src/time/zoneinfo_read.go - go - Git at Google

 // 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.

 // Parse "zoneinfo" time zone file.
 // This is a fairly standard file format used on OS X, Linux, BSD, Sun, and others.
 // See tzfile(5), https://en.wikipedia.org/wiki/Zoneinfo,
 // and ftp://munnari.oz.au/pub/oldtz/

 package time

 import (
 	"errors"
 	"runtime"
 	"syscall"
 )

 // maxFileSize is the max permitted size of files read by readFile.
 // As reference, the zoneinfo.zip distributed by Go is ~350 KB,
 // so 10MB is overkill.
 const maxFileSize = 10 << 20

 type fileSizeError string

 func (f fileSizeError) Error() string {
 	return "time: file " + string(f) + " is too large"
 }

 // Copies of io.Seek* constants to avoid importing "io":
 const (
 	seekStart   = 0
 	seekCurrent = 1
 	seekEnd     = 2
 )

 // Simple I/O interface to binary blob of data.
 type dataIO struct {
 	p     []byte
 	error bool
 }

 func (d *dataIO) read(n int) []byte {
 	if len(d.p) < n {
 		d.p = nil
 		d.error = true
 		return nil
 	}
 	p := d.p[0:n]
 	d.p = d.p[n:]
 	return p
 }

 func (d *dataIO) big4() (n uint32, ok bool) {
 	p := d.read(4)
 	if len(p) < 4 {
 		d.error = true
 		return 0, false
 	}
 	return uint32(p[3]) | uint32(p[2])<<8 | uint32(p[1])<<16 | uint32(p[0])<<24, true
 }

 func (d *dataIO) big8() (n uint64, ok bool) {
 	n1, ok1 := d.big4()
 	n2, ok2 := d.big4()
 	if !ok1 || !ok2 {
 		d.error = true
 		return 0, false
 	}
 	return (uint64(n1) << 32) | uint64(n2), true
 }

 func (d *dataIO) byte() (n byte, ok bool) {
 	p := d.read(1)
 	if len(p) < 1 {
 		d.error = true
 		return 0, false
 	}
 	return p[0], true
 }

 // Make a string by stopping at the first NUL
 func byteString(p []byte) string {
 	for i := 0; i < len(p); i++ {
 		if p[i] == 0 {
 			return string(p[0:i])
 		}
 	}
 	return string(p)
 }

 var badData = errors.New("malformed time zone information")

 // LoadLocationFromTZData returns a Location with the given name
 // initialized from the IANA Time Zone database-formatted data.
 // The data should be in the format of a standard IANA time zone file
 // (for example, the content of /etc/localtime on Unix systems).
 func LoadLocationFromTZData(name string, data []byte) (*Location, error) {
 	d := dataIO{data, false}

 	// 4-byte magic "TZif"
 	if magic := d.read(4); string(magic) != "TZif" {
 		return nil, badData
 	}

 	// 1-byte version, then 15 bytes of padding
 	var version int
 	var p []byte
 	if p = d.read(16); len(p) != 16 {
 		return nil, badData
 	} else {
 		switch p[0] {
 		case 0:
 			version = 1
 		case '2':
 			version = 2
 		case '3':
 			version = 3
 		default:
 			return nil, badData
 		}
 	}

 	// six big-endian 32-bit integers:
 	//	number of UTC/local indicators
 	//	number of standard/wall indicators
 	//	number of leap seconds
 	//	number of transition times
 	//	number of local time zones
 	//	number of characters of time zone abbrev strings
 	const (
 		NUTCLocal = iota
 		NStdWall
 		NLeap
 		NTime
 		NZone
 		NChar
 	)
 	var n [6]int
 	for i := 0; i < 6; i++ {
 		nn, ok := d.big4()
 		if !ok {
 			return nil, badData
 		}
 		if uint32(int(nn)) != nn {
 			return nil, badData
 		}
 		n[i] = int(nn)
 	}

 	// If we have version 2 or 3, then the data is first written out
 	// in a 32-bit format, then written out again in a 64-bit format.
 	// Skip the 32-bit format and read the 64-bit one, as it can
 	// describe a broader range of dates.

