archive/tar: improve handling of directory paths

The USTAR format says:
<<<
Implementors should be aware that the previous file format did not include
a mechanism to archive directory type files.
For this reason, the convention of using a filename ending with
<slash> was adopted to specify a directory on the archive.
>>>

In light of this suggestion, make the following changes:
* Writer.WriteHeader refuses to encode a header where a file that
is obviously a file-type has a trailing slash in the name.
* formatter.formatString avoids encoding a trailing slash in the event
that the string is truncated (the full string will be encoded elsewhere,
so stripping the slash is safe).
* Reader.Next treats a TypeRegA (which is the zero value of Typeflag)
as a TypeDir if the name has a trailing slash.

Change-Id: Ibf27aa8234cce2032d92e5e5b28546c2f2ae5ef6
Reviewed-on: https://go-review.googlesource.com/69293
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

1 files changed, 11 insertions, 0 deletions

diff --git a/src/archive/tar/reader_test.go b/src/archive/tar/reader_test.go
index bbabd96246..3ac81adb4d 100644
--- a/src/archive/tar/reader_test.go
+++ b/src/archive/tar/reader_test.go
@@ -675,6 +675,17 @@ func TestReader(t *testing.T) {
 			},
 			Format: FormatPAX,
 		}},
+	}, {
+		file: "testdata/trailing-slash.tar",
+		headers: []*Header{{
+			Typeflag: TypeDir,
+			Name:     strings.Repeat("123456789/", 30),
+			ModTime:  time.Unix(0, 0),
+			PAXRecords: map[string]string{
+				"path": strings.Repeat("123456789/", 30),
+			},
+			Format: FormatPAX,
+		}},
 	}}
 
 	for _, v := range vectors {