil {
 		return nil, err
@@ -84,9 +79,6 @@ func getValidOptions(options *ExtractOptions) (*ExtractOptions, error) {
 	return options, nil
 }
 
-// Extract extracts from pkgReader the entries listed in options.Extract.
-// openTar opens the tar stream carried by the package and must not be nil; see
-// OpenTarFunc for the contract it must satisfy.
 func Extract(pkgReader io.ReadSeeker, openTar OpenTarFunc, options *ExtractOptions) (err error) {
 	defer func() {
 		if err != nil {
diff --git a/internal/testutil/pkgdata.go b/internal/testutil/pkgdata.go
index 8fb8fb8b2..f901873d3 100644
--- a/internal/testutil/pkgdata.go
+++ b/internal/testutil/pkgdata.go
@@ -8,7 +8,6 @@ import (
 
 	"github.com/blakesmith/ar"
 	"github.com/klauspost/compress/zstd"
-	"github.com/ulikunitz/xz"
 )
 
 var PackageData = map[string][]byte{}
@@ -161,42 +160,6 @@ func MustMakeDeb(entries []TarEntry) []byte {
 	return data
 }
 
-// compressBytesXz compresses the input using XZ, the compression format used by
-// store (bin) packages.
-func compressBytesXz(input []byte) ([]byte, error) {
-	var buf bytes.Buffer
-	writer, err := xz.NewWriter(&buf)
-	if err != nil {
-		return nil, err
-	}
-	if _, err = writer.Write(input); err != nil {
-		return nil, err
-	}
-	if err = writer.Close(); err != nil {
-		return nil, err
-	}
-	return buf.Bytes(), nil
-}
-
-// MakeBin builds a store (bin) package from the given tar entries: an
-// XZ-compressed plain tarball. Unlike MakeDeb, there is no ar container.
-func MakeBin(entries []TarEntry) ([]byte, error) {
-	tarData, err := makeTar(entries)
-	if err != nil {
-		return nil, err
-	}
-	return compressBytesXz(tarData)
-}
-
-// MustMakeBin is the panicking variant of MakeBin.
-func MustMakeBin(entries []TarEntry) []byte {
-	data, err := MakeBin(entries)
-	if err != nil {
-		panic(err)
-	}
-	return data
-}
-
 // Reg is a shortcut for creating a regular file TarEntry structure (with
 // tar.Typeflag set tar.TypeReg). Reg stands for "REGular file".
 func Reg(mode int64, path, content string) TarEntry {

From 09a01bd80b8ef36a30af041c46a2a62dc64c212e Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Wed, 29 Jul 2026 13:30:16 +0200
Subject: [PATCH 04/15] refactor: refining

---
 internal/tarball/extract.go      |  6 +-----
 internal/tarball/extract_test.go | 18 ++++++++++++++----
 2 files changed, 15 insertions(+), 9 deletions(-)

diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index 26c8ec6a9..28d05be9e 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -22,10 +22,6 @@ import (
 // the package format from Extract. An implementation may unwrap a container
 // before decompressing (see deb.OpenTar) or decompress the package itself (see
 // OpenXZ).
-//
-// Extract reads pkgReader from the start of the package, closes the returned
-// reader, and may open the same package more than once, rewinding pkgReader
-// beforehand.
 type OpenTarFunc func(pkgReader io.Reader) (io.ReadCloser, error)
 
 // OpenXZ opens a package which is a plain XZ-compressed tarball, such as a
@@ -190,7 +186,7 @@ func extractData(pkgReader io.ReadSeeker, openTar OpenTarFunc, options *ExtractO
 		}
 
 		var contentCache []byte
-		contentIsCached := len(targetPaths) > 1 && !sourceIsDir
+		var contentIsCached = len(targetPaths) > 1 && !sourceIsDir
 		if contentIsCached {
 			// Read and cache the content so it may be reused.
 			// As an alternative, to avoid having an entire file in
diff --git a/internal/tarball/extract_test.go b/internal/tarball/extract_test.go
index a2246bbec..9c5fe3523 100644
--- a/internal/tarball/extract_test.go
+++ b/internal/tarball/extract_test.go
@@ -19,6 +19,8 @@ import (
 type extractTest struct {
 	summary string
 	pkgdata []byte
+	// openTar must match the format of pkgdata. It defaults to deb.OpenTar.
+	openTar tarball.OpenTarFunc
 	options tarball.ExtractOptions
 	hackopt func(c *C, o *tarball.ExtractOptions)
 	result  map[string]string
@@ -512,8 +514,11 @@ func (s *S) TestExtract(c *C) {
 			test.hackopt(c, &options)
 		}
 
-		// The test fixtures are .deb archives, so use the deb tar opener.
-		err := tarball.Extract(bytes.NewReader(test.pkgdata), deb.OpenTar, &options)
+		openTar := test.openTar
+		if openTar == nil {
+			openTar = deb.OpenTar
+		}
+		err := tarball.Extract(bytes.NewReader(test.pkgdata), openTar, &options)
 		if test.error != "" {
 			c.Assert(err, ErrorMatches, test.error)
 			continue
@@ -541,6 +546,8 @@ func (s *S) TestExtract(c *C) {
 var extractCreateCallbackTests = []struct {
 	summary string
 	pkgdata []byte
+	// openTar must match the format of pkgdata. It defaults to deb.OpenTar.
+	openTar tarball.OpenTarFunc
 	options tarball.ExtractOptions
 	calls   map[string][]tarball.ExtractInfo
 }{{
@@ -624,8 +631,11 @@ func (s *S) TestExtractCreateCallback(c *C) {
 			return nil
 		}
 