 	is64 := false
 	if version > 1 {
 		// Skip the 32-bit data.
 		skip := n[NTime]*4 +
 			n[NTime] +
 			n[NZone]*6 +
 			n[NChar] +
 			n[NLeap]*8 +
 			n[NStdWall] +
 			n[NUTCLocal]
 		// Skip the version 2 header that we just read.
 		skip += 4 + 16
 		d.read(skip)

 		is64 = true

 		// Read the counts again, they can differ.
 		for i := 0; i < 6; i++ {
 			nn, ok := d.big4()
 			if !ok {
 				return nil, badData
 			}
 			if uint32(int(nn)) != nn {
 				return nil, badData
 			}
 			n[i] = int(nn)
 		}
 	}

 	size := 4
 	if is64 {
 		size = 8
 	}

 	// Transition times.
 	txtimes := dataIO{d.read(n[NTime] * size), false}

 	// Time zone indices for transition times.
 	txzones := d.read(n[NTime])

 	// Zone info structures
 	zonedata := dataIO{d.read(n[NZone] * 6), false}

 	// Time zone abbreviations.
 	abbrev := d.read(n[NChar])

 	// Leap-second time pairs
 	d.read(n[NLeap] * (size + 4))

 	// Whether tx times associated with local time types
 	// are specified as standard time or wall time.
 	isstd := d.read(n[NStdWall])

 	// Whether tx times associated with local time types
 	// are specified as UTC or local time.
 	isutc := d.read(n[NUTCLocal])

 	if d.error { // ran out of data
 		return nil, badData
 	}

 	// Now we can build up a useful data structure.
 	// First the zone information.
 	//	utcoff[4] isdst[1] nameindex[1]
 	nzone := n[NZone]
 	if nzone == 0 {
 		// Reject tzdata files with no zones. There's nothing useful in them.
 		// This also avoids a panic later when we add and then use a fake transition (golang.org/issue/29437).
 		return nil, badData
 	}
 	zone := make([]zone, nzone)
 	for i := range zone {
 		var ok bool
 		var n uint32
 		if n, ok = zonedata.big4(); !ok {
 			return nil, badData
 		}
 		if uint32(int(n)) != n {
 			return nil, badData
 		}
 		zone[i].offset = int(int32(n))
 		var b byte
 		if b, ok = zonedata.byte(); !ok {
 			return nil, badData
 		}
 		zone[i].isDST = b != 0
 		if b, ok = zonedata.byte(); !ok || int(b) >= len(abbrev) {
 			return nil, badData
 		}
 		zone[i].name = byteString(abbrev[b:])
 		if runtime.GOOS == "aix" && len(name) > 8 && (name[:8] == "Etc/GMT+" || name[:8] == "Etc/GMT-") {
 			// There is a bug with AIX 7.2 TL 0 with files in Etc,
 			// GMT+1 will return GMT-1 instead of GMT+1 or -01.
 			if name != "Etc/GMT+0" {
 				// GMT+0 is OK
 				zone[i].name = name[4:]
 			}
 		}
 	}

 	// Now the transition time info.
 	tx := make([]zoneTrans, n[NTime])
 	for i := range tx {
 		var n int64
 		if !is64 {
 			if n4, ok := txtimes.big4(); !ok {
 				return nil, badData
 			} else {
 				n = int64(int32(n4))
 			}
 		} else {
 			if n8, ok := txtimes.big8(); !ok {
 				return nil, badData
 			} else {
 				n = int64(n8)
 			}
 		}
 		tx[i].when = n
 		if int(txzones[i]) >= len(zone) {
 			return nil, badData
 		}
 		tx[i].index = txzones[i]
 		if i < len(isstd) {
 			tx[i].isstd = isstd[i] != 0
 		}
 		if i < len(isutc) {
 			tx[i].isutc = isutc[i] != 0
 		}
 	}

 	if len(tx) == 0 {
 		// Build fake transition to cover all time.
 		// This happens in fixed locations like "Etc/GMT0".
 		tx = append(tx, zoneTrans{when: alpha, index: 0})
 	}

 	// Committed to succeed.
 	l := &Location{zone: zone, tx: tx, name: name}

 	// Fill in the cache with information about right now,
 	// since that will be the most common lookup.
 	sec, _, _ := now()
 	for i := range tx {
 		if tx[i].when <= sec && (i+1 == len(tx) || sec < tx[i+1].when) {
 			l.cacheStart = tx[i].when
 			l.cacheEnd = omega
 			if i+1 < len(tx) {
 				l.cacheEnd = tx[i+1].when
 			}
 			l.cacheZone = &l.zone[tx[i].index]
 		}
 	}

 	return l, nil
 }

 // loadTzinfoFromDirOrZip returns the contents of the file with the given name
 // in dir. dir can either be an uncompressed zip file, or a directory.
 func loadTzinfoFromDirOrZip(dir, name string) ([]byte, error) {
 	if len(dir) > 4 && dir[len(dir)-4:] == ".zip" {
 		return loadTzinfoFromZip(dir, name)
 	}
 	if dir != "" {
 		name = dir + "/" + name
 	}
 	return readFile(name)
 }

