// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package tar

import (
	
	
	
	
	
	
)

// Reader provides sequential access to the contents of a tar archive.
// Reader.Next advances to the next file in the archive (including the first),
// and then Reader can be treated as an io.Reader to access the file's data.
type Reader struct {
	r    io.Reader
	pad  int64      // Amount of padding (ignored) after current file entry
	curr fileReader // Reader for current file entry
	blk  block      // Buffer to use as temporary local storage

	// err is a persistent error.
	// It is only the responsibility of every exported method of Reader to
	// ensure that this error is sticky.
	err error
}

type fileReader interface {
	io.Reader
	fileState

	WriteTo(io.Writer) (int64, error)
}

// NewReader creates a new [Reader] reading from r.
func ( io.Reader) *Reader {
	return &Reader{r: , curr: &regFileReader{, 0}}
}

// Next advances to the next entry in the tar archive.
// The Header.Size determines how many bytes can be read for the next file.
// Any remaining data in the current file is automatically discarded.
// At the end of the archive, Next returns the error io.EOF.
//
// If Next encounters a non-local name (as defined by [filepath.IsLocal])
// and the GODEBUG environment variable contains `tarinsecurepath=0`,
// Next returns the header with an [ErrInsecurePath] error.
// A future version of Go may introduce this behavior by default.
// Programs that want to accept non-local names can ignore
// the [ErrInsecurePath] error and use the returned header.
func ( *Reader) () (*Header, error) {
	if .err != nil {
		return nil, .err
	}
	,  := .next()
	.err = 
	if  == nil && !filepath.IsLocal(.Name) {
		if tarinsecurepath.Value() == "0" {
			tarinsecurepath.IncNonDefault()
			 = ErrInsecurePath
		}
	}
	return , 
}

func ( *Reader) () (*Header, error) {
	var  map[string]string
	var ,  string

	// Externally, Next iterates through the tar archive as if it is a series of
	// files. Internally, the tar format often uses fake "files" to add meta
	// data that describes the next file. These meta data "files" should not
	// normally be visible to the outside. As such, this loop iterates through
	// one or more "header files" until it finds a "normal file".
	 := FormatUSTAR | FormatPAX | FormatGNU
	for {
		// Discard the remainder of the file and any padding.
		if  := discard(.r, .curr.physicalRemaining());  != nil {
			return nil, 
		}
		if ,  := tryReadFull(.r, .blk[:.pad]);  != nil {
			return nil, 
		}
		.pad = 0

		, ,  := .readHeader()
		if  != nil {
			return nil, 
		}
		if  := .handleRegularFile();  != nil {
			return nil, 
		}
		.mayOnlyBe(.Format)

		// Check for PAX/GNU special headers and files.
		switch .Typeflag {
		case TypeXHeader, TypeXGlobalHeader:
			.mayOnlyBe(FormatPAX)
			,  = parsePAX()
			if  != nil {
				return nil, 
			}
			if .Typeflag == TypeXGlobalHeader {
				mergePAX(, )
				return &Header{
					Name:       .Name,
					Typeflag:   .Typeflag,
					Xattrs:     .Xattrs,
					PAXRecords: .PAXRecords,
					Format:     ,
				}, nil
			}
			continue // This is a meta header affecting the next header
		case TypeGNULongName, TypeGNULongLink:
			.mayOnlyBe(FormatGNU)
			,  := readSpecialFile()
			if  != nil {
				return nil, 
			}

			var  parser
			switch .Typeflag {
			case TypeGNULongName:
				 = .parseString()
			case TypeGNULongLink:
				 = .parseString()
			}
			continue // This is a meta header affecting the next header
		default:
			// The old GNU sparse format is handled here since it is technically
			// just a regular file with additional attributes.

			if  := mergePAX(, );  != nil {
				return nil, 
			}
			if  != "" {
				.Name = 
			}
			if  != "" {
				.Linkname = 
			}
			if .Typeflag == TypeRegA {
				if strings.HasSuffix(.Name, "/") {
					.Typeflag = TypeDir // Legacy archives use trailing slash for directories
				} else {
					.Typeflag = TypeReg
				}
			}