-		// The test fixtures are .deb archives, so use the deb tar opener.
-		err := tarball.Extract(bytes.NewReader(test.pkgdata), deb.OpenTar, &options)
+		openTar := test.openTar
+		if openTar == nil {
+			openTar = deb.OpenTar
+		}
+		err := tarball.Extract(bytes.NewReader(test.pkgdata), openTar, &options)
 		c.Assert(err, IsNil)
 
 		c.Assert(createExtractInfos, DeepEquals, test.calls)

From 8d87fae3b1ea212fe44dedc2188d1e452c52646f Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Wed, 19 Aug 2026 11:02:15 +0200
Subject: [PATCH 05/15] docs: clean comments

---
 internal/tarball/extract.go      | 2 +-
 internal/tarball/extract_test.go | 2 --
 2 files changed, 1 insertion(+), 3 deletions(-)

diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index 28d05be9e..de014be7e 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -25,7 +25,7 @@ import (
 type OpenTarFunc func(pkgReader io.Reader) (io.ReadCloser, error)
 
 // OpenXZ opens a package which is a plain XZ-compressed tarball, such as a
-// store (bin) package.
+// bin package.
 func OpenXZ(pkgReader io.Reader) (io.ReadCloser, error) {
 	xzReader, err := xz.NewReader(pkgReader)
 	if err != nil {
diff --git a/internal/tarball/extract_test.go b/internal/tarball/extract_test.go
index 9c5fe3523..324936095 100644
--- a/internal/tarball/extract_test.go
+++ b/internal/tarball/extract_test.go
@@ -19,7 +19,6 @@ import (
 type extractTest struct {
 	summary string
 	pkgdata []byte
-	// openTar must match the format of pkgdata. It defaults to deb.OpenTar.
 	openTar tarball.OpenTarFunc
 	options tarball.ExtractOptions
 	hackopt func(c *C, o *tarball.ExtractOptions)
@@ -546,7 +545,6 @@ func (s *S) TestExtract(c *C) {
 var extractCreateCallbackTests = []struct {
 	summary string
 	pkgdata []byte
-	// openTar must match the format of pkgdata. It defaults to deb.OpenTar.
 	openTar tarball.OpenTarFunc
 	options tarball.ExtractOptions
 	calls   map[string][]tarball.ExtractInfo

From 1a38ccdf8f5930bb2da03a8479165827314b76be Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Tue, 1 Sep 2026 09:33:48 +0200
Subject: [PATCH 06/15] docs: remove mentions of packages in tarball pkg

---
 internal/tarball/extract.go | 17 ++++++++---------
 1 file changed, 8 insertions(+), 9 deletions(-)

diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index de014be7e..d98ba8bb8 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -18,14 +18,13 @@ import (
 	"github.com/canonical/chisel/internal/strdist"
 )
 
-// OpenTarFunc opens the uncompressed tar stream carried by a package, hiding
-// the package format from Extract. An implementation may unwrap a container
-// before decompressing (see deb.OpenTar) or decompress the package itself (see
-// OpenXZ).
+// OpenTarFunc returns a reader over the uncompressed tar stream contained in
+// its input, hiding the container and compression details from Extract. An
+// implementation may unwrap a container before decompressing (see deb.OpenTar)
+// or decompress the input directly (see OpenXZ).
 type OpenTarFunc func(pkgReader io.Reader) (io.ReadCloser, error)
 
-// OpenXZ opens a package which is a plain XZ-compressed tarball, such as a
-// bin package.
+// OpenXZ opens a plain XZ-compressed tarball.
 func OpenXZ(pkgReader io.Reader) (io.ReadCloser, error) {
 	xzReader, err := xz.NewReader(pkgReader)
 	if err != nil {
@@ -138,8 +137,8 @@ func extractData(pkgReader io.ReadSeeker, openTar OpenTarFunc, options *ExtractO
 	// create them with the permissions defined in the tarball.
 	//
 	// The assumption is that the tar entries of the parent directories appear
-	// before the entry for the file itself. This is the case for .deb files but
-	// not for all tarballs.
+	// before the entry for the file itself. This is the case for the tarballs
+	// produced by common packaging tools but not for all tarballs.
 	tarDirMode := make(map[string]fs.FileMode)
 	tarReader := tar.NewReader(dataReader)
 	for {
@@ -382,7 +381,7 @@ func extractHardLinks(pkgReader io.ReadSeeker, openTar OpenTarFunc, opts *extrac
 	}
 
 	// If there are pending links, that means the link targets do not come from
-	// this package.
+	// this tarball.
 	if len(opts.pendingLinks) > 0 {
 		var targets []string
 		for target := range opts.pendingLinks {

From ee8732aad0f842ad513f45ea9a6db29e3dcdfef7 Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Thu, 3 Sep 2026 09:30:48 +0200
Subject: [PATCH 07/15] style: refine naming and docs

---
 internal/tarball/extract.go      | 32 +++++++++++++++-----------------
 internal/tarball/extract_test.go |  4 ++--
 2 files changed, 17 insertions(+), 19 deletions(-)

diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index d98ba8bb8..597e8ab96 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -18,14 +18,12 @@ import (
 	"github.com/canonical/chisel/internal/strdist"
 )
 
