packages/deb: add package to extract metadata from .deb files.

Signed-off-by: David Anderson <danderson@tailscale.com>

packages/deb/deb.go

@@ -0,0 +1,184 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Package deb extracts metadata from Debian packages.
+package deb
+
+import (
+	"archive/tar"
+	"bufio"
+	"bytes"
+	"compress/gzip"
+	"crypto/md5"
+	"crypto/sha1"
+	"crypto/sha256"
+	"errors"
+	"fmt"
+	"io"
+	"io/ioutil"
+	"os"
+	"path/filepath"
+	"strconv"
+	"strings"
+)
+
+// Info is the Debian package metadata needed to integrate the package
+// into a repository.
+type Info struct {
+	// Version is the version of the package, as reported by dpkg.
+	Version string
+	// Arch is the Debian CPU architecture the package is for.
+	Arch string
+	// Control is the entire contents of the package's control file,
+	// with leading and trailing whitespace removed.
+	Control []byte
+	// MD5 is the MD5 hash of the package file.
+	MD5 []byte
+	// SHA1 is the SHA1 hash of the package file.
+	SHA1 []byte
+	// SHA256 is the SHA256 hash of the package file.
+	SHA256 []byte
+}
+
+// ReadFile returns Debian package metadata from the .deb file at path.
+func ReadFile(path string) (*Info, error) {
+	f, err := os.Open(path)
+	if err != nil {
+		return nil, err
+	}
+	return Read(f)
+}
+
+// Read returns Debian package metadata from the .deb file in r.
+func Read(r io.Reader) (*Info, error) {
+	b := bufio.NewReader(r)
+
+	m5, s1, s256 := md5.New(), sha1.New(), sha256.New()
+	summers := io.MultiWriter(m5, s1, s256)
+	r = io.TeeReader(b, summers)
+
+	t, err := findControlTar(r)
+	if err != nil {
+		return nil, fmt.Errorf("searching for control.tar.gz: %w", err)
+	}
+
+	control, err := findControlFile(t)
+	if err != nil {
+		return nil, fmt.Errorf("searching for control file in control.tar.gz: %w", err)
+	}
+
+	arch, version, err := findArchAndVersion(control)
+	if err != nil {
+		return nil, fmt.Errorf("extracting version and architecture from control file: %w", err)
+	}
+
+	// Exhaust the remainder of r, so that the summers see the entire file.
+	if _, err := io.Copy(ioutil.Discard, r); err != nil {
+		return nil, fmt.Errorf("hashing file: %w", err)
+	}
+
+	return &Info{
+		Version: version,
+		Arch:    arch,
+		Control: control,
+		MD5:     m5.Sum(nil),
+		SHA1:    s1.Sum(nil),
+		SHA256:  s256.Sum(nil),
+	}, nil
+}
+
+// findControlTar reads r as an `ar` archive, finds a tarball named
+// `control.tar.gz` within, and returns a reader for that file.
+func findControlTar(r io.Reader) (tarReader io.Reader, err error) {
+	var magic [8]byte
+	if _, err := io.ReadFull(r, magic[:]); err != nil {
+		return nil, fmt.Errorf("reading ar magic: %w", err)
+	}
+	if string(magic[:]) != "!<arch>\n" {
+		return nil, fmt.Errorf("not an ar file (bad magic %q)", magic)
+	}
+
+	for {
+		var hdr [60]byte
+		if _, err := io.ReadFull(r, hdr[:]); err != nil {
+			return nil, fmt.Errorf("reading file header: %w", err)
+		}
+		filename := strings.TrimSpace(string(hdr[:16]))
+		size, err := strconv.ParseInt(strings.TrimSpace(string(hdr[48:58])), 10, 64)
+		if err != nil {
+			return nil, fmt.Errorf("reading size of file %q: %w", filename, err)
+		}
+		if filename == "control.tar.gz" {
+			return io.LimitReader(r, size), nil
+		}
+
+		// files in ar are padded out to 2 bytes.
+		if size%2 == 1 {
+			size++
+		}
+		if _, err := io.CopyN(ioutil.Discard, r, size); err != nil {
+			return nil, fmt.Errorf("seeking past file %q: %w", filename, err)
+		}
+	}
+}
+
+// findControlFile reads r as a tar.gz archive, finds a file named
+// `control` within, and returns its contents.
+func findControlFile(r io.Reader) (control []byte, err error) {
+	gz, err := gzip.NewReader(r)
+	if err != nil {
+		return nil, fmt.Errorf("decompressing control.tar.gz: %w", err)
+	}
+	defer gz.Close()
+
+	tr := tar.NewReader(gz)
+	for {
+		hdr, err := tr.Next()
+		if err != nil {
+			if errors.Is(err, io.EOF) {
+				return nil, errors.New("EOF while looking for control file in control.tar.gz")
+			}
+			return nil, fmt.Errorf("reading tar header: %w", err)
+		}
+
+		if filepath.Clean(hdr.Name) != "control" {
+			continue
+		}
+
+		// Found control file
+		break
+	}
+
+	bs, err := ioutil.ReadAll(tr)
+	if err != nil {
+		return nil, fmt.Errorf("reading control file: %w", err)
+	}
+
+	return bytes.TrimSpace(bs), nil
+}
+
+var (
+	archKey    = []byte("Architecture:")
+	versionKey = []byte("Version:")
+)
+
+// findArchAndVersion extracts the architecture and version strings
+// from the given control file.
+func findArchAndVersion(control []byte) (arch string, version string, err error) {
+	b := bytes.NewBuffer(control)
+	for {
+		l, err := b.ReadBytes('\n')
+		if err != nil {
+			return "", "", err
+		}
+		if bytes.HasPrefix(l, archKey) {
+			arch = string(bytes.TrimSpace(l[len(archKey):]))
+		} else if bytes.HasPrefix(l, versionKey) {
+			version = string(bytes.TrimSpace(l[len(versionKey):]))
+		}
+		if arch != "" && version != "" {
+			return arch, version, nil
+		}
+	}
+}