			// The extended headers may have updated the size.
			// Thus, setup the regFileReader again after merging PAX headers.
			if  := .handleRegularFile();  != nil {
				return nil, 
			}

			// Sparse formats rely on being able to read from the logical data
			// section; there must be a preceding call to handleRegularFile.
			if  := .handleSparseFile(, );  != nil {
				return nil, 
			}

			// Set the final guess at the format.
			if .has(FormatUSTAR) && .has(FormatPAX) {
				.mayOnlyBe(FormatUSTAR)
			}
			.Format = 
			return , nil // This is a file, so stop
		}
	}
}

// handleRegularFile sets up the current file reader and padding such that it
// can only read the following logical data section. It will properly handle
// special headers that contain no data section.
func ( *Reader) ( *Header) error {
	 := .Size
	if isHeaderOnlyType(.Typeflag) {
		 = 0
	}
	if  < 0 {
		return ErrHeader
	}

	.pad = blockPadding()
	.curr = &regFileReader{r: .r, nb: }
	return nil
}

// handleSparseFile checks if the current file is a sparse format of any type
// and sets the curr reader appropriately.
func ( *Reader) ( *Header,  *block) error {
	var  sparseDatas
	var  error
	if .Typeflag == TypeGNUSparse {
		,  = .readOldGNUSparseMap(, )
	} else {
		,  = .readGNUSparsePAXHeaders()
	}

	// If sp is non-nil, then this is a sparse file.
	// Note that it is possible for len(sp) == 0.
	if  == nil &&  != nil {
		if isHeaderOnlyType(.Typeflag) || !validateSparseEntries(, .Size) {
			return ErrHeader
		}
		 := invertSparseEntries(, .Size)
		.curr = &sparseFileReader{.curr, , 0}
	}
	return 
}

// readGNUSparsePAXHeaders checks the PAX headers for GNU sparse headers.
// If they are found, then this function reads the sparse map and returns it.
// This assumes that 0.0 headers have already been converted to 0.1 headers
// by the PAX header parsing logic.
func ( *Reader) ( *Header) (sparseDatas, error) {
	// Identify the version of GNU headers.
	var  bool
	,  := .PAXRecords[paxGNUSparseMajor], .PAXRecords[paxGNUSparseMinor]
	switch {
	case  == "0" && ( == "0" ||  == "1"):
		 = false
	case  == "1" &&  == "0":
		 = true
	case  != "" ||  != "":
		return nil, nil // Unknown GNU sparse PAX version
	case .PAXRecords[paxGNUSparseMap] != "":
		 = false // 0.0 and 0.1 did not have explicit version records, so guess
	default:
		return nil, nil // Not a PAX format GNU sparse file.
	}
	.Format.mayOnlyBe(FormatPAX)

	// Update hdr from GNU sparse PAX headers.
	if  := .PAXRecords[paxGNUSparseName];  != "" {
		.Name = 
	}
	 := .PAXRecords[paxGNUSparseSize]
	if  == "" {
		 = .PAXRecords[paxGNUSparseRealSize]
	}
	if  != "" {
		,  := strconv.ParseInt(, 10, 64)
		if  != nil {
			return nil, ErrHeader
		}
		.Size = 
	}

	// Read the sparse map according to the appropriate format.
	if  {
		return readGNUSparseMap1x0(.curr)
	}
	return readGNUSparseMap0x1(.PAXRecords)
}

