http2/write.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 http2

 import (
 	"bytes"
 	"fmt"
 	"log"
 	"net/http"
 	"net/url"

 	"golang.org/x/net/http/httpguts"
 	"golang.org/x/net/http2/hpack"
 )

 // writeFramer is implemented by any type that is used to write frames.
 type writeFramer interface {
 	writeFrame(writeContext) error

 	// staysWithinBuffer reports whether this writer promises that
 	// it will only write less than or equal to size bytes, and it
 	// won't Flush the write context.
 	staysWithinBuffer(size int) bool
 }

 // writeContext is the interface needed by the various frame writer
 // types below. All the writeFrame methods below are scheduled via the
 // frame writing scheduler (see writeScheduler in writesched.go).
 //
 // This interface is implemented by *serverConn.
 //
 // TODO: decide whether to a) use this in the client code (which didn't
 // end up using this yet, because it has a simpler design, not
 // currently implementing priorities), or b) delete this and
 // make the server code a bit more concrete.
 type writeContext interface {
 	Framer() *Framer
 	Flush() error
 	CloseConn() error
 	// HeaderEncoder returns an HPACK encoder that writes to the
 	// returned buffer.
 	HeaderEncoder() (*hpack.Encoder, *bytes.Buffer)
 }

 // writeEndsStream reports whether w writes a frame that will transition
 // the stream to a half-closed local state. This returns false for RST_STREAM,
 // which closes the entire stream (not just the local half).
 func writeEndsStream(w writeFramer) bool {
 	switch v := w.(type) {
 	case *writeData:
 		return v.endStream
 	case *writeResHeaders:
 		return v.endStream
 	case nil:
 		// This can only happen if the caller reuses w after it's
 		// been intentionally nil'ed out to prevent use. Keep this
 		// here to catch future refactoring breaking it.
 		panic("writeEndsStream called on nil writeFramer")
 	}
 	return false
 }

 type flushFrameWriter struct{}

 func (flushFrameWriter) writeFrame(ctx writeContext) error {
 	return ctx.Flush()
 }

 func (flushFrameWriter) staysWithinBuffer(max int) bool { return false }

 type writeSettings []Setting

 func (s writeSettings) staysWithinBuffer(max int) bool {
 	const settingSize = 6 // uint16 + uint32
 	return frameHeaderLen+settingSize*len(s) <= max

 }

 func (s writeSettings) writeFrame(ctx writeContext) error {
 	return ctx.Framer().WriteSettings([]Setting(s)...)
 }

 type writeGoAway struct {
 	maxStreamID uint32
 	code        ErrCode
 }

 func (p *writeGoAway) writeFrame(ctx writeContext) error {
 	err := ctx.Framer().WriteGoAway(p.maxStreamID, p.code, nil)
 	ctx.Flush() // ignore error: we're hanging up on them anyway
 	return err
 }

 func (*writeGoAway) staysWithinBuffer(max int) bool { return false } // flushes

 type writeData struct {
 	streamID  uint32
 	p         []byte
 	endStream bool
 }

 func (w *writeData) String() string {
 	return fmt.Sprintf("writeData(stream=%d, p=%d, endStream=%v)", w.streamID, len(w.p), w.endStream)
 }

 func (w *writeData) writeFrame(ctx writeContext) error {
 	return ctx.Framer().WriteData(w.streamID, w.endStream, w.p)
 }

 func (w *writeData) staysWithinBuffer(max int) bool {
 	return frameHeaderLen+len(w.p) <= max
 }

 // handlerPanicRST is the message sent from handler goroutines when
 // the handler panics.
 type handlerPanicRST struct {
 	StreamID uint32
 }

 func (hp handlerPanicRST) writeFrame(ctx writeContext) error {
 	return ctx.Framer().WriteRSTStream(hp.StreamID, ErrCodeInternal)
 }

 func (hp handlerPanicRST) staysWithinBuffer(max int) bool { return frameHeaderLen+4 <= max }

 func (se StreamError) writeFrame(ctx writeContext) error {
 	return ctx.Framer().WriteRSTStream(se.StreamID, se.Code)
 }

 func (se StreamError) staysWithinBuffer(max int) bool { return frameHeaderLen+4 <= max }