 // There are 500+ zoneinfo files. Rather than distribute them all
 // individually, we ship them in an uncompressed zip file.
 // Used this way, the zip file format serves as a commonly readable
 // container for the individual small files. We choose zip over tar
 // because zip files have a contiguous table of contents, making
 // individual file lookups faster, and because the per-file overhead
 // in a zip file is considerably less than tar's 512 bytes.

 // get4 returns the little-endian 32-bit value in b.
 func get4(b []byte) int {
 	if len(b) < 4 {
 		return 0
 	}
 	return int(b[0]) | int(b[1])<<8 | int(b[2])<<16 | int(b[3])<<24
 }

 // get2 returns the little-endian 16-bit value in b.
 func get2(b []byte) int {
 	if len(b) < 2 {
 		return 0
 	}
 	return int(b[0]) | int(b[1])<<8
 }

 // loadTzinfoFromZip returns the contents of the file with the given name
 // in the given uncompressed zip file.
 func loadTzinfoFromZip(zipfile, name string) ([]byte, error) {
 	fd, err := open(zipfile)
 	if err != nil {
 		return nil, err
 	}
 	defer closefd(fd)

 	const (
 		zecheader = 0x06054b50
 		zcheader  = 0x02014b50
 		ztailsize = 22

 		zheadersize = 30
 		zheader     = 0x04034b50
 	)

 	buf := make([]byte, ztailsize)
 	if err := preadn(fd, buf, -ztailsize); err != nil || get4(buf) != zecheader {
 		return nil, errors.New("corrupt zip file " + zipfile)
 	}
 	n := get2(buf[10:])
 	size := get4(buf[12:])
 	off := get4(buf[16:])

 	buf = make([]byte, size)
 	if err := preadn(fd, buf, off); err != nil {
 		return nil, errors.New("corrupt zip file " + zipfile)
 	}

 	for i := 0; i < n; i++ {
 		// zip entry layout:
 		//	0	magic[4]
 		//	4	madevers[1]
 		//	5	madeos[1]
 		//	6	extvers[1]
 		//	7	extos[1]
 		//	8	flags[2]
 		//	10	meth[2]
 		//	12	modtime[2]
 		//	14	moddate[2]
 		//	16	crc[4]
 		//	20	csize[4]
 		//	24	uncsize[4]
 		//	28	namelen[2]
 		//	30	xlen[2]
 		//	32	fclen[2]
 		//	34	disknum[2]
 		//	36	iattr[2]
 		//	38	eattr[4]
 		//	42	off[4]
 		//	46	name[namelen]
 		//	46+namelen+xlen+fclen - next header
 		//
 		if get4(buf) != zcheader {
 			break
 		}
 		meth := get2(buf[10:])
 		size := get4(buf[24:])
 		namelen := get2(buf[28:])
 		xlen := get2(buf[30:])
 		fclen := get2(buf[32:])
 		off := get4(buf[42:])
 		zname := buf[46 : 46+namelen]
 		buf = buf[46+namelen+xlen+fclen:]
 		if string(zname) != name {
 			continue
 		}
 		if meth != 0 {
 			return nil, errors.New("unsupported compression for " + name + " in " + zipfile)
 		}

 		// zip per-file header layout:
 		//	0	magic[4]
 		//	4	extvers[1]
 		//	5	extos[1]
 		//	6	flags[2]
 		//	8	meth[2]
 		//	10	modtime[2]
 		//	12	moddate[2]
 		//	14	crc[4]
 		//	18	csize[4]
 		//	22	uncsize[4]
 		//	26	namelen[2]
 		//	28	xlen[2]
 		//	30	name[namelen]
 		//	30+namelen+xlen - file data
 		//
 		buf = make([]byte, zheadersize+namelen)
 		if err := preadn(fd, buf, off); err != nil ||
 			get4(buf) != zheader ||
 			get2(buf[8:]) != meth ||
 			get2(buf[26:]) != namelen ||
 			string(buf[30:30+namelen]) != name {
 			return nil, errors.New("corrupt zip file " + zipfile)
 		}
 		xlen = get2(buf[28:])