// mergePAX merges paxHdrs into hdr for all relevant fields of Header.
func mergePAX( *Header,  map[string]string) ( error) {
	for ,  := range  {
		if  == "" {
			continue // Keep the original USTAR value
		}
		var  int64
		switch  {
		case paxPath:
			.Name = 
		case paxLinkpath:
			.Linkname = 
		case paxUname:
			.Uname = 
		case paxGname:
			.Gname = 
		case paxUid:
			,  = strconv.ParseInt(, 10, 64)
			.Uid = int() // Integer overflow possible
		case paxGid:
			,  = strconv.ParseInt(, 10, 64)
			.Gid = int() // Integer overflow possible
		case paxAtime:
			.AccessTime,  = parsePAXTime()
		case paxMtime:
			.ModTime,  = parsePAXTime()
		case paxCtime:
			.ChangeTime,  = parsePAXTime()
		case paxSize:
			.Size,  = strconv.ParseInt(, 10, 64)
		default:
			if strings.HasPrefix(, paxSchilyXattr) {
				if .Xattrs == nil {
					.Xattrs = make(map[string]string)
				}
				.Xattrs[[len(paxSchilyXattr):]] = 
			}
		}
		if  != nil {
			return ErrHeader
		}
	}
	.PAXRecords = 
	return nil
}

// parsePAX parses PAX headers.
// If an extended header (type 'x') is invalid, ErrHeader is returned.
func parsePAX( io.Reader) (map[string]string, error) {
	,  := readSpecialFile()
	if  != nil {
		return nil, 
	}
	 := string()

	// For GNU PAX sparse format 0.0 support.
	// This function transforms the sparse format 0.0 headers into format 0.1
	// headers since 0.0 headers were not PAX compliant.
	var  []string

	 := make(map[string]string)
	for len() > 0 {
		, , ,  := parsePAXRecord()
		if  != nil {
			return nil, ErrHeader
		}
		 = 

		switch  {
		case paxGNUSparseOffset, paxGNUSparseNumBytes:
			// Validate sparse header order and value.
			if (len()%2 == 0 &&  != paxGNUSparseOffset) ||
				(len()%2 == 1 &&  != paxGNUSparseNumBytes) ||
				strings.Contains(, ",") {
				return nil, ErrHeader
			}
			 = append(, )
		default:
			[] = 
		}
	}
	if len() > 0 {
		[paxGNUSparseMap] = strings.Join(, ",")
	}
	return , nil
}

// readHeader reads the next block header and assumes that the underlying reader
// is already aligned to a block boundary. It returns the raw block of the
// header in case further processing is required.
//
// The err will be set to io.EOF only when one of the following occurs:
//   - Exactly 0 bytes are read and EOF is hit.
//   - Exactly 1 block of zeros is read and EOF is hit.
//   - At least 2 blocks of zeros are read.
func ( *Reader) () (*Header, *block, error) {
	// Two blocks of zero bytes marks the end of the archive.
	if ,  := io.ReadFull(.r, .blk[:]);  != nil {
		return nil, nil,  // EOF is okay here; exactly 0 bytes read
	}
	if bytes.Equal(.blk[:], zeroBlock[:]) {
		if ,  := io.ReadFull(.r, .blk[:]);  != nil {
			return nil, nil,  // EOF is okay here; exactly 1 block of zeros read
		}
		if bytes.Equal(.blk[:], zeroBlock[:]) {
			return nil, nil, io.EOF // normal EOF; exactly 2 block of zeros read
		}
		return nil, nil, ErrHeader // Zero block and then non-zero block
	}

	// Verify the header matches a known format.
	 := .blk.getFormat()
	if  == FormatUnknown {
		return nil, nil, ErrHeader
	}

	var  parser
	 := new(Header)

	// Unpack the V7 header.
	 := .blk.toV7()
	.Typeflag = .typeFlag()[0]
	.Name = .parseString(.name())
	.Linkname = .parseString(.linkName())
	.Size = .parseNumeric(.size())
	.Mode = .parseNumeric(.mode())
	.Uid = int(.parseNumeric(.uid()))
	.Gid = int(.parseNumeric(.gid()))
	.ModTime = time.Unix(.parseNumeric(.modTime()), 0)