 type writePing struct {
 	data [8]byte
 }

 func (w writePing) writeFrame(ctx writeContext) error {
 	return ctx.Framer().WritePing(false, w.data)
 }

 func (w writePing) staysWithinBuffer(max int) bool { return frameHeaderLen+len(w.data) <= max }

 type writePingAck struct{ pf *PingFrame }

 func (w writePingAck) writeFrame(ctx writeContext) error {
 	return ctx.Framer().WritePing(true, w.pf.Data)
 }

 func (w writePingAck) staysWithinBuffer(max int) bool { return frameHeaderLen+len(w.pf.Data) <= max }

 type writeSettingsAck struct{}

 func (writeSettingsAck) writeFrame(ctx writeContext) error {
 	return ctx.Framer().WriteSettingsAck()
 }

 func (writeSettingsAck) staysWithinBuffer(max int) bool { return frameHeaderLen <= max }

 // splitHeaderBlock splits headerBlock into fragments so that each fragment fits
 // in a single frame, then calls fn for each fragment. firstFrag/lastFrag are true
 // for the first/last fragment, respectively.
 func splitHeaderBlock(ctx writeContext, headerBlock []byte, fn func(ctx writeContext, frag []byte, firstFrag, lastFrag bool) error) error {
 	// For now we're lazy and just pick the minimum MAX_FRAME_SIZE
 	// that all peers must support (16KB). Later we could care
 	// more and send larger frames if the peer advertised it, but
 	// there's little point. Most headers are small anyway (so we
 	// generally won't have CONTINUATION frames), and extra frames
 	// only waste 9 bytes anyway.
 	const maxFrameSize = 16384

 	first := true
 	for len(headerBlock) > 0 {
 		frag := headerBlock
 		if len(frag) > maxFrameSize {
 			frag = frag[:maxFrameSize]
 		}
 		headerBlock = headerBlock[len(frag):]
 		if err := fn(ctx, frag, first, len(headerBlock) == 0); err != nil {
 			return err
 		}
 		first = false
 	}
 	return nil
 }

 // writeResHeaders is a request to write a HEADERS and 0+ CONTINUATION frames
 // for HTTP response headers or trailers from a server handler.
 type writeResHeaders struct {
 	streamID    uint32
 	httpResCode int         // 0 means no ":status" line
 	h           http.Header // may be nil
 	trailers    []string    // if non-nil, which keys of h to write. nil means all.
 	endStream   bool

 	date          string
 	contentType   string
 	contentLength string
 }

 func encKV(enc *hpack.Encoder, k, v string) {
 	if VerboseLogs {
 		log.Printf("http2: server encoding header %q = %q", k, v)
 	}
 	enc.WriteField(hpack.HeaderField{Name: k, Value: v})
 }

 func (w *writeResHeaders) staysWithinBuffer(max int) bool {
 	// TODO: this is a common one. It'd be nice to return true
 	// here and get into the fast path if we could be clever and
 	// calculate the size fast enough, or at least a conservative
 	// upper bound that usually fires. (Maybe if w.h and
 	// w.trailers are nil, so we don't need to enumerate it.)
 	// Otherwise I'm afraid that just calculating the length to
 	// answer this question would be slower than the ~2µs benefit.
 	return false
 }

 func (w *writeResHeaders) writeFrame(ctx writeContext) error {
 	enc, buf := ctx.HeaderEncoder()
 	buf.Reset()

 	if w.httpResCode != 0 {
 		encKV(enc, ":status", httpCodeString(w.httpResCode))
 	}

 	encodeHeaders(enc, w.h, w.trailers)

 	if w.contentType != "" {
 		encKV(enc, "content-type", w.contentType)
 	}
 	if w.contentLength != "" {
 		encKV(enc, "content-length", w.contentLength)
 	}
 	if w.date != "" {
 		encKV(enc, "date", w.date)
 	}

 	headerBlock := buf.Bytes()
 	if len(headerBlock) == 0 && w.trailers == nil {
 		panic("unexpected empty hpack")
 	}

 	return splitHeaderBlock(ctx, headerBlock, w.writeHeaderBlock)
 }

 func (w *writeResHeaders) writeHeaderBlock(ctx writeContext, frag []byte, firstFrag, lastFrag bool) error {
 	if firstFrag {
 		return ctx.Framer().WriteHeaders(HeadersFrameParam{
 			StreamID:      w.streamID,
 			BlockFragment: frag,
 			EndStream:     w.endStream,
 			EndHeaders:    lastFrag,
 		})
 	} else {
 		return ctx.Framer().WriteContinuation(w.streamID, lastFrag, frag)
 	}
 }

