net/http: document, test, define, clean up Request.Trailer

Go's had pretty decent HTTP Trailer support for a long time, but
the docs have been largely non-existent. Fix that.

In the process, re-learn the Trailer code, clean some stuff
up, add some error checks, remove some TODOs, fix a minor bug
or two, and add tests.

LGTM=adg
R=golang-codereviews, adg
CC=dsymonds, golang-codereviews, rsc
https://golang.org/cl/86660043

src/pkg/net/http/client_test.go

@@ -20,6 +20,8 @@ import (
 	. "net/http"
 	"net/http/httptest"
 	"net/url"
+	"reflect"
+	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -944,3 +946,93 @@ func TestClientRedirectEatsBody(t *testing.T) {
 		t.Fatal("server saw different client ports before & after the redirect")
 	}
 }
+
+// eofReaderFunc is an io.Reader that runs itself, and then returns io.EOF.
+type eofReaderFunc func()
+
+func (f eofReaderFunc) Read(p []byte) (n int, err error) {
+	f()
+	return 0, io.EOF
+}
+
+func TestClientTrailers(t *testing.T) {
+	defer afterTest(t)
+	ts := httptest.NewServer(HandlerFunc(func(w ResponseWriter, r *Request) {
+		w.Header().Set("Connection", "close")
+		w.Header().Set("Trailer", "Server-Trailer-A, Server-Trailer-B")
+		w.Header().Add("Trailer", "Server-Trailer-C")
+
+		var decl []string
+		for k := range r.Trailer {
+			decl = append(decl, k)
+		}
+		sort.Strings(decl)
+
+		slurp, err := ioutil.ReadAll(r.Body)
+		if err != nil {
+			t.Errorf("Server reading request body: %v", err)
+		}
+		if string(slurp) != "foo" {
+			t.Errorf("Server read request body %q; want foo", slurp)
+		}
+		if r.Trailer == nil {
+			io.WriteString(w, "nil Trailer")
+		} else {
+			fmt.Fprintf(w, "decl: %v, vals: %s, %s",
+				decl,
+				r.Trailer.Get("Client-Trailer-A"),
+				r.Trailer.Get("Client-Trailer-B"))
+		}
+
+		// TODO: golang.org/issue/7759: there's no way yet for
+		// the server to set trailers without hijacking, so do
+		// that for now, just to test the client.  Later, in
+		// Go 1.4, it should be be implicit that any mutations
+		// to w.Header() after the initial write are the
+		// trailers to be sent, if and only if they were
+		// previously declared with w.Header().Set("Trailer",
+		// ..keys..)
+		w.(Flusher).Flush()
+		conn, buf, _ := w.(Hijacker).Hijack()
+		t := Header{}
+		t.Set("Server-Trailer-A", "valuea")
+		t.Set("Server-Trailer-C", "valuec") // skipping B
+		buf.WriteString("0\r\n")            // eof
+		t.Write(buf)
+		buf.WriteString("\r\n") // end of trailers
+		buf.Flush()
+		conn.Close()
+	}))
+	defer ts.Close()
+
+	var req *Request
+	req, _ = NewRequest("POST", ts.URL, io.MultiReader(
+		eofReaderFunc(func() {
+			req.Trailer["Client-Trailer-A"] = []string{"valuea"}
+		}),
+		strings.NewReader("foo"),
+		eofReaderFunc(func() {
+			req.Trailer["Client-Trailer-B"] = []string{"valueb"}
+		}),
+	))
+	req.Trailer = Header{
+		"Client-Trailer-A": nil, //  to be set later
+		"Client-Trailer-B": nil, //  to be set later
+	}
+	req.ContentLength = -1
+	res, err := DefaultClient.Do(req)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if err := wantBody(res, err, "decl: [Client-Trailer-A Client-Trailer-B], vals: valuea, valueb"); err != nil {
+		t.Error(err)
+	}
+	want := Header{
+		"Server-Trailer-A": []string{"valuea"},
+		"Server-Trailer-B": nil,
+		"Server-Trailer-C": []string{"valuec"},
+	}
+	if !reflect.DeepEqual(res.Trailer, want) {
+		t.Errorf("Response trailers = %#v; want %#v", res.Trailer, want)
+	}
+}

src/pkg/net/http/request.go

@@ -182,12 +182,24 @@ type Request struct {
 	// The HTTP client ignores MultipartForm and uses Body instead.
 	MultipartForm *multipart.Form
 