	// Unpack format specific fields.
	if  > formatV7 {
		 := .blk.toUSTAR()
		.Uname = .parseString(.userName())
		.Gname = .parseString(.groupName())
		.Devmajor = .parseNumeric(.devMajor())
		.Devminor = .parseNumeric(.devMinor())

		var  string
		switch {
		case .has(FormatUSTAR | FormatPAX):
			.Format = 
			 := .blk.toUSTAR()
			 = .parseString(.prefix())

			// For Format detection, check if block is properly formatted since
			// the parser is more liberal than what USTAR actually permits.
			 := func( rune) bool { return  >= 0x80 }
			if bytes.IndexFunc(.blk[:], ) >= 0 {
				.Format = FormatUnknown // Non-ASCII characters in block.
			}
			 := func( []byte) bool { return int([len()-1]) == 0 }
			if !((.size()) && (.mode()) && (.uid()) && (.gid()) &&
				(.modTime()) && (.devMajor()) && (.devMinor())) {
				.Format = FormatUnknown // Numeric fields must end in NUL
			}
		case .has(formatSTAR):
			 := .blk.toSTAR()
			 = .parseString(.prefix())
			.AccessTime = time.Unix(.parseNumeric(.accessTime()), 0)
			.ChangeTime = time.Unix(.parseNumeric(.changeTime()), 0)
		case .has(FormatGNU):
			.Format = 
			var  parser
			 := .blk.toGNU()
			if  := .accessTime(); [0] != 0 {
				.AccessTime = time.Unix(.parseNumeric(), 0)
			}
			if  := .changeTime(); [0] != 0 {
				.ChangeTime = time.Unix(.parseNumeric(), 0)
			}

			// Prior to Go1.8, the Writer had a bug where it would output
			// an invalid tar file in certain rare situations because the logic
			// incorrectly believed that the old GNU format had a prefix field.
			// This is wrong and leads to an output file that mangles the
			// atime and ctime fields, which are often left unused.
			//
			// In order to continue reading tar files created by former, buggy
			// versions of Go, we skeptically parse the atime and ctime fields.
			// If we are unable to parse them and the prefix field looks like
			// an ASCII string, then we fallback on the pre-Go1.8 behavior
			// of treating these fields as the USTAR prefix field.
			//
			// Note that this will not use the fallback logic for all possible
			// files generated by a pre-Go1.8 toolchain. If the generated file
			// happened to have a prefix field that parses as valid
			// atime and ctime fields (e.g., when they are valid octal strings),
			// then it is impossible to distinguish between a valid GNU file
			// and an invalid pre-Go1.8 file.
			//
			// See https://golang.org/issues/12594
			// See https://golang.org/issues/21005
			if .err != nil {
				.AccessTime, .ChangeTime = time.Time{}, time.Time{}
				 := .blk.toUSTAR()
				if  := .parseString(.prefix()); isASCII() {
					 = 
				}
				.Format = FormatUnknown // Buggy file is not GNU
			}
		}
		if len() > 0 {
			.Name =  + "/" + .Name
		}
	}
	return , &.blk, .err
}

// readOldGNUSparseMap reads the sparse map from the old GNU sparse format.
// The sparse map is stored in the tar header if it's small enough.
// If it's larger than four entries, then one or more extension headers are used
// to store the rest of the sparse map.
//
// The Header.Size does not reflect the size of any extended headers used.
// Thus, this function will read from the raw io.Reader to fetch extra headers.
// This method mutates blk in the process.
func ( *Reader) ( *Header,  *block) (sparseDatas, error) {
	// Make sure that the input format is GNU.
	// Unfortunately, the STAR format also has a sparse header format that uses
	// the same type flag but has a completely different layout.
	if .getFormat() != FormatGNU {
		return nil, ErrHeader
	}
	.Format.mayOnlyBe(FormatGNU)

