http2/hpack/hpack.go - net - Git at Google

 // Copyright 2014 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 hpack implements HPACK, a compression format for
 // efficiently representing HTTP header fields in the context of HTTP/2.
 //
 // See http://tools.ietf.org/html/draft-ietf-httpbis-header-compression-09
 package hpack

 import (
 	"bytes"
 	"errors"
 	"fmt"
 )

 // A DecodingError is something the spec defines as a decoding error.
 type DecodingError struct {
 	Err error
 }

 func (de DecodingError) Error() string {
 	return fmt.Sprintf("decoding error: %v", de.Err)
 }

 // An InvalidIndexError is returned when an encoder references a table
 // entry before the static table or after the end of the dynamic table.
 type InvalidIndexError int

 func (e InvalidIndexError) Error() string {
 	return fmt.Sprintf("invalid indexed representation index %d", int(e))
 }

 // A HeaderField is a name-value pair. Both the name and value are
 // treated as opaque sequences of octets.
 type HeaderField struct {
 	Name, Value string

 	// Sensitive means that this header field should never be
 	// indexed.
 	Sensitive bool
 }

 // IsPseudo reports whether the header field is an http2 pseudo header.
 // That is, it reports whether it starts with a colon.
 // It is not otherwise guaranteed to be a valid pseudo header field,
 // though.
 func (hf HeaderField) IsPseudo() bool {
 	return len(hf.Name) != 0 && hf.Name[0] == ':'
 }

 func (hf HeaderField) String() string {
 	var suffix string
 	if hf.Sensitive {
 		suffix = " (sensitive)"
 	}
 	return fmt.Sprintf("header field %q = %q%s", hf.Name, hf.Value, suffix)
 }

 // Size returns the size of an entry per RFC 7541 section 4.1.
 func (hf HeaderField) Size() uint32 {
 	// http://http2.github.io/http2-spec/compression.html#rfc.section.4.1
 	// "The size of the dynamic table is the sum of the size of
 	// its entries. The size of an entry is the sum of its name's
 	// length in octets (as defined in Section 5.2), its value's
 	// length in octets (see Section 5.2), plus 32.  The size of
 	// an entry is calculated using the length of the name and
 	// value without any Huffman encoding applied."

 	// This can overflow if somebody makes a large HeaderField
 	// Name and/or Value by hand, but we don't care, because that
 	// won't happen on the wire because the encoding doesn't allow
 	// it.
 	return uint32(len(hf.Name) + len(hf.Value) + 32)
 }

 // A Decoder is the decoding context for incremental processing of
 // header blocks.
 type Decoder struct {
 	dynTab dynamicTable
 	emit   func(f HeaderField)

 	emitEnabled bool // whether calls to emit are enabled
 	maxStrLen   int  // 0 means unlimited

 	// buf is the unparsed buffer. It's only written to
 	// saveBuf if it was truncated in the middle of a header
 	// block. Because it's usually not owned, we can only
 	// process it under Write.
 	buf []byte // not owned; only valid during Write

 	// saveBuf is previous data passed to Write which we weren't able
 	// to fully parse before. Unlike buf, we own this data.
 	saveBuf bytes.Buffer
 }

 // NewDecoder returns a new decoder with the provided maximum dynamic
 // table size. The emitFunc will be called for each valid field
 // parsed, in the same goroutine as calls to Write, before Write returns.
 func NewDecoder(maxDynamicTableSize uint32, emitFunc func(f HeaderField)) *Decoder {
 	d := &Decoder{
 		emit:        emitFunc,
 		emitEnabled: true,
 	}
 	d.dynTab.table.init()
 	d.dynTab.allowedMaxSize = maxDynamicTableSize
 	d.dynTab.setMaxSize(maxDynamicTableSize)
 	return d
 }

 // ErrStringLength is returned by Decoder.Write when the max string length
 // (as configured by Decoder.SetMaxStringLength) would be violated.
 var ErrStringLength = errors.New("hpack: string too long")

 // SetMaxStringLength sets the maximum size of a HeaderField name or
 // value string. If a string exceeds this length (even after any
 // decompression), Write will return ErrStringLength.
 // A value of 0 means unlimited and is the default from NewDecoder.
 func (d *Decoder) SetMaxStringLength(n int) {
 	d.maxStrLen = n
 }