-// OpenTarFunc returns a reader over the uncompressed tar stream contained in
-// its input, hiding the container and compression details from Extract. An
-// implementation may unwrap a container before decompressing (see deb.OpenTar)
-// or decompress the input directly (see OpenXZ).
-type OpenTarFunc func(pkgReader io.Reader) (io.ReadCloser, error)
-
-// OpenXZ opens a plain XZ-compressed tarball.
-func OpenXZ(pkgReader io.Reader) (io.ReadCloser, error) {
+// TarOpener returns a reader over the uncompressed tar stream contained in
+// its input, hiding the container and compression details from Extract.
+type TarOpener func(pkgReader io.Reader) (io.ReadCloser, error)
+
+// OpenXZTar returns a reader over the decompressed XZ stream.
+func OpenXZTar(pkgReader io.Reader) (io.ReadCloser, error) {
 	xzReader, err := xz.NewReader(pkgReader)
 	if err != nil {
 		return nil, err
@@ -74,7 +72,7 @@ func getValidOptions(options *ExtractOptions) (*ExtractOptions, error) {
 	return options, nil
 }
 
-func Extract(pkgReader io.ReadSeeker, openTar OpenTarFunc, options *ExtractOptions) (err error) {
+func Extract(pkgReader io.ReadSeeker, opener TarOpener, options *ExtractOptions) (err error) {
 	defer func() {
 		if err != nil {
 			err = fmt.Errorf("cannot extract from package %q: %w", options.Package, err)
@@ -83,7 +81,7 @@ func Extract(pkgReader io.ReadSeeker, openTar OpenTarFunc, options *ExtractOptio
 
 	logf("Extracting files from package %q...", options.Package)
 
-	if openTar == nil {
+	if opener == nil {
 		return fmt.Errorf("internal error: no tar opener provided")
 	}
 
@@ -99,11 +97,11 @@ func Extract(pkgReader io.ReadSeeker, openTar OpenTarFunc, options *ExtractOptio
 		return err
 	}
 
-	return extractData(pkgReader, openTar, validOpts)
+	return extractData(pkgReader, opener, validOpts)
 }
 
-func extractData(pkgReader io.ReadSeeker, openTar OpenTarFunc, options *ExtractOptions) error {
-	dataReader, err := openTar(pkgReader)
+func extractData(pkgReader io.ReadSeeker, opener TarOpener, options *ExtractOptions) error {
+	dataReader, err := opener(pkgReader)
 	if err != nil {
 		return err
 	}
@@ -285,7 +283,7 @@ func extractData(pkgReader io.ReadSeeker, openTar OpenTarFunc, options *ExtractO
 		if err != nil {
 			return err
 		}
-		err = extractHardLinks(pkgReader, openTar, extractHardLinkOptions)
+		err = extractHardLinks(pkgReader, opener, extractHardLinkOptions)
 		if err != nil {
 			return err
 		}
@@ -319,8 +317,8 @@ type extractHardLinkOptions struct {
 
 // extractHardLinks iterates through the tarball a second time to extract the
 // hard links that were not extracted in the first pass.
-func extractHardLinks(pkgReader io.ReadSeeker, openTar OpenTarFunc, opts *extractHardLinkOptions) error {
-	dataReader, err := openTar(pkgReader)
+func extractHardLinks(pkgReader io.ReadSeeker, opener TarOpener, opts *extractHardLinkOptions) error {
+	dataReader, err := opener(pkgReader)
 	if err != nil {
 		return err
 	}
@@ -381,7 +379,7 @@ func extractHardLinks(pkgReader io.ReadSeeker, openTar OpenTarFunc, opts *extrac
 	}
 
 	// If there are pending links, that means the link targets do not come from
-	// this tarball.
+	// this package.
 	if len(opts.pendingLinks) > 0 {
 		var targets []string
 		for target := range opts.pendingLinks {
diff --git a/internal/tarball/extract_test.go b/internal/tarball/extract_test.go
index 324936095..436bea074 100644
--- a/internal/tarball/extract_test.go
+++ b/internal/tarball/extract_test.go
@@ -19,7 +19,7 @@ import (
 type extractTest struct {
 	summary string
 	pkgdata []byte
-	openTar tarball.OpenTarFunc
+	openTar tarball.TarOpener
 	options tarball.ExtractOptions
 	hackopt func(c *C, o *tarball.ExtractOptions)
 	result  map[string]string
@@ -545,7 +545,7 @@ func (s *S) TestExtract(c *C) {
 var extractCreateCallbackTests = []struct {
 	summary string
 	pkgdata []byte
-	openTar tarball.OpenTarFunc
+	openTar tarball.TarOpener
 	options tarball.ExtractOptions
 	calls   map[string][]tarball.ExtractInfo
 }{{

From ca72fd65d108d3e9d1a10fbadf78228e921b832b Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Tue, 8 Sep 2026 15:24:49 +0200
Subject: [PATCH 08/15] refactor: rework approach

---
 .../cmd_debug_check_release_archives.go       |  4 +-
 internal/archive/archive_test.go              |  3 +-
 internal/deb/extract.go                       | 49 ----------
 internal/slicer/slicer.go                     |  4 +-
 internal/tarball/extract.go                   | 92 +++++++++++++++----
 internal/tarball/extract_test.go              | 35 ++-----
 6 files changed, 88 insertions(+), 99 deletions(-)
 delete mode 100644 internal/deb/extract.go

diff --git a/cmd/chisel/cmd_debug_check_release_archives.go b/cmd/chisel/cmd_debug_check_release_archives.go
index 29a8189a8..0225e29f0 100644
--- a/cmd/chisel/cmd_debug_check_release_archives.go
+++ b/cmd/chisel/cmd_debug_check_release_archives.go
@@ -13,8 +13,8 @@ import (
 