	var  parser
	.Size = .parseNumeric(.toGNU().realSize())
	if .err != nil {
		return nil, .err
	}
	 := .toGNU().sparse()
	 := make(sparseDatas, 0, .maxEntries())
	for {
		for  := 0;  < .maxEntries(); ++ {
			// This termination condition is identical to GNU and BSD tar.
			if .entry().offset()[0] == 0x00 {
				break // Don't return, need to process extended headers (even if empty)
			}
			 := .parseNumeric(.entry().offset())
			 := .parseNumeric(.entry().length())
			if .err != nil {
				return nil, .err
			}
			 = append(, sparseEntry{Offset: , Length: })
		}

		if .isExtended()[0] > 0 {
			// There are more entries. Read an extension header and parse its entries.
			if ,  := mustReadFull(.r, [:]);  != nil {
				return nil, 
			}
			 = .toSparse()
			continue
		}
		return , nil // Done
	}
}

// readGNUSparseMap1x0 reads the sparse map as stored in GNU's PAX sparse format
// version 1.0. The format of the sparse map consists of a series of
// newline-terminated numeric fields. The first field is the number of entries
// and is always present. Following this are the entries, consisting of two
// fields (offset, length). This function must stop reading at the end
// boundary of the block containing the last newline.
//
// Note that the GNU manual says that numeric values should be encoded in octal
// format. However, the GNU tar utility itself outputs these values in decimal.
// As such, this library treats values as being encoded in decimal.
func readGNUSparseMap1x0( io.Reader) (sparseDatas, error) {
	var (
		 int64
		        bytes.Buffer
		        block
	)

	// feedTokens copies data in blocks from r into buf until there are
	// at least cnt newlines in buf. It will not read more blocks than needed.
	 := func( int64) error {
		for  <  {
			if ,  := mustReadFull(, [:]);  != nil {
				return 
			}
			.Write([:])
			for ,  := range  {
				if  == '\n' {
					++
				}
			}
		}
		return nil
	}

	// nextToken gets the next token delimited by a newline. This assumes that
	// at least one newline exists in the buffer.
	 := func() string {
		--
		,  := .ReadString('\n')
		return strings.TrimRight(, "\n")
	}

	// Parse for the number of entries.
	// Use integer overflow resistant math to check this.
	if  := (1);  != nil {
		return nil, 
	}
	,  := strconv.ParseInt((), 10, 0) // Intentionally parse as native int
	if  != nil ||  < 0 || int(2*) < int() {
		return nil, ErrHeader
	}

	// Parse for all member entries.
	// numEntries is trusted after this since a potential attacker must have
	// committed resources proportional to what this library used.
	if  := (2 * );  != nil {
		return nil, 
	}
	 := make(sparseDatas, 0, )
	for  := int64(0);  < ; ++ {
		,  := strconv.ParseInt((), 10, 64)
		,  := strconv.ParseInt((), 10, 64)
		if  != nil ||  != nil {
			return nil, ErrHeader
		}
		 = append(, sparseEntry{Offset: , Length: })
	}
	return , nil
}

// readGNUSparseMap0x1 reads the sparse map as stored in GNU's PAX sparse format
// version 0.1. The sparse map is stored in the PAX headers.
func readGNUSparseMap0x1( map[string]string) (sparseDatas, error) {
	// Get number of entries.
	// Use integer overflow resistant math to check this.
	 := [paxGNUSparseNumBlocks]
	,  := strconv.ParseInt(, 10, 0) // Intentionally parse as native int
	if  != nil ||  < 0 || int(2*) < int() {
		return nil, ErrHeader
	}

	// There should be two numbers in sparseMap for each entry.
	 := strings.Split([paxGNUSparseMap], ",")
	if len() == 1 && [0] == "" {
		 = [:0]
	}
	if int64(len()) != 2* {
		return nil, ErrHeader
	}

	// Loop through the entries in the sparse map.
	// numEntries is trusted now.
	 := make(sparseDatas, 0, )
	for len() >= 2 {
		,  := strconv.ParseInt([0], 10, 64)
		,  := strconv.ParseInt([1], 10, 64)
		if  != nil ||  != nil {
			return nil, ErrHeader
		}
		 = append(, sparseEntry{Offset: , Length: })
		 = [2:]
	}
	return , nil
}