 		buf = make([]byte, size)
 		if err := preadn(fd, buf, off+30+namelen+xlen); err != nil {
 			return nil, errors.New("corrupt zip file " + zipfile)
 		}

 		return buf, nil
 	}

 	return nil, syscall.ENOENT
 }

 // loadTzinfoFromTzdata returns the time zone information of the time zone
 // with the given name, from a tzdata database file as they are typically
 // found on android.
 var loadTzinfoFromTzdata func(file, name string) ([]byte, error)

 // loadTzinfo returns the time zone information of the time zone
 // with the given name, from a given source. A source may be a
 // timezone database directory, tzdata database file or an uncompressed
 // zip file, containing the contents of such a directory.
 func loadTzinfo(name string, source string) ([]byte, error) {
 	if len(source) >= 6 && source[len(source)-6:] == "tzdata" {
 		return loadTzinfoFromTzdata(source, name)
 	}
 	return loadTzinfoFromDirOrZip(source, name)
 }

 // loadLocation returns the Location with the given name from one of
 // the specified sources. See loadTzinfo for a list of supported sources.
 // The first timezone data matching the given name that is successfully loaded
 // and parsed is returned as a Location.
 func loadLocation(name string, sources []string) (z *Location, firstErr error) {
 	for _, source := range sources {
 		var zoneData, err = loadTzinfo(name, source)
 		if err == nil {
 			if z, err = LoadLocationFromTZData(name, zoneData); err == nil {
 				return z, nil
 			}
 		}
 		if firstErr == nil && err != syscall.ENOENT {
 			firstErr = err
 		}
 	}
 	if firstErr != nil {
 		return nil, firstErr
 	}
 	return nil, errors.New("unknown time zone " + name)
 }

 // readFile reads and returns the content of the named file.
 // It is a trivial implementation of ioutil.ReadFile, reimplemented
 // here to avoid depending on io/ioutil or os.
 // It returns an error if name exceeds maxFileSize bytes.
 func readFile(name string) ([]byte, error) {
 	f, err := open(name)
 	if err != nil {
 		return nil, err
 	}
 	defer closefd(f)
 	var (
 		buf [4096]byte
 		ret []byte
 		n   int
 	)
 	for {
 		n, err = read(f, buf[:])
 		if n > 0 {
 			ret = append(ret, buf[:n]...)
 		}
 		if n == 0 || err != nil {
 			break
 		}
 		if len(ret) > maxFileSize {
 			return nil, fileSizeError(name)
 		}
 	}
 	return ret, err
 }
	// 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.

	// Parse "zoneinfo" time zone file.
	// This is a fairly standard file format used on OS X, Linux, BSD, Sun, and others.
	// See tzfile(5), https://en.wikipedia.org/wiki/Zoneinfo,
	// and ftp://munnari.oz.au/pub/oldtz/

	package time

	import (
	"errors"
	"runtime"
	"syscall"
	)

	// maxFileSize is the max permitted size of files read by readFile.
	// As reference, the zoneinfo.zip distributed by Go is ~350 KB,
	// so 10MB is overkill.
	const maxFileSize = 10 << 20

	type fileSizeError string

	func (f fileSizeError) Error() string {
	return "time: file " + string(f) + " is too large"
	}

	// Copies of io.Seek* constants to avoid importing "io":
	const (
	seekStart = 0
	seekCurrent = 1
	seekEnd = 2
	)

	// Simple I/O interface to binary blob of data.
	type dataIO struct {
	p []byte
	error bool
	}

	func (d *dataIO) read(n int) []byte {
	if len(d.p) < n {
	d.p = nil
	d.error = true
	return nil
	}
	p := d.p[0:n]
	d.p = d.p[n:]
	return p
	}

	func (d *dataIO) big4() (n uint32, ok bool) {
	p := d.read(4)
	if len(p) < 4 {
	d.error = true
	return 0, false
	}
	return uint32(p[3]) \| uint32(p[2])<<8 \| uint32(p[1])<<16 \| uint32(p[0])<<24, true
	}

	func (d *dataIO) big8() (n uint64, ok bool) {
	n1, ok1 := d.big4()
	n2, ok2 := d.big4()
	if !ok1 \|\| !ok2 {
	d.error = true
	return 0, false
	}
	return (uint64(n1) << 32) \| uint64(n2), true
	}

	func (d *dataIO) byte() (n byte, ok bool) {
	p := d.read(1)
	if len(p) < 1 {
	d.error = true
	return 0, false
	}
	return p[0], true
	}