 	"github.com/canonical/chisel/internal/archive"
 	"github.com/canonical/chisel/internal/cache"
-	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/setup"
+	"github.com/canonical/chisel/internal/tarball"
 )
 
 var shortCheckReleaseArchivesHelp = "Check the release's archives"
@@ -150,7 +150,7 @@ func computePathObservations(release *setup.Release, archives map[string]archive
 			if err != nil {
 				return nil, err
 			}
-			dataReader, err := deb.OpenTar(pkgReader)
+			dataReader, err := tarball.DataReader(pkgReader, tarball.DebFormat)
 			if err != nil {
 				return nil, err
 			}
diff --git a/internal/archive/archive_test.go b/internal/archive/archive_test.go
index 8c40eeced..bc5687213 100644
--- a/internal/archive/archive_test.go
+++ b/internal/archive/archive_test.go
@@ -1236,8 +1236,9 @@ func (s *S) testOpenArchiveArch(c *C, test realArchiveTest, arch string) {
 	c.Assert(info.Name, DeepEquals, test.pkg)
 	c.Assert(info.Arch, DeepEquals, arch)
 
-	err = tarball.Extract(pkg, deb.OpenTar, &tarball.ExtractOptions{
+	err = tarball.Extract(pkg, &tarball.ExtractOptions{
 		Package:   test.pkg,
+		Format:    tarball.DebFormat,
 		TargetDir: extractDir,
 		Extract: map[string][]tarball.ExtractInfo{
 			fmt.Sprintf("/usr/share/doc/%s/copyright", test.pkg): {
diff --git a/internal/deb/extract.go b/internal/deb/extract.go
deleted file mode 100644
index d25f1f883..000000000
--- a/internal/deb/extract.go
+++ /dev/null
@@ -1,49 +0,0 @@
-package deb
-
-import (
-	"compress/gzip"
-	"fmt"
-	"io"
-
-	"github.com/blakesmith/ar"
-	"github.com/klauspost/compress/zstd"
-	"github.com/ulikunitz/xz"
-)
-
-// OpenTar takes a Reader for the ar file belonging to a Debian package and
-// returns a Reader to the uncompressed inner tarball.
-func OpenTar(pkgReader io.Reader) (io.ReadCloser, error) {
-	arReader := ar.NewReader(pkgReader)
-	var dataReader io.ReadCloser
-	for dataReader == nil {
-		arHeader, err := arReader.Next()
-		if err == io.EOF {
-			return nil, fmt.Errorf("no data payload")
-		}
-		if err != nil {
-			return nil, err
-		}
-		switch arHeader.Name {
-		case "data.tar.gz":
-			gzipReader, err := gzip.NewReader(arReader)
-			if err != nil {
-				return nil, err
-			}
-			dataReader = gzipReader
-		case "data.tar.xz":
-			xzReader, err := xz.NewReader(arReader)
-			if err != nil {
-				return nil, err
-			}
-			dataReader = io.NopCloser(xzReader)
-		case "data.tar.zst":
-			zstdReader, err := zstd.NewReader(arReader)
-			if err != nil {
-				return nil, err
-			}
-			dataReader = zstdReader.IOReadCloser()
-		}
-	}
-
-	return dataReader, nil
-}
diff --git a/internal/slicer/slicer.go b/internal/slicer/slicer.go
index 994f65ba2..6c528c651 100644
--- a/internal/slicer/slicer.go
+++ b/internal/slicer/slicer.go
@@ -16,7 +16,6 @@ import (
 	"github.com/klauspost/compress/zstd"
 
 	"github.com/canonical/chisel/internal/archive"
-	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/fsutil"
 	"github.com/canonical/chisel/internal/manifestutil"
 	"github.com/canonical/chisel/internal/scripts"
@@ -240,8 +239,9 @@ func Run(options *RunOptions) error {
 		if reader == nil {
 			continue
 		}
-		err := tarball.Extract(reader, deb.OpenTar, &tarball.ExtractOptions{
+		err := tarball.Extract(reader, &tarball.ExtractOptions{
 			Package:   slice.Package,
+			Format:    tarball.DebFormat,
 			Extract:   extract[slice.Package],
 			TargetDir: targetDir,
 			Create:    create,
diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index 597e8ab96..c8b79c859 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -3,6 +3,7 @@ package tarball
 import (
 	"archive/tar"
 	"bytes"
+	"compress/gzip"
 	"fmt"
 	"io"
 	"io/fs"
@@ -12,27 +13,82 @@ import (
 	"strings"
 	"syscall"
 
+	"github.com/blakesmith/ar"
+	"github.com/klauspost/compress/zstd"
 	"github.com/ulikunitz/xz"
 
 	"github.com/canonical/chisel/internal/fsutil"
 	"github.com/canonical/chisel/internal/strdist"
 )
 
-// TarOpener returns a reader over the uncompressed tar stream contained in
-// its input, hiding the container and compression details from Extract.
-type TarOpener func(pkgReader io.Reader) (io.ReadCloser, error)
+// Format identifies the format of a package.
+type Format string
 
