Fix hardlinks with unmodified files

Previously hardlinking to an unmodified file or linking to a file
which was touched by not detected as modified caused a new file
to be created on unpack. This new file and the original source file
were not linked since no link record was created in the tar. This
change addresses this by adding links for all hardlinks to a file
when it is detected as changed. These links will be written after
the source file is written and may occur out of order in regard to
file name.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

archive/tar.go

@@ -260,18 +260,20 @@ func Apply(ctx context.Context, root string, r io.Reader) (int64, error) {
 }
 
 type changeWriter struct {
-	tw         *tar.Writer
-	source     string
-	whiteoutT  time.Time
-	inodeCache map[uint64]string
+	tw        *tar.Writer
+	source    string
+	whiteoutT time.Time
+	inodeSrc  map[uint64]string
+	inodeRefs map[uint64][]string
 }
 
 func newChangeWriter(w io.Writer, source string) *changeWriter {
 	return &changeWriter{
-		tw:         tar.NewWriter(w),
-		source:     source,
-		whiteoutT:  time.Now(),
-		inodeCache: map[uint64]string{},
+		tw:        tar.NewWriter(w),
+		source:    source,
+		whiteoutT: time.Now(),
+		inodeSrc:  map[uint64]string{},
+		inodeRefs: map[uint64][]string{},
 	}
 }
 
@@ -334,15 +336,28 @@ func (cw *changeWriter) HandleChange(k fs.ChangeKind, p string, f os.FileInfo, e
 			return errors.Wrap(err, "failed to set device headers")
 		}
 
-		linkname, err := fs.GetLinkSource(name, f, cw.inodeCache)
-		if err != nil {
-			return errors.Wrap(err, "failed to get hardlink")
-		}
-
-		if linkname != "" {
-			hdr.Typeflag = tar.TypeLink
-			hdr.Linkname = linkname
-			hdr.Size = 0
+		// additionalLinks stores file names which must be linked to
+		// this file when this file is added
+		var additionalLinks []string
+		inode, isHardlink := fs.GetLinkInfo(f)
+		if isHardlink {
+			// If the inode has a source, always link to it
+			if source, ok := cw.inodeSrc[inode]; ok {
+				hdr.Typeflag = tar.TypeLink
+				hdr.Linkname = source
+				hdr.Size = 0
+			} else {
+				if k == fs.ChangeKindUnmodified {
+					cw.inodeRefs[inode] = append(cw.inodeRefs[inode], name)
+					return nil
+				}
+				cw.inodeSrc[inode] = name
+				additionalLinks = cw.inodeRefs[inode]
+				delete(cw.inodeRefs, inode)
+			}
+		} else if k == fs.ChangeKindUnmodified {
+			// Nothing to write to diff
+			return nil
 		}
 
 		if capability, err := getxattr(source, "security.capability"); err != nil {
@@ -374,6 +389,19 @@ func (cw *changeWriter) HandleChange(k fs.ChangeKind, p string, f os.FileInfo, e
 				return errors.New("short write copying file")
 			}
 		}
+
+		if additionalLinks != nil {
+			source = hdr.Name
+			for _, extra := range additionalLinks {
+				hdr.Name = extra
+				hdr.Typeflag = tar.TypeLink
+				hdr.Linkname = source
+				hdr.Size = 0
+				if err := cw.tw.WriteHeader(hdr); err != nil {
+					return errors.Wrap(err, "failed to write file header")
+				}
+			}
+		}
 	}
 	return nil
 }