 // SetEmitFunc changes the callback used when new header fields
 // are decoded.
 // It must be non-nil. It does not affect EmitEnabled.
 func (d *Decoder) SetEmitFunc(emitFunc func(f HeaderField)) {
 	d.emit = emitFunc
 }

 // SetEmitEnabled controls whether the emitFunc provided to NewDecoder
 // should be called. The default is true.
 //
 // This facility exists to let servers enforce MAX_HEADER_LIST_SIZE
 // while still decoding and keeping in-sync with decoder state, but
 // without doing unnecessary decompression or generating unnecessary
 // garbage for header fields past the limit.
 func (d *Decoder) SetEmitEnabled(v bool) { d.emitEnabled = v }

 // EmitEnabled reports whether calls to the emitFunc provided to NewDecoder
 // are currently enabled. The default is true.
 func (d *Decoder) EmitEnabled() bool { return d.emitEnabled }

 // TODO: add method *Decoder.Reset(maxSize, emitFunc) to let callers re-use Decoders and their
 // underlying buffers for garbage reasons.

 func (d *Decoder) SetMaxDynamicTableSize(v uint32) {
 	d.dynTab.setMaxSize(v)
 }

 // SetAllowedMaxDynamicTableSize sets the upper bound that the encoded
 // stream (via dynamic table size updates) may set the maximum size
 // to.
 func (d *Decoder) SetAllowedMaxDynamicTableSize(v uint32) {
 	d.dynTab.allowedMaxSize = v
 }

 type dynamicTable struct {
 	// http://http2.github.io/http2-spec/compression.html#rfc.section.2.3.2
 	table          headerFieldTable
 	size           uint32 // in bytes
 	maxSize        uint32 // current maxSize
 	allowedMaxSize uint32 // maxSize may go up to this, inclusive
 }

 func (dt *dynamicTable) setMaxSize(v uint32) {
 	dt.maxSize = v
 	dt.evict()
 }

 func (dt *dynamicTable) add(f HeaderField) {
 	dt.table.addEntry(f)
 	dt.size += f.Size()
 	dt.evict()
 }

 // If we're too big, evict old stuff.
 func (dt *dynamicTable) evict() {
 	var n int
 	for dt.size > dt.maxSize && n < dt.table.len() {
 		dt.size -= dt.table.ents[n].Size()
 		n++
 	}
 	dt.table.evictOldest(n)
 }

 func (d *Decoder) maxTableIndex() int {
 	// This should never overflow. RFC 7540 Section 6.5.2 limits the size of
 	// the dynamic table to 2^32 bytes, where each entry will occupy more than
 	// one byte. Further, the staticTable has a fixed, small length.
 	return d.dynTab.table.len() + staticTable.len()
 }

 func (d *Decoder) at(i uint64) (hf HeaderField, ok bool) {
 	// See Section 2.3.3.
 	if i == 0 {
 		return
 	}
 	if i <= uint64(staticTable.len()) {
 		return staticTable.ents[i-1], true
 	}
 	if i > uint64(d.maxTableIndex()) {
 		return
 	}
 	// In the dynamic table, newer entries have lower indices.
 	// However, dt.ents[0] is the oldest entry. Hence, dt.ents is
 	// the reversed dynamic table.
 	dt := d.dynTab.table
 	return dt.ents[dt.len()-(int(i)-staticTable.len())], true
 }

 // Decode decodes an entire block.
 //
 // TODO: remove this method and make it incremental later? This is
 // easier for debugging now.
 func (d *Decoder) DecodeFull(p []byte) ([]HeaderField, error) {
 	var hf []HeaderField
 	saveFunc := d.emit
 	defer func() { d.emit = saveFunc }()
 	d.emit = func(f HeaderField) { hf = append(hf, f) }
 	if _, err := d.Write(p); err != nil {
 		return nil, err
 	}
 	if err := d.Close(); err != nil {
 		return nil, err
 	}
 	return hf, nil
 }

 func (d *Decoder) Close() error {
 	if d.saveBuf.Len() > 0 {
 		d.saveBuf.Reset()
 		return DecodingError{errors.New("truncated headers")}
 	}
 	return nil
 }