	// Make a string by stopping at the first NUL
	func byteString(p []byte) string {
	for i := 0; i < len(p); i++ {
	if p[i] == 0 {
	return string(p[0:i])
	}
	}
	return string(p)
	}

	var badData = errors.New("malformed time zone information")

	// LoadLocationFromTZData returns a Location with the given name
	// initialized from the IANA Time Zone database-formatted data.
	// The data should be in the format of a standard IANA time zone file
	// (for example, the content of /etc/localtime on Unix systems).
	func LoadLocationFromTZData(name string, data []byte) (*Location, error) {
	d := dataIO{data, false}

	// 4-byte magic "TZif"
	if magic := d.read(4); string(magic) != "TZif" {
	return nil, badData
	}

	// 1-byte version, then 15 bytes of padding
	var version int
	var p []byte
	if p = d.read(16); len(p) != 16 {
	return nil, badData
	} else {
	switch p[0] {
	case 0:
	version = 1
	case '2':
	version = 2
	case '3':
	version = 3
	default:
	return nil, badData
	}
	}

	// six big-endian 32-bit integers:
	// number of UTC/local indicators
	// number of standard/wall indicators
	// number of leap seconds
	// number of transition times
	// number of local time zones
	// number of characters of time zone abbrev strings
	const (
	NUTCLocal = iota
	NStdWall
	NLeap
	NTime
	NZone
	NChar
	)
	var n [6]int
	for i := 0; i < 6; i++ {
	nn, ok := d.big4()
	if !ok {
	return nil, badData
	}
	if uint32(int(nn)) != nn {
	return nil, badData
	}
	n[i] = int(nn)
	}

	// If we have version 2 or 3, then the data is first written out
	// in a 32-bit format, then written out again in a 64-bit format.
	// Skip the 32-bit format and read the 64-bit one, as it can
	// describe a broader range of dates.

	is64 := false
	if version > 1 {
	// Skip the 32-bit data.
	skip := n[NTime]*4 +
	n[NTime] +
	n[NZone]*6 +
	n[NChar] +
	n[NLeap]*8 +
	n[NStdWall] +
	n[NUTCLocal]
	// Skip the version 2 header that we just read.
	skip += 4 + 16
	d.read(skip)

	is64 = true

	// Read the counts again, they can differ.
	for i := 0; i < 6; i++ {
	nn, ok := d.big4()
	if !ok {
	return nil, badData
	}
	if uint32(int(nn)) != nn {
	return nil, badData
	}
	n[i] = int(nn)
	}
	}

	size := 4
	if is64 {
	size = 8
	}

	// Transition times.
	txtimes := dataIO{d.read(n[NTime] * size), false}

	// Time zone indices for transition times.
	txzones := d.read(n[NTime])

	// Zone info structures
	zonedata := dataIO{d.read(n[NZone] * 6), false}

	// Time zone abbreviations.
	abbrev := d.read(n[NChar])

	// Leap-second time pairs
	d.read(n[NLeap] * (size + 4))

	// Whether tx times associated with local time types
	// are specified as standard time or wall time.
	isstd := d.read(n[NStdWall])

	// Whether tx times associated with local time types
	// are specified as UTC or local time.
	isutc := d.read(n[NUTCLocal])

	if d.error { // ran out of data
	return nil, badData
	}

	// Now we can build up a useful data structure.
	// First the zone information.
	// utcoff[4] isdst[1] nameindex[1]
	nzone := n[NZone]
	if nzone == 0 {
	// Reject tzdata files with no zones. There's nothing useful in them.
	// This also avoids a panic later when we add and then use a fake transition (golang.org/issue/29437).
	return nil, badData
	}
	zone := make([]zone, nzone)
	for i := range zone {
	var ok bool
	var n uint32
	if n, ok = zonedata.big4(); !ok {
	return nil, badData
	}
	if uint32(int(n)) != n {
	return nil, badData
	}
	zone[i].offset = int(int32(n))
	var b byte
	if b, ok = zonedata.byte(); !ok {
	return nil, badData
	}
	zone[i].isDST = b != 0
	if b, ok = zonedata.byte(); !ok \|\| int(b) >= len(abbrev) {
	return nil, badData
	}
	zone[i].name = byteString(abbrev[b:])
	if runtime.GOOS == "aix" && len(name) > 8 && (name[:8] == "Etc/GMT+" \|\| name[:8] == "Etc/GMT-") {
	// There is a bug with AIX 7.2 TL 0 with files in Etc,
	// GMT+1 will return GMT-1 instead of GMT+1 or -01.
	if name != "Etc/GMT+0" {
	// GMT+0 is OK
	zone[i].name = name[4:]
	}
	}
	}