 // writePushPromise is a request to write a PUSH_PROMISE and 0+ CONTINUATION frames.
 type writePushPromise struct {
 	streamID uint32   // pusher stream
 	method   string   // for :method
 	url      *url.URL // for :scheme, :authority, :path
 	h        http.Header

 	// Creates an ID for a pushed stream. This runs on serveG just before
 	// the frame is written. The returned ID is copied to promisedID.
 	allocatePromisedID func() (uint32, error)
 	promisedID         uint32
 }

 func (w *writePushPromise) staysWithinBuffer(max int) bool {
 	// TODO: see writeResHeaders.staysWithinBuffer
 	return false
 }

 func (w *writePushPromise) writeFrame(ctx writeContext) error {
 	enc, buf := ctx.HeaderEncoder()
 	buf.Reset()

 	encKV(enc, ":method", w.method)
 	encKV(enc, ":scheme", w.url.Scheme)
 	encKV(enc, ":authority", w.url.Host)
 	encKV(enc, ":path", w.url.RequestURI())
 	encodeHeaders(enc, w.h, nil)

 	headerBlock := buf.Bytes()
 	if len(headerBlock) == 0 {
 		panic("unexpected empty hpack")
 	}

 	return splitHeaderBlock(ctx, headerBlock, w.writeHeaderBlock)
 }

 func (w *writePushPromise) writeHeaderBlock(ctx writeContext, frag []byte, firstFrag, lastFrag bool) error {
 	if firstFrag {
 		return ctx.Framer().WritePushPromise(PushPromiseParam{
 			StreamID:      w.streamID,
 			PromiseID:     w.promisedID,
 			BlockFragment: frag,
 			EndHeaders:    lastFrag,
 		})
 	} else {
 		return ctx.Framer().WriteContinuation(w.streamID, lastFrag, frag)
 	}
 }

 type write100ContinueHeadersFrame struct {
 	streamID uint32
 }

 func (w write100ContinueHeadersFrame) writeFrame(ctx writeContext) error {
 	enc, buf := ctx.HeaderEncoder()
 	buf.Reset()
 	encKV(enc, ":status", "100")
 	return ctx.Framer().WriteHeaders(HeadersFrameParam{
 		StreamID:      w.streamID,
 		BlockFragment: buf.Bytes(),
 		EndStream:     false,
 		EndHeaders:    true,
 	})
 }

 func (w write100ContinueHeadersFrame) staysWithinBuffer(max int) bool {
 	// Sloppy but conservative:
 	return 9+2*(len(":status")+len("100")) <= max
 }

 type writeWindowUpdate struct {
 	streamID uint32 // or 0 for conn-level
 	n        uint32
 }

 func (wu writeWindowUpdate) staysWithinBuffer(max int) bool { return frameHeaderLen+4 <= max }

 func (wu writeWindowUpdate) writeFrame(ctx writeContext) error {
 	return ctx.Framer().WriteWindowUpdate(wu.streamID, wu.n)
 }

 // encodeHeaders encodes an http.Header. If keys is not nil, then (k, h[k])
 // is encoded only if k is in keys.
 func encodeHeaders(enc *hpack.Encoder, h http.Header, keys []string) {
 	if keys == nil {
 		sorter := sorterPool.Get().(*sorter)
 		// Using defer here, since the returned keys from the
 		// sorter.Keys method is only valid until the sorter
 		// is returned:
 		defer sorterPool.Put(sorter)
 		keys = sorter.Keys(h)
 	}
 	for _, k := range keys {
 		vv := h[k]
 		k, ascii := lowerHeader(k)
 		if !ascii {
 			// Skip writing invalid headers. Per RFC 7540, Section 8.1.2, header
 			// field names have to be ASCII characters (just as in HTTP/1.x).
 			continue
 		}
 		if !validWireHeaderFieldName(k) {
 			// Skip it as backup paranoia. Per
 			// golang.org/issue/14048, these should
 			// already be rejected at a higher level.
 			continue
 		}
 		isTE := k == "transfer-encoding"
 		for _, v := range vv {
 			if !httpguts.ValidHeaderFieldValue(v) {
 				// TODO: return an error? golang.org/issue/14048
 				// For now just omit it.
 				continue
 			}
 			// TODO: more of "8.1.2.2 Connection-Specific Header Fields"
 			if isTE && v != "trailers" {
 				continue
 			}
 			encKV(enc, k, v)
 		}
 	}
 }
	// 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 http2

	import (
	"bytes"
	"fmt"
	"log"
	"net/http"
	"net/url"