 func (d *Decoder) Write(p []byte) (n int, err error) {
 	if len(p) == 0 {
 		// Prevent state machine CPU attacks (making us redo
 		// work up to the point of finding out we don't have
 		// enough data)
 		return
 	}
 	// Only copy the data if we have to. Optimistically assume
 	// that p will contain a complete header block.
 	if d.saveBuf.Len() == 0 {
 		d.buf = p
 	} else {
 		d.saveBuf.Write(p)
 		d.buf = d.saveBuf.Bytes()
 		d.saveBuf.Reset()
 	}

 	for len(d.buf) > 0 {
 		err = d.parseHeaderFieldRepr()
 		if err == errNeedMore {
 			// Extra paranoia, making sure saveBuf won't
 			// get too large. All the varint and string
 			// reading code earlier should already catch
 			// overlong things and return ErrStringLength,
 			// but keep this as a last resort.
 			const varIntOverhead = 8 // conservative
 			if d.maxStrLen != 0 && int64(len(d.buf)) > 2*(int64(d.maxStrLen)+varIntOverhead) {
 				return 0, ErrStringLength
 			}
 			d.saveBuf.Write(d.buf)
 			return len(p), nil
 		}
 		if err != nil {
 			break
 		}
 	}
 	return len(p), err
 }

 // errNeedMore is an internal sentinel error value that means the
 // buffer is truncated and we need to read more data before we can
 // continue parsing.
 var errNeedMore = errors.New("need more data")

 type indexType int

 const (
 	indexedTrue indexType = iota
 	indexedFalse
 	indexedNever
 )

 func (v indexType) indexed() bool   { return v == indexedTrue }
 func (v indexType) sensitive() bool { return v == indexedNever }

 // returns errNeedMore if there isn't enough data available.
 // any other error is fatal.
 // consumes d.buf iff it returns nil.
 // precondition: must be called with len(d.buf) > 0
 func (d *Decoder) parseHeaderFieldRepr() error {
 	b := d.buf[0]
 	switch {
 	case b&128 != 0:
 		// Indexed representation.
 		// High bit set?
 		// http://http2.github.io/http2-spec/compression.html#rfc.section.6.1
 		return d.parseFieldIndexed()
 	case b&192 == 64:
 		// 6.2.1 Literal Header Field with Incremental Indexing
 		// 0b10xxxxxx: top two bits are 10
 		// http://http2.github.io/http2-spec/compression.html#rfc.section.6.2.1
 		return d.parseFieldLiteral(6, indexedTrue)
 	case b&240 == 0:
 		// 6.2.2 Literal Header Field without Indexing
 		// 0b0000xxxx: top four bits are 0000
 		// http://http2.github.io/http2-spec/compression.html#rfc.section.6.2.2
 		return d.parseFieldLiteral(4, indexedFalse)
 	case b&240 == 16:
 		// 6.2.3 Literal Header Field never Indexed
 		// 0b0001xxxx: top four bits are 0001
 		// http://http2.github.io/http2-spec/compression.html#rfc.section.6.2.3
 		return d.parseFieldLiteral(4, indexedNever)
 	case b&224 == 32:
 		// 6.3 Dynamic Table Size Update
 		// Top three bits are '001'.
 		// http://http2.github.io/http2-spec/compression.html#rfc.section.6.3
 		return d.parseDynamicTableSizeUpdate()
 	}

 	return DecodingError{errors.New("invalid encoding")}
 }

 // (same invariants and behavior as parseHeaderFieldRepr)
 func (d *Decoder) parseFieldIndexed() error {
 	buf := d.buf
 	idx, buf, err := readVarInt(7, buf)
 	if err != nil {
 		return err
 	}
 	hf, ok := d.at(idx)
 	if !ok {
 		return DecodingError{InvalidIndexError(idx)}
 	}
 	d.buf = buf
 	return d.callEmit(HeaderField{Name: hf.Name, Value: hf.Value})
 }