	// Now the transition time info.
	tx := make([]zoneTrans, n[NTime])
	for i := range tx {
	var n int64
	if !is64 {
	if n4, ok := txtimes.big4(); !ok {
	return nil, badData
	} else {
	n = int64(int32(n4))
	}
	} else {
	if n8, ok := txtimes.big8(); !ok {
	return nil, badData
	} else {
	n = int64(n8)
	}
	}
	tx[i].when = n
	if int(txzones[i]) >= len(zone) {
	return nil, badData
	}
	tx[i].index = txzones[i]
	if i < len(isstd) {
	tx[i].isstd = isstd[i] != 0
	}
	if i < len(isutc) {
	tx[i].isutc = isutc[i] != 0
	}
	}

	if len(tx) == 0 {
	// Build fake transition to cover all time.
	// This happens in fixed locations like "Etc/GMT0".
	tx = append(tx, zoneTrans{when: alpha, index: 0})
	}

	// Committed to succeed.
	l := &Location{zone: zone, tx: tx, name: name}

	// Fill in the cache with information about right now,
	// since that will be the most common lookup.
	sec, _, _ := now()
	for i := range tx {
	if tx[i].when <= sec && (i+1 == len(tx) \|\| sec < tx[i+1].when) {
	l.cacheStart = tx[i].when
	l.cacheEnd = omega
	if i+1 < len(tx) {
	l.cacheEnd = tx[i+1].when
	}
	l.cacheZone = &l.zone[tx[i].index]
	}
	}

	return l, nil
	}

	// loadTzinfoFromDirOrZip returns the contents of the file with the given name
	// in dir. dir can either be an uncompressed zip file, or a directory.
	func loadTzinfoFromDirOrZip(dir, name string) ([]byte, error) {
	if len(dir) > 4 && dir[len(dir)-4:] == ".zip" {
	return loadTzinfoFromZip(dir, name)
	}
	if dir != "" {
	name = dir + "/" + name
	}
	return readFile(name)
	}

	// There are 500+ zoneinfo files. Rather than distribute them all
	// individually, we ship them in an uncompressed zip file.
	// Used this way, the zip file format serves as a commonly readable
	// container for the individual small files. We choose zip over tar
	// because zip files have a contiguous table of contents, making
	// individual file lookups faster, and because the per-file overhead
	// in a zip file is considerably less than tar's 512 bytes.

	// get4 returns the little-endian 32-bit value in b.
	func get4(b []byte) int {
	if len(b) < 4 {
	return 0
	}
	return int(b[0]) \| int(b[1])<<8 \| int(b[2])<<16 \| int(b[3])<<24
	}

	// get2 returns the little-endian 16-bit value in b.
	func get2(b []byte) int {
	if len(b) < 2 {
	return 0
	}
	return int(b[0]) \| int(b[1])<<8
	}

	// loadTzinfoFromZip returns the contents of the file with the given name
	// in the given uncompressed zip file.
	func loadTzinfoFromZip(zipfile, name string) ([]byte, error) {
	fd, err := open(zipfile)
	if err != nil {
	return nil, err
	}
	defer closefd(fd)

	const (
	zecheader = 0x06054b50
	zcheader = 0x02014b50
	ztailsize = 22

	zheadersize = 30
	zheader = 0x04034b50
	)

	buf := make([]byte, ztailsize)
	if err := preadn(fd, buf, -ztailsize); err != nil \|\| get4(buf) != zecheader {
	return nil, errors.New("corrupt zip file " + zipfile)
	}
	n := get2(buf[10:])
	size := get4(buf[12:])
	off := get4(buf[16:])

	buf = make([]byte, size)
	if err := preadn(fd, buf, off); err != nil {
	return nil, errors.New("corrupt zip file " + zipfile)
	}