	"golang.org/x/net/http/httpguts"
	"golang.org/x/net/http2/hpack"
	)

	// writeFramer is implemented by any type that is used to write frames.
	type writeFramer interface {
	writeFrame(writeContext) error

	// staysWithinBuffer reports whether this writer promises that
	// it will only write less than or equal to size bytes, and it
	// won't Flush the write context.
	staysWithinBuffer(size int) bool
	}

	// writeContext is the interface needed by the various frame writer
	// types below. All the writeFrame methods below are scheduled via the
	// frame writing scheduler (see writeScheduler in writesched.go).
	//
	// This interface is implemented by *serverConn.
	//
	// TODO: decide whether to a) use this in the client code (which didn't
	// end up using this yet, because it has a simpler design, not
	// currently implementing priorities), or b) delete this and
	// make the server code a bit more concrete.
	type writeContext interface {
	Framer() *Framer
	Flush() error
	CloseConn() error
	// HeaderEncoder returns an HPACK encoder that writes to the
	// returned buffer.
	HeaderEncoder() (hpack.Encoder, bytes.Buffer)
	}

	// writeEndsStream reports whether w writes a frame that will transition
	// the stream to a half-closed local state. This returns false for RST_STREAM,
	// which closes the entire stream (not just the local half).
	func writeEndsStream(w writeFramer) bool {
	switch v := w.(type) {
	case *writeData:
	return v.endStream
	case *writeResHeaders:
	return v.endStream
	case nil:
	// This can only happen if the caller reuses w after it's
	// been intentionally nil'ed out to prevent use. Keep this
	// here to catch future refactoring breaking it.
	panic("writeEndsStream called on nil writeFramer")
	}
	return false
	}

	type flushFrameWriter struct{}

	func (flushFrameWriter) writeFrame(ctx writeContext) error {
	return ctx.Flush()
	}

	func (flushFrameWriter) staysWithinBuffer(max int) bool { return false }

	type writeSettings []Setting

	func (s writeSettings) staysWithinBuffer(max int) bool {
	const settingSize = 6 // uint16 + uint32
	return frameHeaderLen+settingSize*len(s) <= max

	}

	func (s writeSettings) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteSettings([]Setting(s)...)
	}

	type writeGoAway struct {
	maxStreamID uint32
	code ErrCode
	}

	func (p *writeGoAway) writeFrame(ctx writeContext) error {
	err := ctx.Framer().WriteGoAway(p.maxStreamID, p.code, nil)
	ctx.Flush() // ignore error: we're hanging up on them anyway
	return err
	}

	func (*writeGoAway) staysWithinBuffer(max int) bool { return false } // flushes

	type writeData struct {
	streamID uint32
	p []byte
	endStream bool
	}

	func (w *writeData) String() string {
	return fmt.Sprintf("writeData(stream=%d, p=%d, endStream=%v)", w.streamID, len(w.p), w.endStream)
	}

	func (w *writeData) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteData(w.streamID, w.endStream, w.p)
	}

	func (w *writeData) staysWithinBuffer(max int) bool {
	return frameHeaderLen+len(w.p) <= max
	}