 // (same invariants and behavior as parseHeaderFieldRepr)
 func (d *Decoder) parseFieldLiteral(n uint8, it indexType) error {
 	buf := d.buf
 	nameIdx, buf, err := readVarInt(n, buf)
 	if err != nil {
 		return err
 	}

 	var hf HeaderField
 	wantStr := d.emitEnabled || it.indexed()
 	if nameIdx > 0 {
 		ihf, ok := d.at(nameIdx)
 		if !ok {
 			return DecodingError{InvalidIndexError(nameIdx)}
 		}
 		hf.Name = ihf.Name
 	} else {
 		hf.Name, buf, err = d.readString(buf, wantStr)
 		if err != nil {
 			return err
 		}
 	}
 	hf.Value, buf, err = d.readString(buf, wantStr)
 	if err != nil {
 		return err
 	}
 	d.buf = buf
 	if it.indexed() {
 		d.dynTab.add(hf)
 	}
 	hf.Sensitive = it.sensitive()
 	return d.callEmit(hf)
 }

 func (d *Decoder) callEmit(hf HeaderField) error {
 	if d.maxStrLen != 0 {
 		if len(hf.Name) > d.maxStrLen || len(hf.Value) > d.maxStrLen {
 			return ErrStringLength
 		}
 	}
 	if d.emitEnabled {
 		d.emit(hf)
 	}
 	return nil
 }

 // (same invariants and behavior as parseHeaderFieldRepr)
 func (d *Decoder) parseDynamicTableSizeUpdate() error {
 	buf := d.buf
 	size, buf, err := readVarInt(5, buf)
 	if err != nil {
 		return err
 	}
 	if size > uint64(d.dynTab.allowedMaxSize) {
 		return DecodingError{errors.New("dynamic table size update too large")}
 	}
 	d.dynTab.setMaxSize(uint32(size))
 	d.buf = buf
 	return nil
 }

 var errVarintOverflow = DecodingError{errors.New("varint integer overflow")}

 // readVarInt reads an unsigned variable length integer off the
 // beginning of p. n is the parameter as described in
 // http://http2.github.io/http2-spec/compression.html#rfc.section.5.1.
 //
 // n must always be between 1 and 8.
 //
 // The returned remain buffer is either a smaller suffix of p, or err != nil.
 // The error is errNeedMore if p doesn't contain a complete integer.
 func readVarInt(n byte, p []byte) (i uint64, remain []byte, err error) {
 	if n < 1 || n > 8 {
 		panic("bad n")
 	}
 	if len(p) == 0 {
 		return 0, p, errNeedMore
 	}
 	i = uint64(p[0])
 	if n < 8 {
 		i &= (1 << uint64(n)) - 1
 	}
 	if i < (1<<uint64(n))-1 {
 		return i, p[1:], nil
 	}

 	origP := p
 	p = p[1:]
 	var m uint64
 	for len(p) > 0 {
 		b := p[0]
 		p = p[1:]
 		i += uint64(b&127) << m
 		if b&128 == 0 {
 			return i, p, nil
 		}
 		m += 7
 		if m >= 63 { // TODO: proper overflow check. making this up.
 			return 0, origP, errVarintOverflow
 		}
 	}
 	return 0, origP, errNeedMore
 }

 // readString decodes an hpack string from p.
 //
 // wantStr is whether s will be used. If false, decompression and
 // []byte->string garbage are skipped if s will be ignored
 // anyway. This does mean that huffman decoding errors for non-indexed
 // strings past the MAX_HEADER_LIST_SIZE are ignored, but the server
 // is returning an error anyway, and because they're not indexed, the error
 // won't affect the decoding state.
 func (d *Decoder) readString(p []byte, wantStr bool) (s string, remain []byte, err error) {
 	if len(p) == 0 {
 		return "", p, errNeedMore
 	}
 	isHuff := p[0]&128 != 0
 	strLen, p, err := readVarInt(7, p)
 	if err != nil {
 		return "", p, err
 	}
 	if d.maxStrLen != 0 && strLen > uint64(d.maxStrLen) {
 		return "", nil, ErrStringLength
 	}
 	if uint64(len(p)) < strLen {
 		return "", p, errNeedMore
 	}
 	if !isHuff {
 		if wantStr {
 			s = string(p[:strLen])
 		}
 		return s, p[strLen:], nil
 	}