-// OpenXZTar returns a reader over the decompressed XZ stream.
-func OpenXZTar(pkgReader io.Reader) (io.ReadCloser, error) {
-	xzReader, err := xz.NewReader(pkgReader)
-	if err != nil {
-		return nil, err
+const (
+	// DebFormat is the Debian package format: an ar archive holding a
+	// compressed data tarball.
+	DebFormat Format = "deb"
+	// BinFormat is the bin package format: a plain XZ-compressed tarball.
+	BinFormat Format = "bin"
+)
+
+// DataReader returns a reader over the uncompressed tar stream contained in
+// the given package, based on its format.
+func DataReader(pkgReader io.Reader, format Format) (io.ReadCloser, error) {
+	switch format {
+	case DebFormat:
+		return debDataReader(pkgReader)
+	case BinFormat:
+		xzReader, err := xz.NewReader(pkgReader)
+		if err != nil {
+			return nil, err
+		}
+		return io.NopCloser(xzReader), nil
+	}
+	return nil, fmt.Errorf("internal error: unsupported package format: %q", format)
+}
+
+// debDataReader returns a reader over the data tarball contained in the ar
+// file of a Debian package.
+func debDataReader(pkgReader io.Reader) (io.ReadCloser, error) {
+	arReader := ar.NewReader(pkgReader)
+	var dataReader io.ReadCloser
+	for dataReader == nil {
+		arHeader, err := arReader.Next()
+		if err == io.EOF {
+			return nil, fmt.Errorf("no data payload")
+		}
+		if err != nil {
+			return nil, err
+		}
+		switch arHeader.Name {
+		case "data.tar.gz":
+			gzipReader, err := gzip.NewReader(arReader)
+			if err != nil {
+				return nil, err
+			}
+			dataReader = gzipReader
+		case "data.tar.xz":
+			xzReader, err := xz.NewReader(arReader)
+			if err != nil {
+				return nil, err
+			}
+			dataReader = io.NopCloser(xzReader)
+		case "data.tar.zst":
+			zstdReader, err := zstd.NewReader(arReader)
+			if err != nil {
+				return nil, err
+			}
+			dataReader = zstdReader.IOReadCloser()
+		}
 	}
-	return io.NopCloser(xzReader), nil
+
+	return dataReader, nil
 }
 
 type ExtractOptions struct {
 	Package   string
+	Format    Format
 	TargetDir string
 	Extract   map[string][]ExtractInfo
 	// Create can optionally be set to control the creation of extracted entries.
@@ -72,7 +128,7 @@ func getValidOptions(options *ExtractOptions) (*ExtractOptions, error) {
 	return options, nil
 }
 
-func Extract(pkgReader io.ReadSeeker, opener TarOpener, options *ExtractOptions) (err error) {
+func Extract(pkgReader io.ReadSeeker, options *ExtractOptions) (err error) {
 	defer func() {
 		if err != nil {
 			err = fmt.Errorf("cannot extract from package %q: %w", options.Package, err)
@@ -81,10 +137,6 @@ func Extract(pkgReader io.ReadSeeker, opener TarOpener, options *ExtractOptions)
 
 	logf("Extracting files from package %q...", options.Package)
 
-	if opener == nil {
-		return fmt.Errorf("internal error: no tar opener provided")
-	}
-
 	validOpts, err := getValidOptions(options)
 	if err != nil {
 		return err
@@ -97,11 +149,11 @@ func Extract(pkgReader io.ReadSeeker, opener TarOpener, options *ExtractOptions)
 		return err
 	}
 
-	return extractData(pkgReader, opener, validOpts)
+	return extractData(pkgReader, validOpts)
 }
 
-func extractData(pkgReader io.ReadSeeker, opener TarOpener, options *ExtractOptions) error {
-	dataReader, err := opener(pkgReader)
+func extractData(pkgReader io.ReadSeeker, options *ExtractOptions) error {
+	dataReader, err := DataReader(pkgReader, options.Format)
 	if err != nil {
 		return err
 	}
@@ -283,7 +335,7 @@ func extractData(pkgReader io.ReadSeeker, opener TarOpener, options *ExtractOpti
 		if err != nil {
 			return err
 		}
-		err = extractHardLinks(pkgReader, opener, extractHardLinkOptions)
+		err = extractHardLinks(pkgReader, extractHardLinkOptions)
 		if err != nil {
 			return err
 		}
@@ -317,8 +369,8 @@ type extractHardLinkOptions struct {
 
 // extractHardLinks iterates through the tarball a second time to extract the
 // hard links that were not extracted in the first pass.
-func extractHardLinks(pkgReader io.ReadSeeker, opener TarOpener, opts *extractHardLinkOptions) error {
-	dataReader, err := opener(pkgReader)
+func extractHardLinks(pkgReader io.ReadSeeker, opts *extractHardLinkOptions) error {
+	dataReader, err := DataReader(pkgReader, opts.Format)
 	if err != nil {
 		return err
 	}
diff --git a/internal/tarball/extract_test.go b/internal/tarball/extract_test.go
index 436bea074..0b3b670cb 100644
--- a/internal/tarball/extract_test.go
+++ b/internal/tarball/extract_test.go
@@ -10,7 +10,6 @@ import (
 
 	. "gopkg.in/check.v1"
 
-	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/fsutil"
 	"github.com/canonical/chisel/internal/tarball"
 	"github.com/canonical/chisel/internal/testutil"
@@ -19,7 +18,7 @@ import (
 type extractTest struct {
 	summary string
 	pkgdata []byte
-	openTar tarball.TarOpener
+	format  tarball.Format
 	options tarball.ExtractOptions
 	hackopt func(c *C, o *tarball.ExtractOptions)
 	result  map[string]string
@@ -498,6 +497,9 @@ func (s *S) TestExtract(c *C) {
 		options := test.options
 		options.Package = "test-package"
 		options.TargetDir = dir
+		if options.Format == "" {
+			options.Format = tarball.DebFormat
+		}
 		createdPaths := make(map[string]bool)
 		options.Create = func(_ []tarball.ExtractInfo, o *fsutil.CreateOptions) error {
 			relPath := filepath.Clean("/" + strings.TrimPrefix(o.Path, dir))
@@ -513,11 +515,7 @@ func (s *S) TestExtract(c *C) {
 			test.hackopt(c, &options)
 		}
 