	for i := 0; i < n; i++ {
	// zip entry layout:
	// 0 magic[4]
	// 4 madevers[1]
	// 5 madeos[1]
	// 6 extvers[1]
	// 7 extos[1]
	// 8 flags[2]
	// 10 meth[2]
	// 12 modtime[2]
	// 14 moddate[2]
	// 16 crc[4]
	// 20 csize[4]
	// 24 uncsize[4]
	// 28 namelen[2]
	// 30 xlen[2]
	// 32 fclen[2]
	// 34 disknum[2]
	// 36 iattr[2]
	// 38 eattr[4]
	// 42 off[4]
	// 46 name[namelen]
	// 46+namelen+xlen+fclen - next header
	//
	if get4(buf) != zcheader {
	break
	}
	meth := get2(buf[10:])
	size := get4(buf[24:])
	namelen := get2(buf[28:])
	xlen := get2(buf[30:])
	fclen := get2(buf[32:])
	off := get4(buf[42:])
	zname := buf[46 : 46+namelen]
	buf = buf[46+namelen+xlen+fclen:]
	if string(zname) != name {
	continue
	}
	if meth != 0 {
	return nil, errors.New("unsupported compression for " + name + " in " + zipfile)
	}

	// zip per-file header layout:
	// 0 magic[4]
	// 4 extvers[1]
	// 5 extos[1]
	// 6 flags[2]
	// 8 meth[2]
	// 10 modtime[2]
	// 12 moddate[2]
	// 14 crc[4]
	// 18 csize[4]
	// 22 uncsize[4]
	// 26 namelen[2]
	// 28 xlen[2]
	// 30 name[namelen]
	// 30+namelen+xlen - file data
	//
	buf = make([]byte, zheadersize+namelen)
	if err := preadn(fd, buf, off); err != nil \|\|
	get4(buf) != zheader \|\|
	get2(buf[8:]) != meth \|\|
	get2(buf[26:]) != namelen \|\|
	string(buf[30:30+namelen]) != name {
	return nil, errors.New("corrupt zip file " + zipfile)
	}
	xlen = get2(buf[28:])

	buf = make([]byte, size)
	if err := preadn(fd, buf, off+30+namelen+xlen); err != nil {
	return nil, errors.New("corrupt zip file " + zipfile)
	}

	return buf, nil
	}

	return nil, syscall.ENOENT
	}

	// loadTzinfoFromTzdata returns the time zone information of the time zone
	// with the given name, from a tzdata database file as they are typically
	// found on android.
	var loadTzinfoFromTzdata func(file, name string) ([]byte, error)

	// loadTzinfo returns the time zone information of the time zone
	// with the given name, from a given source. A source may be a
	// timezone database directory, tzdata database file or an uncompressed
	// zip file, containing the contents of such a directory.
	func loadTzinfo(name string, source string) ([]byte, error) {
	if len(source) >= 6 && source[len(source)-6:] == "tzdata" {
	return loadTzinfoFromTzdata(source, name)
	}
	return loadTzinfoFromDirOrZip(source, name)
	}

	// loadLocation returns the Location with the given name from one of
	// the specified sources. See loadTzinfo for a list of supported sources.
	// The first timezone data matching the given name that is successfully loaded
	// and parsed is returned as a Location.
	func loadLocation(name string, sources []string) (z *Location, firstErr error) {
	for _, source := range sources {
	var zoneData, err = loadTzinfo(name, source)
	if err == nil {
	if z, err = LoadLocationFromTZData(name, zoneData); err == nil {
	return z, nil
	}
	}
	if firstErr == nil && err != syscall.ENOENT {
	firstErr = err
	}
	}
	if firstErr != nil {
	return nil, firstErr
	}
	return nil, errors.New("unknown time zone " + name)
	}

	// readFile reads and returns the content of the named file.
	// It is a trivial implementation of ioutil.ReadFile, reimplemented
	// here to avoid depending on io/ioutil or os.
	// It returns an error if name exceeds maxFileSize bytes.
	func readFile(name string) ([]byte, error) {
	f, err := open(name)
	if err != nil {
	return nil, err
	}
	defer closefd(f)
	var (
	buf [4096]byte
	ret []byte
	n int
	)
	for {
	n, err = read(f, buf[:])
	if n > 0 {
	ret = append(ret, buf[:n]...)
	}
	if n == 0 \|\| err != nil {
	break
	}
	if len(ret) > maxFileSize {
	return nil, fileSizeError(name)
	}
	}
	return ret, err
	}