 	if wantStr {
 		buf := bufPool.Get().(*bytes.Buffer)
 		buf.Reset() // don't trust others
 		defer bufPool.Put(buf)
 		if err := huffmanDecode(buf, d.maxStrLen, p[:strLen]); err != nil {
 			buf.Reset()
 			return "", nil, err
 		}
 		s = buf.String()
 		buf.Reset() // be nice to GC
 	}
 	return s, p[strLen:], nil
 }
	// Copyright 2014 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 hpack implements HPACK, a compression format for
	// efficiently representing HTTP header fields in the context of HTTP/2.
	//
	// See http://tools.ietf.org/html/draft-ietf-httpbis-header-compression-09
	package hpack

	import (
	"bytes"
	"errors"
	"fmt"
	)

	// A DecodingError is something the spec defines as a decoding error.
	type DecodingError struct {
	Err error
	}

	func (de DecodingError) Error() string {
	return fmt.Sprintf("decoding error: %v", de.Err)
	}

	// An InvalidIndexError is returned when an encoder references a table
	// entry before the static table or after the end of the dynamic table.
	type InvalidIndexError int

	func (e InvalidIndexError) Error() string {
	return fmt.Sprintf("invalid indexed representation index %d", int(e))
	}

	// A HeaderField is a name-value pair. Both the name and value are
	// treated as opaque sequences of octets.
	type HeaderField struct {
	Name, Value string

	// Sensitive means that this header field should never be
	// indexed.
	Sensitive bool
	}

	// IsPseudo reports whether the header field is an http2 pseudo header.
	// That is, it reports whether it starts with a colon.
	// It is not otherwise guaranteed to be a valid pseudo header field,
	// though.
	func (hf HeaderField) IsPseudo() bool {
	return len(hf.Name) != 0 && hf.Name[0] == ':'
	}

	func (hf HeaderField) String() string {
	var suffix string
	if hf.Sensitive {
	suffix = " (sensitive)"
	}
	return fmt.Sprintf("header field %q = %q%s", hf.Name, hf.Value, suffix)
	}

	// Size returns the size of an entry per RFC 7541 section 4.1.
	func (hf HeaderField) Size() uint32 {
	// http://http2.github.io/http2-spec/compression.html#rfc.section.4.1
	// "The size of the dynamic table is the sum of the size of
	// its entries. The size of an entry is the sum of its name's
	// length in octets (as defined in Section 5.2), its value's
	// length in octets (see Section 5.2), plus 32. The size of
	// an entry is calculated using the length of the name and
	// value without any Huffman encoding applied."

	// This can overflow if somebody makes a large HeaderField
	// Name and/or Value by hand, but we don't care, because that
	// won't happen on the wire because the encoding doesn't allow
	// it.
	return uint32(len(hf.Name) + len(hf.Value) + 32)
	}

	// A Decoder is the decoding context for incremental processing of
	// header blocks.
	type Decoder struct {
	dynTab dynamicTable
	emit func(f HeaderField)

	emitEnabled bool // whether calls to emit are enabled
	maxStrLen int // 0 means unlimited

	// buf is the unparsed buffer. It's only written to
	// saveBuf if it was truncated in the middle of a header
	// block. Because it's usually not owned, we can only
	// process it under Write.
	buf []byte // not owned; only valid during Write

	// saveBuf is previous data passed to Write which we weren't able
	// to fully parse before. Unlike buf, we own this data.
	saveBuf bytes.Buffer
	}

	// NewDecoder returns a new decoder with the provided maximum dynamic
	// table size. The emitFunc will be called for each valid field
	// parsed, in the same goroutine as calls to Write, before Write returns.
	func NewDecoder(maxDynamicTableSize uint32, emitFunc func(f HeaderField)) *Decoder {
	d := &Decoder{
	emit: emitFunc,
	emitEnabled: true,
	}
	d.dynTab.table.init()
	d.dynTab.allowedMaxSize = maxDynamicTableSize
	d.dynTab.setMaxSize(maxDynamicTableSize)
	return d
	}

	// ErrStringLength is returned by Decoder.Write when the max string length
	// (as configured by Decoder.SetMaxStringLength) would be violated.
	var ErrStringLength = errors.New("hpack: string too long")