	// handlerPanicRST is the message sent from handler goroutines when
	// the handler panics.
	type handlerPanicRST struct {
	StreamID uint32
	}

	func (hp handlerPanicRST) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteRSTStream(hp.StreamID, ErrCodeInternal)
	}

	func (hp handlerPanicRST) staysWithinBuffer(max int) bool { return frameHeaderLen+4 <= max }

	func (se StreamError) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteRSTStream(se.StreamID, se.Code)
	}

	func (se StreamError) staysWithinBuffer(max int) bool { return frameHeaderLen+4 <= max }

	type writePing struct {
	data [8]byte
	}

	func (w writePing) writeFrame(ctx writeContext) error {
	return ctx.Framer().WritePing(false, w.data)
	}

	func (w writePing) staysWithinBuffer(max int) bool { return frameHeaderLen+len(w.data) <= max }

	type writePingAck struct{ pf *PingFrame }

	func (w writePingAck) writeFrame(ctx writeContext) error {
	return ctx.Framer().WritePing(true, w.pf.Data)
	}

	func (w writePingAck) staysWithinBuffer(max int) bool { return frameHeaderLen+len(w.pf.Data) <= max }

	type writeSettingsAck struct{}

	func (writeSettingsAck) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteSettingsAck()
	}

	func (writeSettingsAck) staysWithinBuffer(max int) bool { return frameHeaderLen <= max }

	// splitHeaderBlock splits headerBlock into fragments so that each fragment fits
	// in a single frame, then calls fn for each fragment. firstFrag/lastFrag are true
	// for the first/last fragment, respectively.
	func splitHeaderBlock(ctx writeContext, headerBlock []byte, fn func(ctx writeContext, frag []byte, firstFrag, lastFrag bool) error) error {
	// For now we're lazy and just pick the minimum MAX_FRAME_SIZE
	// that all peers must support (16KB). Later we could care
	// more and send larger frames if the peer advertised it, but
	// there's little point. Most headers are small anyway (so we
	// generally won't have CONTINUATION frames), and extra frames
	// only waste 9 bytes anyway.
	const maxFrameSize = 16384

	first := true
	for len(headerBlock) > 0 {
	frag := headerBlock
	if len(frag) > maxFrameSize {
	frag = frag[:maxFrameSize]
	}
	headerBlock = headerBlock[len(frag):]
	if err := fn(ctx, frag, first, len(headerBlock) == 0); err != nil {
	return err
	}
	first = false
	}
	return nil
	}

	// writeResHeaders is a request to write a HEADERS and 0+ CONTINUATION frames
	// for HTTP response headers or trailers from a server handler.
	type writeResHeaders struct {
	streamID uint32
	httpResCode int // 0 means no ":status" line
	h http.Header // may be nil
	trailers []string // if non-nil, which keys of h to write. nil means all.
	endStream bool

	date string
	contentType string
	contentLength string
	}

	func encKV(enc *hpack.Encoder, k, v string) {
	if VerboseLogs {
	log.Printf("http2: server encoding header %q = %q", k, v)
	}
	enc.WriteField(hpack.HeaderField{Name: k, Value: v})
	}

	func (w *writeResHeaders) staysWithinBuffer(max int) bool {
	// TODO: this is a common one. It'd be nice to return true
	// here and get into the fast path if we could be clever and
	// calculate the size fast enough, or at least a conservative
	// upper bound that usually fires. (Maybe if w.h and
	// w.trailers are nil, so we don't need to enumerate it.)
	// Otherwise I'm afraid that just calculating the length to
	// answer this question would be slower than the ~2µs benefit.
	return false
	}

	func (w *writeResHeaders) writeFrame(ctx writeContext) error {
	enc, buf := ctx.HeaderEncoder()
	buf.Reset()

	if w.httpResCode != 0 {
	encKV(enc, ":status", httpCodeString(w.httpResCode))
	}

	encodeHeaders(enc, w.h, w.trailers)

	if w.contentType != "" {
	encKV(enc, "content-type", w.contentType)
	}
	if w.contentLength != "" {
	encKV(enc, "content-length", w.contentLength)
	}
	if w.date != "" {
	encKV(enc, "date", w.date)
	}

	headerBlock := buf.Bytes()
	if len(headerBlock) == 0 && w.trailers == nil {
	panic("unexpected empty hpack")
	}

	return splitHeaderBlock(ctx, headerBlock, w.writeHeaderBlock)
	}

	func (w *writeResHeaders) writeHeaderBlock(ctx writeContext, frag []byte, firstFrag, lastFrag bool) error {
	if firstFrag {
	return ctx.Framer().WriteHeaders(HeadersFrameParam{
	StreamID: w.streamID,
	BlockFragment: frag,
	EndStream: w.endStream,
	EndHeaders: lastFrag,
	})
	} else {
	return ctx.Framer().WriteContinuation(w.streamID, lastFrag, frag)
	}
	}