-		openTar := test.openTar
-		if openTar == nil {
-			openTar = deb.OpenTar
-		}
-		err := tarball.Extract(bytes.NewReader(test.pkgdata), openTar, &options)
+		err := tarball.Extract(bytes.NewReader(test.pkgdata), &options)
 		if test.error != "" {
 			c.Assert(err, ErrorMatches, test.error)
 			continue
@@ -545,7 +543,7 @@ func (s *S) TestExtract(c *C) {
 var extractCreateCallbackTests = []struct {
 	summary string
 	pkgdata []byte
-	openTar tarball.TarOpener
+	format  tarball.Format
 	options tarball.ExtractOptions
 	calls   map[string][]tarball.ExtractInfo
 }{{
@@ -612,6 +610,9 @@ func (s *S) TestExtractCreateCallback(c *C) {
 		options := test.options
 		options.Package = "test-package"
 		options.TargetDir = dir
+		if options.Format == "" {
+			options.Format = tarball.DebFormat
+		}
 		createExtractInfos := map[string][]tarball.ExtractInfo{}
 		options.Create = func(extractInfos []tarball.ExtractInfo, o *fsutil.CreateOptions) error {
 			if extractInfos == nil {
@@ -629,25 +630,9 @@ func (s *S) TestExtractCreateCallback(c *C) {
 			return nil
 		}
 
-		openTar := test.openTar
-		if openTar == nil {
-			openTar = deb.OpenTar
-		}
-		err := tarball.Extract(bytes.NewReader(test.pkgdata), openTar, &options)
+		err := tarball.Extract(bytes.NewReader(test.pkgdata), &options)
 		c.Assert(err, IsNil)
 
 		c.Assert(createExtractInfos, DeepEquals, test.calls)
 	}
 }
-
-func (s *S) TestExtractMissingOpenTar(c *C) {
-	options := tarball.ExtractOptions{
-		Package:   "test-package",
-		TargetDir: c.MkDir(),
-		Extract: map[string][]tarball.ExtractInfo{
-			"/dir/file": {{Path: "/dir/file"}},
-		},
-	}
-	err := tarball.Extract(bytes.NewReader(testutil.PackageData["test-package"]), nil, &options)
-	c.Assert(err, ErrorMatches, `cannot extract from package "test-package": internal error: no tar opener provided`)
-}

From dc8e94bd6caedca91cb578482c7fbeef33c6266c Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Tue, 8 Sep 2026 15:40:05 +0200
Subject: [PATCH 09/15] fix: clean superfluous import

---
 internal/archive/archive_test.go | 1 -
 1 file changed, 1 deletion(-)

diff --git a/internal/archive/archive_test.go b/internal/archive/archive_test.go
index bc5687213..3a83d73d5 100644
--- a/internal/archive/archive_test.go
+++ b/internal/archive/archive_test.go
@@ -20,7 +20,6 @@ import (
 
 	"github.com/canonical/chisel/internal/archive"
 	"github.com/canonical/chisel/internal/archive/testarchive"
-	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/tarball"
 	"github.com/canonical/chisel/internal/testutil"
 )

From f882d4711b3bf0c15f56f9ed419a4409deccbf7a Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Tue, 8 Sep 2026 15:50:27 +0200
Subject: [PATCH 10/15] test: remove unused field for now

---
 internal/tarball/extract_test.go | 1 -
 1 file changed, 1 deletion(-)

diff --git a/internal/tarball/extract_test.go b/internal/tarball/extract_test.go
index 0b3b670cb..5f233a92a 100644
--- a/internal/tarball/extract_test.go
+++ b/internal/tarball/extract_test.go
@@ -18,7 +18,6 @@ import (
 type extractTest struct {
 	summary string
 	pkgdata []byte
-	format  tarball.Format
 	options tarball.ExtractOptions
 	hackopt func(c *C, o *tarball.ExtractOptions)
 	result  map[string]string

From 116752e32f3f83aac412e8a512be3dfe1ea74063 Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Wed, 9 Sep 2026 08:32:37 +0200
Subject: [PATCH 11/15] refactor: put deb DataReader back in deb package

---
 internal/deb/extract.go     | 49 +++++++++++++++++++++++++++++++++++++
 internal/tarball/extract.go | 44 ++-------------------------------
 2 files changed, 51 insertions(+), 42 deletions(-)
 create mode 100644 internal/deb/extract.go