	// SetMaxStringLength sets the maximum size of a HeaderField name or
	// value string. If a string exceeds this length (even after any
	// decompression), Write will return ErrStringLength.
	// A value of 0 means unlimited and is the default from NewDecoder.
	func (d *Decoder) SetMaxStringLength(n int) {
	d.maxStrLen = n
	}

	// SetEmitFunc changes the callback used when new header fields
	// are decoded.
	// It must be non-nil. It does not affect EmitEnabled.
	func (d *Decoder) SetEmitFunc(emitFunc func(f HeaderField)) {
	d.emit = emitFunc
	}

	// SetEmitEnabled controls whether the emitFunc provided to NewDecoder
	// should be called. The default is true.
	//
	// This facility exists to let servers enforce MAX_HEADER_LIST_SIZE
	// while still decoding and keeping in-sync with decoder state, but
	// without doing unnecessary decompression or generating unnecessary
	// garbage for header fields past the limit.
	func (d *Decoder) SetEmitEnabled(v bool) { d.emitEnabled = v }

	// EmitEnabled reports whether calls to the emitFunc provided to NewDecoder
	// are currently enabled. The default is true.
	func (d *Decoder) EmitEnabled() bool { return d.emitEnabled }

	// TODO: add method *Decoder.Reset(maxSize, emitFunc) to let callers re-use Decoders and their
	// underlying buffers for garbage reasons.

	func (d *Decoder) SetMaxDynamicTableSize(v uint32) {
	d.dynTab.setMaxSize(v)
	}

	// SetAllowedMaxDynamicTableSize sets the upper bound that the encoded
	// stream (via dynamic table size updates) may set the maximum size
	// to.
	func (d *Decoder) SetAllowedMaxDynamicTableSize(v uint32) {
	d.dynTab.allowedMaxSize = v
	}

	type dynamicTable struct {
	// http://http2.github.io/http2-spec/compression.html#rfc.section.2.3.2
	table headerFieldTable
	size uint32 // in bytes
	maxSize uint32 // current maxSize
	allowedMaxSize uint32 // maxSize may go up to this, inclusive
	}

	func (dt *dynamicTable) setMaxSize(v uint32) {
	dt.maxSize = v
	dt.evict()
	}

	func (dt *dynamicTable) add(f HeaderField) {
	dt.table.addEntry(f)
	dt.size += f.Size()
	dt.evict()
	}

	// If we're too big, evict old stuff.
	func (dt *dynamicTable) evict() {
	var n int
	for dt.size > dt.maxSize && n < dt.table.len() {
	dt.size -= dt.table.ents[n].Size()
	n++
	}
	dt.table.evictOldest(n)
	}

	func (d *Decoder) maxTableIndex() int {
	// This should never overflow. RFC 7540 Section 6.5.2 limits the size of
	// the dynamic table to 2^32 bytes, where each entry will occupy more than
	// one byte. Further, the staticTable has a fixed, small length.
	return d.dynTab.table.len() + staticTable.len()
	}

	func (d *Decoder) at(i uint64) (hf HeaderField, ok bool) {
	// See Section 2.3.3.
	if i == 0 {
	return
	}
	if i <= uint64(staticTable.len()) {
	return staticTable.ents[i-1], true
	}
	if i > uint64(d.maxTableIndex()) {
	return
	}
	// In the dynamic table, newer entries have lower indices.
	// However, dt.ents[0] is the oldest entry. Hence, dt.ents is
	// the reversed dynamic table.
	dt := d.dynTab.table
	return dt.ents[dt.len()-(int(i)-staticTable.len())], true
	}

	// Decode decodes an entire block.
	//
	// TODO: remove this method and make it incremental later? This is
	// easier for debugging now.
	func (d *Decoder) DecodeFull(p []byte) ([]HeaderField, error) {
	var hf []HeaderField
	saveFunc := d.emit
	defer func() { d.emit = saveFunc }()
	d.emit = func(f HeaderField) { hf = append(hf, f) }
	if _, err := d.Write(p); err != nil {
	return nil, err
	}
	if err := d.Close(); err != nil {
	return nil, err
	}
	return hf, nil
	}

