feat: handle hardlinks

internal/deb/extract.go

@@ -246,11 +246,17 @@ func extractData(dataReader io.Reader, options *ExtractOptions) error {
 				}
 			}
 			// Create the entry itself.
+			link := tarHeader.Linkname
+			if tarHeader.Typeflag == tar.TypeLink {
+				// The hard link requires the real path of the target file.
+				link = filepath.Join(options.TargetDir, link)
+			}
+
 			createOptions := &fsutil.CreateOptions{
 				Path:        filepath.Join(options.TargetDir, targetPath),
 				Mode:        tarHeader.FileInfo().Mode(),
 				Data:        pathReader,
-				Link:        tarHeader.Linkname,
+				Link:        link,
 				MakeParents: true,
 			}
 			err := options.Create(extractInfos, createOptions)

internal/deb/extract_test.go

@@ -352,6 +352,40 @@ var extractTests = []extractTest{{
 		},
 	},
 	error: `cannot extract from package "test-package": path /dir/ requested twice with diverging mode: 0777 != 0000`,
+}, {
+	summary: "Hard link must be created if specified in the tarball",
+	pkgdata: testutil.MustMakeDeb([]testutil.TarEntry{
+		testutil.Dir(0755, "./"),
+		testutil.Reg(0644, "./file1.txt", "text for file1.txt"),
+		testutil.Hln(0644, "./file2.txt", "./file1.txt"),
+	}),
+	options: deb.ExtractOptions{
+		Extract: map[string][]deb.ExtractInfo{
+			"/*.txt": []deb.ExtractInfo{{
+				Path: "/*.txt",
+			}},
+		},
+	},
+	result: map[string]string{
+		"/file1.txt": "file 0644 e926a7fb",
+		"/file2.txt": "file 0644 e926a7fb",
+	},
+	notCreated: []string{},
+}, {
+	summary: "Dangling hard link must raise an error",
+	pkgdata: testutil.MustMakeDeb([]testutil.TarEntry{
+		testutil.Dir(0755, "./"),
+		// testutil.Reg(0644, "./file1.txt", "text for file1.txt"),
+		testutil.Hln(0644, "./file2.txt", "./file1.txt"),
+	}),
+	options: deb.ExtractOptions{
+		Extract: map[string][]deb.ExtractInfo{
+			"/*.txt": []deb.ExtractInfo{{
+				Path: "/*.txt",
+			}},
+		},
+	},
+	error: "cannot extract from package \"test-package\": the target file does not exist: .*/file1.txt",
 }}
 
 func (s *S) TestExtract(c *C) {

internal/fsutil/create.go

@@ -48,8 +48,12 @@ func Create(options *CreateOptions) (*Entry, error) {
 
 	switch o.Mode & fs.ModeType {
 	case 0:
-		err = createFile(o)
-		hash = hex.EncodeToString(rp.h.Sum(nil))
+		if o.Link != "" {
+			err = createHardLink(o)
+		} else {
+			err = createFile(o)
+			hash = hex.EncodeToString(rp.h.Sum(nil))
+		}
 	case fs.ModeDir:
 		err = createDir(o)
 	case fs.ModeSymlink:
@@ -121,6 +125,28 @@ func createSymlink(o *CreateOptions) error {
 	return os.Symlink(o.Link, o.Path)
 }
 
+func createHardLink(o *CreateOptions) error {
+	debugf("Creating hard link: %s => %s", o.Path, o.Link)
+	targetInfo, err := os.Lstat(o.Link)
+	if err != nil && os.IsNotExist(err) {
+		return fmt.Errorf("the target file does not exist: %s", o.Link)
+	} else if err != nil {
+		return err
+	}
+
+	linkInfo, err := os.Lstat(o.Path)
+	if err == nil || os.IsExist(err) {
+		if os.SameFile(targetInfo, linkInfo) {
+			return nil
+		}
+		return fmt.Errorf("the link already exists: %s", o.Path)
+	} else if !os.IsNotExist(err) {
+		return err
+	}
+
+	return os.Link(o.Link, o.Path)
+}
+
 // readerProxy implements the io.Reader interface proxying the calls to its
 // inner io.Reader. On each read, the proxy keeps track of the file size and hash.
 type readerProxy struct {

internal/testutil/pkgdata.go

@@ -197,3 +197,16 @@ func Lnk(mode int64, path, target string) TarEntry {
 		},
 	}
 }
+
+// Hln is a shortcut for creating a hard link TarEntry structure (with
+// tar.Typeflag set to tar.TypeLink). Hln stands for "Hard LiNk".
+func Hln(mode int64, path, target string) TarEntry {
+	return TarEntry{
+		Header: tar.Header{
+			Typeflag: tar.TypeLink,
+			Name:     path,
+			Mode:     mode,
+			Linkname: target,
+		},
+	}
+}