diff --git a/internal/deb/extract.go b/internal/deb/extract.go
new file mode 100644
index 000000000..2bce509f4
--- /dev/null
+++ b/internal/deb/extract.go
@@ -0,0 +1,49 @@
+package deb
+
+import (
+	"compress/gzip"
+	"fmt"
+	"io"
+
+	"github.com/blakesmith/ar"
+	"github.com/klauspost/compress/zstd"
+	"github.com/ulikunitz/xz"
+)
+
+// DataReader takes a Reader for the ar file belonging to a Debian package and
+// returns a Reader to the inner tarball.
+func DataReader(pkgReader io.Reader) (io.ReadCloser, error) {
+	arReader := ar.NewReader(pkgReader)
+	var dataReader io.ReadCloser
+	for dataReader == nil {
+		arHeader, err := arReader.Next()
+		if err == io.EOF {
+			return nil, fmt.Errorf("no data payload")
+		}
+		if err != nil {
+			return nil, err
+		}
+		switch arHeader.Name {
+		case "data.tar.gz":
+			gzipReader, err := gzip.NewReader(arReader)
+			if err != nil {
+				return nil, err
+			}
+			dataReader = gzipReader
+		case "data.tar.xz":
+			xzReader, err := xz.NewReader(arReader)
+			if err != nil {
+				return nil, err
+			}
+			dataReader = io.NopCloser(xzReader)
+		case "data.tar.zst":
+			zstdReader, err := zstd.NewReader(arReader)
+			if err != nil {
+				return nil, err
+			}
+			dataReader = zstdReader.IOReadCloser()
+		}
+	}
+
+	return dataReader, nil
+}
\ No newline at end of file
diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index c8b79c859..3807d1544 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -3,7 +3,6 @@ package tarball
 import (
 	"archive/tar"
 	"bytes"
-	"compress/gzip"
 	"fmt"
 	"io"
 	"io/fs"
@@ -13,10 +12,9 @@ import (
 	"strings"
 	"syscall"
 
-	"github.com/blakesmith/ar"
-	"github.com/klauspost/compress/zstd"
 	"github.com/ulikunitz/xz"
 
+	"github.com/canonical/chisel/internal/deb"
 	"github.com/canonical/chisel/internal/fsutil"
 	"github.com/canonical/chisel/internal/strdist"
 )
@@ -37,7 +35,7 @@ const (
 func DataReader(pkgReader io.Reader, format Format) (io.ReadCloser, error) {
 	switch format {
 	case DebFormat:
-		return debDataReader(pkgReader)
+		return deb.DataReader(pkgReader)
 	case BinFormat:
 		xzReader, err := xz.NewReader(pkgReader)
 		if err != nil {
@@ -48,44 +46,6 @@ func DataReader(pkgReader io.Reader, format Format) (io.ReadCloser, error) {
 	return nil, fmt.Errorf("internal error: unsupported package format: %q", format)
 }
 
-// debDataReader returns a reader over the data tarball contained in the ar
-// file of a Debian package.
-func debDataReader(pkgReader io.Reader) (io.ReadCloser, error) {
-	arReader := ar.NewReader(pkgReader)
-	var dataReader io.ReadCloser
-	for dataReader == nil {
-		arHeader, err := arReader.Next()
-		if err == io.EOF {
-			return nil, fmt.Errorf("no data payload")
-		}
-		if err != nil {
-			return nil, err
-		}
-		switch arHeader.Name {
-		case "data.tar.gz":
-			gzipReader, err := gzip.NewReader(arReader)
-			if err != nil {
-				return nil, err
-			}
-			dataReader = gzipReader
-		case "data.tar.xz":
-			xzReader, err := xz.NewReader(arReader)
-			if err != nil {
-				return nil, err
-			}
-			dataReader = io.NopCloser(xzReader)
-		case "data.tar.zst":
-			zstdReader, err := zstd.NewReader(arReader)
-			if err != nil {
-				return nil, err
-			}
-			dataReader = zstdReader.IOReadCloser()
-		}
-	}
-
-	return dataReader, nil
-}
-
 type ExtractOptions struct {
 	Package   string
 	Format    Format

From e9c4d881ab915066a7a8807bb78a506418d44341 Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Wed, 9 Sep 2026 08:34:03 +0200
Subject: [PATCH 12/15] style: add missing trailing newline

---
 internal/deb/extract.go | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/internal/deb/extract.go b/internal/deb/extract.go
index 2bce509f4..88fc4ad56 100644
--- a/internal/deb/extract.go
+++ b/internal/deb/extract.go
@@ -46,4 +46,4 @@ func DataReader(pkgReader io.Reader) (io.ReadCloser, error) {
 	}
 
 	return dataReader, nil
-}
\ No newline at end of file
+}

From b1ec3903276a6de50ddeec6703775323858ca56d Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Wed, 9 Sep 2026 13:40:51 +0200
Subject: [PATCH 13/15] style: refine naming

---
 cmd/chisel/cmd_debug_check_release_archives.go |  2 +-
 internal/tarball/extract.go                    | 10 +++++-----
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/cmd/chisel/cmd_debug_check_release_archives.go b/cmd/chisel/cmd_debug_check_release_archives.go
index 0225e29f0..c7931d9a6 100644
--- a/cmd/chisel/cmd_debug_check_release_archives.go
+++ b/cmd/chisel/cmd_debug_check_release_archives.go
@@ -150,7 +150,7 @@ func computePathObservations(release *setup.Release, archives map[string]archive
 			if err != nil {
 				return nil, err
 			}
-			dataReader, err := tarball.DataReader(pkgReader, tarball.DebFormat)
+			dataReader, err := tarball.DebFormat.TarStream(pkgReader)
 			if err != nil {
 				return nil, err
 			}
diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index 3807d1544..0760a1fef 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -30,9 +30,9 @@ const (
 	BinFormat Format = "bin"
 )
 