	// writePushPromise is a request to write a PUSH_PROMISE and 0+ CONTINUATION frames.
	type writePushPromise struct {
	streamID uint32 // pusher stream
	method string // for :method
	url *url.URL // for :scheme, :authority, :path
	h http.Header

	// Creates an ID for a pushed stream. This runs on serveG just before
	// the frame is written. The returned ID is copied to promisedID.
	allocatePromisedID func() (uint32, error)
	promisedID uint32
	}

	func (w *writePushPromise) staysWithinBuffer(max int) bool {
	// TODO: see writeResHeaders.staysWithinBuffer
	return false
	}

	func (w *writePushPromise) writeFrame(ctx writeContext) error {
	enc, buf := ctx.HeaderEncoder()
	buf.Reset()

	encKV(enc, ":method", w.method)
	encKV(enc, ":scheme", w.url.Scheme)
	encKV(enc, ":authority", w.url.Host)
	encKV(enc, ":path", w.url.RequestURI())
	encodeHeaders(enc, w.h, nil)

	headerBlock := buf.Bytes()
	if len(headerBlock) == 0 {
	panic("unexpected empty hpack")
	}

	return splitHeaderBlock(ctx, headerBlock, w.writeHeaderBlock)
	}

	func (w *writePushPromise) writeHeaderBlock(ctx writeContext, frag []byte, firstFrag, lastFrag bool) error {
	if firstFrag {
	return ctx.Framer().WritePushPromise(PushPromiseParam{
	StreamID: w.streamID,
	PromiseID: w.promisedID,
	BlockFragment: frag,
	EndHeaders: lastFrag,
	})
	} else {
	return ctx.Framer().WriteContinuation(w.streamID, lastFrag, frag)
	}
	}

	type write100ContinueHeadersFrame struct {
	streamID uint32
	}

	func (w write100ContinueHeadersFrame) writeFrame(ctx writeContext) error {
	enc, buf := ctx.HeaderEncoder()
	buf.Reset()
	encKV(enc, ":status", "100")
	return ctx.Framer().WriteHeaders(HeadersFrameParam{
	StreamID: w.streamID,
	BlockFragment: buf.Bytes(),
	EndStream: false,
	EndHeaders: true,
	})
	}

	func (w write100ContinueHeadersFrame) staysWithinBuffer(max int) bool {
	// Sloppy but conservative:
	return 9+2*(len(":status")+len("100")) <= max
	}

	type writeWindowUpdate struct {
	streamID uint32 // or 0 for conn-level
	n uint32
	}

	func (wu writeWindowUpdate) staysWithinBuffer(max int) bool { return frameHeaderLen+4 <= max }

	func (wu writeWindowUpdate) writeFrame(ctx writeContext) error {
	return ctx.Framer().WriteWindowUpdate(wu.streamID, wu.n)
	}

	// encodeHeaders encodes an http.Header. If keys is not nil, then (k, h[k])
	// is encoded only if k is in keys.
	func encodeHeaders(enc *hpack.Encoder, h http.Header, keys []string) {
	if keys == nil {
	sorter := sorterPool.Get().(*sorter)
	// Using defer here, since the returned keys from the
	// sorter.Keys method is only valid until the sorter
	// is returned:
	defer sorterPool.Put(sorter)
	keys = sorter.Keys(h)
	}
	for _, k := range keys {
	vv := h[k]
	k, ascii := lowerHeader(k)
	if !ascii {
	// Skip writing invalid headers. Per RFC 7540, Section 8.1.2, header
	// field names have to be ASCII characters (just as in HTTP/1.x).
	continue
	}
	if !validWireHeaderFieldName(k) {
	// Skip it as backup paranoia. Per
	// golang.org/issue/14048, these should
	// already be rejected at a higher level.
	continue
	}
	isTE := k == "transfer-encoding"
	for _, v := range vv {
	if !httpguts.ValidHeaderFieldValue(v) {
	// TODO: return an error? golang.org/issue/14048
	// For now just omit it.
	continue
	}
	// TODO: more of "8.1.2.2 Connection-Specific Header Fields"
	if isTE && v != "trailers" {
	continue
	}
	encKV(enc, k, v)
	}
	}
	}