	func (d *Decoder) Close() error {
	if d.saveBuf.Len() > 0 {
	d.saveBuf.Reset()
	return DecodingError{errors.New("truncated headers")}
	}
	return nil
	}

	func (d *Decoder) Write(p []byte) (n int, err error) {
	if len(p) == 0 {
	// Prevent state machine CPU attacks (making us redo
	// work up to the point of finding out we don't have
	// enough data)
	return
	}
	// Only copy the data if we have to. Optimistically assume
	// that p will contain a complete header block.
	if d.saveBuf.Len() == 0 {
	d.buf = p
	} else {
	d.saveBuf.Write(p)
	d.buf = d.saveBuf.Bytes()
	d.saveBuf.Reset()
	}

	for len(d.buf) > 0 {
	err = d.parseHeaderFieldRepr()
	if err == errNeedMore {
	// Extra paranoia, making sure saveBuf won't
	// get too large. All the varint and string
	// reading code earlier should already catch
	// overlong things and return ErrStringLength,
	// but keep this as a last resort.
	const varIntOverhead = 8 // conservative
	if d.maxStrLen != 0 && int64(len(d.buf)) > 2*(int64(d.maxStrLen)+varIntOverhead) {
	return 0, ErrStringLength
	}
	d.saveBuf.Write(d.buf)
	return len(p), nil
	}
	if err != nil {
	break
	}
	}
	return len(p), err
	}

	// errNeedMore is an internal sentinel error value that means the
	// buffer is truncated and we need to read more data before we can
	// continue parsing.
	var errNeedMore = errors.New("need more data")

	type indexType int

	const (
	indexedTrue indexType = iota
	indexedFalse
	indexedNever
	)

	func (v indexType) indexed() bool { return v == indexedTrue }
	func (v indexType) sensitive() bool { return v == indexedNever }

	// returns errNeedMore if there isn't enough data available.
	// any other error is fatal.
	// consumes d.buf iff it returns nil.
	// precondition: must be called with len(d.buf) > 0
	func (d *Decoder) parseHeaderFieldRepr() error {
	b := d.buf[0]
	switch {
	case b&128 != 0:
	// Indexed representation.
	// High bit set?
	// http://http2.github.io/http2-spec/compression.html#rfc.section.6.1
	return d.parseFieldIndexed()
	case b&192 == 64:
	// 6.2.1 Literal Header Field with Incremental Indexing
	// 0b10xxxxxx: top two bits are 10
	// http://http2.github.io/http2-spec/compression.html#rfc.section.6.2.1
	return d.parseFieldLiteral(6, indexedTrue)
	case b&240 == 0:
	// 6.2.2 Literal Header Field without Indexing
	// 0b0000xxxx: top four bits are 0000
	// http://http2.github.io/http2-spec/compression.html#rfc.section.6.2.2
	return d.parseFieldLiteral(4, indexedFalse)
	case b&240 == 16:
	// 6.2.3 Literal Header Field never Indexed
	// 0b0001xxxx: top four bits are 0001
	// http://http2.github.io/http2-spec/compression.html#rfc.section.6.2.3
	return d.parseFieldLiteral(4, indexedNever)
	case b&224 == 32:
	// 6.3 Dynamic Table Size Update
	// Top three bits are '001'.
	// http://http2.github.io/http2-spec/compression.html#rfc.section.6.3
	return d.parseDynamicTableSizeUpdate()
	}

	return DecodingError{errors.New("invalid encoding")}
	}

	// (same invariants and behavior as parseHeaderFieldRepr)
	func (d *Decoder) parseFieldIndexed() error {
	buf := d.buf
	idx, buf, err := readVarInt(7, buf)
	if err != nil {
	return err
	}
	hf, ok := d.at(idx)
	if !ok {
	return DecodingError{InvalidIndexError(idx)}
	}
	d.buf = buf
	return d.callEmit(HeaderField{Name: hf.Name, Value: hf.Value})
	}