// Read reads from the current file in the tar archive.
// It returns (0, io.EOF) when it reaches the end of that file,
// until [Next] is called to advance to the next file.
//
// If the current file is sparse, then the regions marked as a hole
// are read back as NUL-bytes.
//
// Calling Read on special types like [TypeLink], [TypeSymlink], [TypeChar],
// [TypeBlock], [TypeDir], and [TypeFifo] returns (0, [io.EOF]) regardless of what
// the [Header.Size] claims.
func ( *Reader) ( []byte) (int, error) {
	if .err != nil {
		return 0, .err
	}
	,  := .curr.Read()
	if  != nil &&  != io.EOF {
		.err = 
	}
	return , 
}

// writeTo writes the content of the current file to w.
// The bytes written matches the number of remaining bytes in the current file.
//
// If the current file is sparse and w is an io.WriteSeeker,
// then writeTo uses Seek to skip past holes defined in Header.SparseHoles,
// assuming that skipped regions are filled with NULs.
// This always writes the last byte to ensure w is the right size.
//
// TODO(dsnet): Re-export this when adding sparse file support.
// See https://golang.org/issue/22735
func ( *Reader) ( io.Writer) (int64, error) {
	if .err != nil {
		return 0, .err
	}
	,  := .curr.WriteTo()
	if  != nil {
		.err = 
	}
	return , 
}

// regFileReader is a fileReader for reading data from a regular file entry.
type regFileReader struct {
	r  io.Reader // Underlying Reader
	nb int64     // Number of remaining bytes to read
}

func ( *regFileReader) ( []byte) ( int,  error) {
	if int64(len()) > .nb {
		 = [:.nb]
	}
	if len() > 0 {
		,  = .r.Read()
		.nb -= int64()
	}
	switch {
	case  == io.EOF && .nb > 0:
		return , io.ErrUnexpectedEOF
	case  == nil && .nb == 0:
		return , io.EOF
	default:
		return , 
	}
}

func ( *regFileReader) ( io.Writer) (int64, error) {
	return io.Copy(, struct{ io.Reader }{})
}

// logicalRemaining implements fileState.logicalRemaining.
func ( regFileReader) () int64 {
	return .nb
}

// physicalRemaining implements fileState.physicalRemaining.
func ( regFileReader) () int64 {
	return .nb
}

// sparseFileReader is a fileReader for reading data from a sparse file entry.
type sparseFileReader struct {
	fr  fileReader  // Underlying fileReader
	sp  sparseHoles // Normalized list of sparse holes
	pos int64       // Current position in sparse file
}

func ( *sparseFileReader) ( []byte) ( int,  error) {
	 := int64(len()) >= .logicalRemaining()
	if  {
		 = [:.logicalRemaining()]
	}

	 := 
	 := .pos + int64(len())
	for  > .pos &&  == nil {
		var  int // Bytes read in fragment
		,  := .sp[0].Offset, .sp[0].endOffset()
		if .pos <  { // In a data fragment
			 := [:min(int64(len()), -.pos)]
			,  = tryReadFull(.fr, )
		} else { // In a hole fragment
			 := [:min(int64(len()), -.pos)]
			,  = tryReadFull(zeroReader{}, )
		}
		 = [:]
		.pos += int64()
		if .pos >=  && len(.sp) > 1 {
			.sp = .sp[1:] // Ensure last fragment always remains
		}
	}

