archive/zip: if non-zero base offset fails, fall back to zero

This permits us to read files that earlier Go releases could read.
It is also compatible with other zip programs.

Change-Id: I7e2999f1073c4db5ba3f51f92681e0b149d55b3e
Reviewed-on: https://go-review.googlesource.com/c/go/+/408734
Run-TryBot: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>

src/archive/zip/reader.go

@@ -123,6 +123,20 @@ func (z *Reader) init(r io.ReaderAt, size int64) error {
 	for {
 		f := &File{zip: z, zipr: r}
 		err = readDirectoryHeader(f, buf)
+
+		// For compatibility with other zip programs,
+		// if we have a non-zero base offset and can't read
+		// the first directory header, try again with a zero
+		// base offset.
+		if err == ErrFormat && z.baseOffset != 0 && len(z.File) == 0 {
+			z.baseOffset = 0
+			if _, err = rs.Seek(int64(end.directoryOffset), io.SeekStart); err != nil {
+				return err
+			}
+			buf = bufio.NewReader(rs)
+			continue
+		}
+
 		if err == ErrFormat || err == io.ErrUnexpectedEOF {
 			break
 		}

src/archive/zip/reader_test.go

@@ -108,6 +108,24 @@ var tests = []ZipTest{
 			},
 		},
 	},
+	{
+		Name:    "test-baddirsz.zip",
+		Comment: "This is a zipfile comment.",
+		File: []ZipTestFile{
+			{
+				Name:     "test.txt",
+				Content:  []byte("This is a test text file.\n"),
+				Modified: time.Date(2010, 9, 5, 12, 12, 1, 0, timeZone(+10*time.Hour)),
+				Mode:     0644,
+			},
+			{
+				Name:     "gophercolor16x16.png",
+				File:     "gophercolor16x16.png",
+				Modified: time.Date(2010, 9, 5, 15, 52, 58, 0, timeZone(+10*time.Hour)),
+				Mode:     0644,
+			},
+		},
+	},
 	{
 		Name:   "r.zip",
 		Source: returnRecursiveZip,