-// DataReader returns a reader over the uncompressed tar stream contained in
-// the given package, based on its format.
-func DataReader(pkgReader io.Reader, format Format) (io.ReadCloser, error) {
+// TarStream returns a reader over the uncompressed tar stream contained
+// in the given package.
+func (format Format) TarStream(pkgReader io.Reader) (io.ReadCloser, error) {
 	switch format {
 	case DebFormat:
 		return deb.DataReader(pkgReader)
@@ -113,7 +113,7 @@ func Extract(pkgReader io.ReadSeeker, options *ExtractOptions) (err error) {
 }
 
 func extractData(pkgReader io.ReadSeeker, options *ExtractOptions) error {
-	dataReader, err := DataReader(pkgReader, options.Format)
+	dataReader, err := options.Format.TarStream(pkgReader)
 	if err != nil {
 		return err
 	}
@@ -330,7 +330,7 @@ type extractHardLinkOptions struct {
 // extractHardLinks iterates through the tarball a second time to extract the
 // hard links that were not extracted in the first pass.
 func extractHardLinks(pkgReader io.ReadSeeker, opts *extractHardLinkOptions) error {
-	dataReader, err := DataReader(pkgReader, opts.Format)
+	dataReader, err := opts.Format.TarStream(pkgReader)
 	if err != nil {
 		return err
 	}

From a8ad8137611ca866f6f3d33f2be2d44fa6ad3082 Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Wed, 9 Sep 2026 13:46:18 +0200
Subject: [PATCH 14/15] style: improve naming consistency

---
 internal/tarball/extract.go | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index 0760a1fef..f61ed21e1 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -113,11 +113,11 @@ func Extract(pkgReader io.ReadSeeker, options *ExtractOptions) (err error) {
 }
 
 func extractData(pkgReader io.ReadSeeker, options *ExtractOptions) error {
-	dataReader, err := options.Format.TarStream(pkgReader)
+	tarStream, err := options.Format.TarStream(pkgReader)
 	if err != nil {
 		return err
 	}
-	defer dataReader.Close()
+	defer tarStream.Close()
 
 	oldUmask := syscall.Umask(0)
 	defer func() {
@@ -150,7 +150,7 @@ func extractData(pkgReader io.ReadSeeker, options *ExtractOptions) error {
 	// before the entry for the file itself. This is the case for the tarballs
 	// produced by common packaging tools but not for all tarballs.
 	tarDirMode := make(map[string]fs.FileMode)
-	tarReader := tar.NewReader(dataReader)
+	tarReader := tar.NewReader(tarStream)
 	for {
 		tarHeader, err := tarReader.Next()
 		if err == io.EOF {
@@ -330,13 +330,13 @@ type extractHardLinkOptions struct {
 // extractHardLinks iterates through the tarball a second time to extract the
 // hard links that were not extracted in the first pass.
 func extractHardLinks(pkgReader io.ReadSeeker, opts *extractHardLinkOptions) error {
-	dataReader, err := opts.Format.TarStream(pkgReader)
+	tarStream, err := opts.Format.TarStream(pkgReader)
 	if err != nil {
 		return err
 	}
-	defer dataReader.Close()
+	defer tarStream.Close()
 
-	tarReader := tar.NewReader(dataReader)
+	tarReader := tar.NewReader(tarStream)
 	for {
 		tarHeader, err := tarReader.Next()
 		if err == io.EOF {

From 6ad029e50f078420f157ddcfa850e11f070c53d7 Mon Sep 17 00:00:00 2001
From: Paul Mars <paul.mars@canonical.com>
Date: Wed, 9 Sep 2026 13:54:03 +0200
Subject: [PATCH 15/15] style: avoid ambiguity

---
 internal/tarball/extract.go      | 16 ++++++++--------
 internal/tarball/extract_test.go |  2 +-
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/internal/tarball/extract.go b/internal/tarball/extract.go
index f61ed21e1..a0c7af11a 100644
--- a/internal/tarball/extract.go
+++ b/internal/tarball/extract.go
@@ -19,20 +19,20 @@ import (
 	"github.com/canonical/chisel/internal/strdist"
 )
 
-// Format identifies the format of a package.
-type Format string
+// PkgFormat identifies the format of a package.
+type PkgFormat string
 
 const (
 	// DebFormat is the Debian package format: an ar archive holding a
 	// compressed data tarball.
-	DebFormat Format = "deb"
+	DebFormat PkgFormat = "deb"
 	// BinFormat is the bin package format: a plain XZ-compressed tarball.
-	BinFormat Format = "bin"
+	BinFormat PkgFormat = "bin"
 )
 
-// TarStream returns a reader over the uncompressed tar stream contained
-// in the given package.
-func (format Format) TarStream(pkgReader io.Reader) (io.ReadCloser, error) {
+// TarStream returns a reader over the raw, uncompressed tar stream contained
+// in the given package. The stream is not parsed.
+func (format PkgFormat) TarStream(pkgReader io.Reader) (io.ReadCloser, error) {
 	switch format {
 	case DebFormat:
 		return deb.DataReader(pkgReader)
@@ -48,7 +48,7 @@ func (format Format) TarStream(pkgReader io.Reader) (io.ReadCloser, error) {
 
 type ExtractOptions struct {
 	Package   string
-	Format    Format
+	Format    PkgFormat
 	TargetDir string
 	Extract   map[string][]ExtractInfo
 	// Create can optionally be set to control the creation of extracted entries.
diff --git a/internal/tarball/extract_test.go b/internal/tarball/extract_test.go
index 5f233a92a..ed43414ef 100644
--- a/internal/tarball/extract_test.go
+++ b/internal/tarball/extract_test.go
@@ -542,7 +542,7 @@ func (s *S) TestExtract(c *C) {
 var extractCreateCallbackTests = []struct {
 	summary string
 	pkgdata []byte
-	format  tarball.Format
+	format  tarball.PkgFormat
 	options tarball.ExtractOptions
 	calls   map[string][]tarball.ExtractInfo
 }{{