	 = len() - len()
	switch {
	case  == io.EOF:
		return , errMissData // Less data in dense file than sparse file
	case  != nil:
		return , 
	case .logicalRemaining() == 0 && .physicalRemaining() > 0:
		return , errUnrefData // More data in dense file than sparse file
	case :
		return , io.EOF
	default:
		return , nil
	}
}

func ( *sparseFileReader) ( io.Writer) ( int64,  error) {
	,  := .(io.WriteSeeker)
	if  {
		if ,  := .Seek(0, io.SeekCurrent);  != nil {
			 = false // Not all io.Seeker can really seek
		}
	}
	if ! {
		return io.Copy(, struct{ io.Reader }{})
	}

	var  bool
	 := .pos
	for .logicalRemaining() > 0 && ! &&  == nil {
		var  int64 // Size of fragment
		,  := .sp[0].Offset, .sp[0].endOffset()
		if .pos <  { // In a data fragment
			 =  - .pos
			,  = io.CopyN(, .fr, )
		} else { // In a hole fragment
			 =  - .pos
			if .physicalRemaining() == 0 {
				 = true
				--
			}
			_,  = .Seek(, io.SeekCurrent)
		}
		.pos += 
		if .pos >=  && len(.sp) > 1 {
			.sp = .sp[1:] // Ensure last fragment always remains
		}
	}

	// If the last fragment is a hole, then seek to 1-byte before EOF, and
	// write a single byte to ensure the file is the right size.
	if  &&  == nil {
		_,  = .Write([]byte{0})
		.pos++
	}

	 = .pos - 
	switch {
	case  == io.EOF:
		return , errMissData // Less data in dense file than sparse file
	case  != nil:
		return , 
	case .logicalRemaining() == 0 && .physicalRemaining() > 0:
		return , errUnrefData // More data in dense file than sparse file
	default:
		return , nil
	}
}

func ( sparseFileReader) () int64 {
	return .sp[len(.sp)-1].endOffset() - .pos
}
func ( sparseFileReader) () int64 {
	return .fr.physicalRemaining()
}

type zeroReader struct{}

func (zeroReader) ( []byte) (int, error) {
	clear()
	return len(), nil
}

// mustReadFull is like io.ReadFull except it returns
// io.ErrUnexpectedEOF when io.EOF is hit before len(b) bytes are read.
func mustReadFull( io.Reader,  []byte) (int, error) {
	,  := tryReadFull(, )
	if  == io.EOF {
		 = io.ErrUnexpectedEOF
	}
	return , 
}

// tryReadFull is like io.ReadFull except it returns
// io.EOF when it is hit before len(b) bytes are read.
func tryReadFull( io.Reader,  []byte) ( int,  error) {
	for len() >  &&  == nil {
		var  int
		,  = .Read([:])
		 += 
	}
	if len() ==  &&  == io.EOF {
		 = nil
	}
	return , 
}

// readSpecialFile is like io.ReadAll except it returns
// ErrFieldTooLong if more than maxSpecialFileSize is read.
func readSpecialFile( io.Reader) ([]byte, error) {
	,  := io.ReadAll(io.LimitReader(, maxSpecialFileSize+1))
	if len() > maxSpecialFileSize {
		return nil, ErrFieldTooLong
	}
	return , 
}

// discard skips n bytes in r, reporting an error if unable to do so.
func discard( io.Reader,  int64) error {
	// If possible, Seek to the last byte before the end of the data section.
	// Do this because Seek is often lazy about reporting errors; this will mask
	// the fact that the stream may be truncated. We can rely on the
	// io.CopyN done shortly afterwards to trigger any IO errors.
	var  int64 // Number of bytes skipped via Seek
	if ,  := .(io.Seeker);  &&  > 1 {
		// Not all io.Seeker can actually Seek. For example, os.Stdin implements
		// io.Seeker, but calling Seek always returns an error and performs
		// no action. Thus, we try an innocent seek to the current position
		// to see if Seek is really supported.
		,  := .Seek(0, io.SeekCurrent)
		if  >= 0 &&  == nil {
			// Seek seems supported, so perform the real Seek.
			,  := .Seek(-1, io.SeekCurrent)
			if  < 0 ||  != nil {
				return 
			}
			 =  - 
		}
	}

	,  := io.CopyN(io.Discard, , -)
	if  == io.EOF && + <  {
		 = io.ErrUnexpectedEOF
	}
	return 
}