archive/tar: remove dead code with USTAR path splitting

Convert splitUSTARPath to return a bool rather than an error since
the caller never ever uses the error other than to check if it is
nil. Thus, we can remove errNameTooLong as well.

Also, fold the checking of the length <= fileNameSize and whether
the string is ASCII into the split function itself.

Lastly, remove logic to set the MAGIC since that's already done on
L200. Thus, setting the magic is redundant.

There is no overall logic change.

Updates #12638

Change-Id: I26b6992578199abad723c2a2af7f4fc078af9c17
Reviewed-on: https://go-review.googlesource.com/14723Reviewed-by: David Symonds <dsymonds@golang.org>
Run-TryBot: David Symonds <dsymonds@golang.org>

src/archive/tar/writer.go

@@ -23,7 +23,6 @@ var (
 	ErrWriteTooLong    = errors.New("archive/tar: write too long")
 	ErrFieldTooLong    = errors.New("archive/tar: header field too long")
 	ErrWriteAfterClose = errors.New("archive/tar: write after close")
-	errNameTooLong     = errors.New("archive/tar: name too long")
 	errInvalidHeader   = errors.New("archive/tar: header field too long or contains invalid values")
 )
 
@@ -215,26 +214,14 @@ func (tw *Writer) writeHeader(hdr *Header, allowPax bool) error {
 	_, paxPathUsed := paxHeaders[paxPath]
 	// try to use a ustar header when only the name is too long
 	if !tw.preferPax && len(paxHeaders) == 1 && paxPathUsed {
-		suffix := hdr.Name
-		prefix := ""
-		if len(hdr.Name) > fileNameSize && isASCII(hdr.Name) {
-			var err error
-			prefix, suffix, err = tw.splitUSTARLongName(hdr.Name)
-			if err == nil {
-				// ok we can use a ustar long name instead of pax, now correct the fields
-
-				// remove the path field from the pax header. this will suppress the pax header
+		prefix, suffix, ok := splitUSTARPath(hdr.Name)
+		if ok {
+			// Since we can encode in USTAR format, disable PAX header.
 			delete(paxHeaders, paxPath)
 
-				// update the path fields
+			// Update the path fields
 			tw.cString(pathHeaderBytes, suffix, false, paxNone, nil)
 			tw.cString(prefixHeaderBytes, prefix, false, paxNone, nil)
-
-				// Use the ustar magic if we used ustar long names.
-				if len(prefix) > 0 && !tw.usedBinary {
-					copy(header[257:265], []byte("ustar\x00"))
-				}
-			}
 		}
 	}
 
@@ -270,28 +257,25 @@ func (tw *Writer) writeHeader(hdr *Header, allowPax bool) error {
 	return tw.err
 }
 
-// writeUSTARLongName splits a USTAR long name hdr.Name.
-// name must be < 256 characters. errNameTooLong is returned
-// if hdr.Name can't be split. The splitting heuristic
-// is compatible with gnu tar.
-func (tw *Writer) splitUSTARLongName(name string) (prefix, suffix string, err error) {
+// splitUSTARPath splits a path according to USTAR prefix and suffix rules.
+// If the path is not splittable, then it will return ("", "", false).
+func splitUSTARPath(name string) (prefix, suffix string, ok bool) {
 	length := len(name)
-	if length > fileNamePrefixSize+1 {
+	if length <= fileNameSize || !isASCII(name) {
+		return "", "", false
+	} else if length > fileNamePrefixSize+1 {
 		length = fileNamePrefixSize + 1
 	} else if name[length-1] == '/' {
 		length--
 	}
+
 	i := strings.LastIndex(name[:length], "/")
-	// nlen contains the resulting length in the name field.
-	// plen contains the resulting length in the prefix field.
-	nlen := len(name) - i - 1
-	plen := i
+	nlen := len(name) - i - 1 // nlen is length of suffix
+	plen := i                 // plen is length of prefix
 	if i <= 0 || nlen > fileNameSize || nlen == 0 || plen > fileNamePrefixSize {
-		err = errNameTooLong
-		return
+		return "", "", false
 	}
-	prefix, suffix = name[:i], name[i+1:]
-	return
+	return name[:i], name[i+1:], true
 }
 
 // writePaxHeader writes an extended pax header to the

src/archive/tar/writer_test.go

@@ -544,3 +544,37 @@ func TestWriteAfterClose(t *testing.T) {
 		t.Fatalf("Write: got %v; want ErrWriteAfterClose", err)
 	}
 }
+
+func TestSplitUSTARPath(t *testing.T) {
+	var sr = strings.Repeat
+
+	var vectors = []struct {
+		input  string // Input path
+		prefix string // Expected output prefix
+		suffix string // Expected output suffix
+		ok     bool   // Split success?
+	}{
+		{"", "", "", false},
+		{"abc", "", "", false},
+		{"用戶名", "", "", false},
+		{sr("a", fileNameSize), "", "", false},
+		{sr("a", fileNameSize) + "/", "", "", false},
+		{sr("a", fileNameSize) + "/a", sr("a", fileNameSize), "a", true},
+		{sr("a", fileNamePrefixSize) + "/", "", "", false},
+		{sr("a", fileNamePrefixSize) + "/a", sr("a", fileNamePrefixSize), "a", true},
+		{sr("a", fileNameSize+1), "", "", false},
+		{sr("/", fileNameSize+1), sr("/", fileNameSize-1), "/", true},
+		{sr("a", fileNamePrefixSize) + "/" + sr("b", fileNameSize),
+			sr("a", fileNamePrefixSize), sr("b", fileNameSize), true},
+		{sr("a", fileNamePrefixSize) + "//" + sr("b", fileNameSize), "", "", false},
+		{sr("a/", fileNameSize), sr("a/", 77) + "a", sr("a/", 22), true},
+	}
+
+	for _, v := range vectors {
+		prefix, suffix, ok := splitUSTARPath(v.input)
+		if prefix != v.prefix || suffix != v.suffix || ok != v.ok {
+			t.Errorf("splitUSTARPath(%q):\ngot  (%q, %q, %v)\nwant (%q, %q, %v)",
+				v.input, prefix, suffix, ok, v.prefix, v.suffix, v.ok)
+		}
+	}
+}