// 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
firstField bool // processing the first field of the header block
}
// 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,
firstField: 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
}
// Close declares that the decoding is complete and resets the Decoder
// to be reused again for a new header block. If there is any remaining
// data in the decoder's buffer, Close returns an error.
func (d *Decoder) Close() error {
if d.saveBuf.Len() > 0 {
d.saveBuf.Reset()
return DecodingError{errors.New("truncated headers")}
}
d.firstField = true
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
}
d.firstField = false
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 {
// RFC 7541, sec 4.2: This dynamic table size update MUST occur at the
// beginning of the first header block following the change to the dynamic table size.
if !d.firstField && d.dynTab.size > 0 {
return DecodingError{errors.New("dynamic table size update MUST occur at the beginning of a header block")}
}
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
}
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