	// (same invariants and behavior as parseHeaderFieldRepr)
	func (d *Decoder) parseFieldLiteral(n uint8, it indexType) error {
	buf := d.buf
	nameIdx, buf, err := readVarInt(n, buf)
	if err != nil {
	return err
	}

	var hf HeaderField
	wantStr := d.emitEnabled \|\| it.indexed()
	if nameIdx > 0 {
	ihf, ok := d.at(nameIdx)
	if !ok {
	return DecodingError{InvalidIndexError(nameIdx)}
	}
	hf.Name = ihf.Name
	} else {
	hf.Name, buf, err = d.readString(buf, wantStr)
	if err != nil {
	return err
	}
	}
	hf.Value, buf, err = d.readString(buf, wantStr)
	if err != nil {
	return err
	}
	d.buf = buf
	if it.indexed() {
	d.dynTab.add(hf)
	}
	hf.Sensitive = it.sensitive()
	return d.callEmit(hf)
	}

	func (d *Decoder) callEmit(hf HeaderField) error {
	if d.maxStrLen != 0 {
	if len(hf.Name) > d.maxStrLen \|\| len(hf.Value) > d.maxStrLen {
	return ErrStringLength
	}
	}
	if d.emitEnabled {
	d.emit(hf)
	}
	return nil
	}

	// (same invariants and behavior as parseHeaderFieldRepr)
	func (d *Decoder) parseDynamicTableSizeUpdate() error {
	buf := d.buf
	size, buf, err := readVarInt(5, buf)
	if err != nil {
	return err
	}
	if size > uint64(d.dynTab.allowedMaxSize) {
	return DecodingError{errors.New("dynamic table size update too large")}
	}
	d.dynTab.setMaxSize(uint32(size))
	d.buf = buf
	return nil
	}

	var errVarintOverflow = DecodingError{errors.New("varint integer overflow")}

	// readVarInt reads an unsigned variable length integer off the
	// beginning of p. n is the parameter as described in
	// http://http2.github.io/http2-spec/compression.html#rfc.section.5.1.
	//
	// n must always be between 1 and 8.
	//
	// The returned remain buffer is either a smaller suffix of p, or err != nil.
	// The error is errNeedMore if p doesn't contain a complete integer.
	func readVarInt(n byte, p []byte) (i uint64, remain []byte, err error) {
	if n < 1 \|\| n > 8 {
	panic("bad n")
	}
	if len(p) == 0 {
	return 0, p, errNeedMore
	}
	i = uint64(p[0])
	if n < 8 {
	i &= (1 << uint64(n)) - 1
	}
	if i < (1<<uint64(n))-1 {
	return i, p[1:], nil
	}

	origP := p
	p = p[1:]
	var m uint64
	for len(p) > 0 {
	b := p[0]
	p = p[1:]
	i += uint64(b&127) << m
	if b&128 == 0 {
	return i, p, nil
	}
	m += 7
	if m >= 63 { // TODO: proper overflow check. making this up.
	return 0, origP, errVarintOverflow
	}
	}
	return 0, origP, errNeedMore
	}

	// readString decodes an hpack string from p.
	//
	// wantStr is whether s will be used. If false, decompression and
	// []byte->string garbage are skipped if s will be ignored
	// anyway. This does mean that huffman decoding errors for non-indexed
	// strings past the MAX_HEADER_LIST_SIZE are ignored, but the server
	// is returning an error anyway, and because they're not indexed, the error
	// won't affect the decoding state.
	func (d *Decoder) readString(p []byte, wantStr bool) (s string, remain []byte, err error) {
	if len(p) == 0 {
	return "", p, errNeedMore
	}
	isHuff := p[0]&128 != 0
	strLen, p, err := readVarInt(7, p)
	if err != nil {
	return "", p, err
	}
	if d.maxStrLen != 0 && strLen > uint64(d.maxStrLen) {
	return "", nil, ErrStringLength
	}
	if uint64(len(p)) < strLen {
	return "", p, errNeedMore
	}
	if !isHuff {
	if wantStr {
	s = string(p[:strLen])
	}
	return s, p[strLen:], nil
	}

	if wantStr {
	buf := bufPool.Get().(*bytes.Buffer)
	buf.Reset() // don't trust others
	defer bufPool.Put(buf)
	if err := huffmanDecode(buf, d.maxStrLen, p[:strLen]); err != nil {
	buf.Reset()
	return "", nil, err
	}
	s = buf.String()
	buf.Reset() // be nice to GC
	}
	return s, p[strLen:], nil
	}