-	// Trailer maps trailer keys to values.  Like for Header, if the
-	// response has multiple trailer lines with the same key, they will be
-	// concatenated, delimited by commas.
-	// For server requests Trailer is only populated after Body has been
-	// closed or fully consumed.
-	// Trailer support is only partially complete.
+	// Trailer specifies additional headers that are sent after the request
+	// body.
+	//
+	// For server requests the Trailer map initially contains only the
+	// trailer keys, with nil values. (The client declares which trailers it
+	// will later send.)  While the handler is reading from Body, it must
+	// not reference Trailer. After reading from Body returns EOF, Trailer
+	// can be read again and will contain non-nil values, if they were sent
+	// by the client.
+	//
+	// For client requests Trailer must be initialized to a map containing
+	// the trailer keys to later send. The values may be nil or their final
+	// values. The ContentLength must be 0 or -1, to send a chunked request.
+	// After the HTTP request is sent the map values can be updated while
+	// the request body is read. Once the body returns EOF, the caller must
+	// not mutate Trailer.
+	//
+	// Few HTTP clients, servers, or proxies support HTTP trailers.
 	Trailer Header
 
 	// RemoteAddr allows HTTP servers and other software to record
@@ -405,7 +417,6 @@ func (req *Request) write(w io.Writer, usingProxy bool, extraHeaders Header) err
 		return err
 	}
 
-	// TODO: split long values?  (If so, should share code with Conn.Write)
 	err = req.Header.WriteSubset(w, reqWriteExcludeHeader)
 	if err != nil {
 		return err
@@ -607,32 +618,6 @@ func ReadRequest(b *bufio.Reader) (req *Request, err error) {
 
 	fixPragmaCacheControl(req.Header)
 
-	// TODO: Parse specific header values:
-	//	Accept
-	//	Accept-Encoding
-	//	Accept-Language
-	//	Authorization
-	//	Cache-Control
-	//	Connection
-	//	Date
-	//	Expect
-	//	From
-	//	If-Match
-	//	If-Modified-Since
-	//	If-None-Match
-	//	If-Range
-	//	If-Unmodified-Since
-	//	Max-Forwards
-	//	Proxy-Authorization
-	//	Referer [sic]
-	//	TE (transfer-codings)
-	//	Trailer
-	//	Transfer-Encoding
-	//	Upgrade
-	//	User-Agent
-	//	Via
-	//	Warning
-
 	err = readTransfer(req, b)
 	if err != nil {
 		return nil, err

src/pkg/net/http/transfer.go

@@ -12,6 +12,7 @@ import (
 	"io"
 	"io/ioutil"
 	"net/textproto"
+	"sort"
 	"strconv"
 	"strings"
 	"sync"
@@ -143,11 +144,10 @@ func (t *transferWriter) shouldSendContentLength() bool {
 	return false
 }
 
-func (t *transferWriter) WriteHeader(w io.Writer) (err error) {
+func (t *transferWriter) WriteHeader(w io.Writer) error {
 	if t.Close {
-		_, err = io.WriteString(w, "Connection: close\r\n")
-		if err != nil {
-			return
+		if _, err := io.WriteString(w, "Connection: close\r\n"); err != nil {
+			return err
 		}
 	}
 
@@ -156,42 +156,41 @@ func (t *transferWriter) WriteHeader(w io.Writer) (err error) {
 	// TransferEncoding)
 	if t.shouldSendContentLength() {
 		io.WriteString(w, "Content-Length: ")
-		_, err = io.WriteString(w, strconv.FormatInt(t.ContentLength, 10)+"\r\n")
-		if err != nil {
-			return
+		if _, err := io.WriteString(w, strconv.FormatInt(t.ContentLength, 10)+"\r\n"); err != nil {
+			return err
 		}
 	} else if chunked(t.TransferEncoding) {
-		_, err = io.WriteString(w, "Transfer-Encoding: chunked\r\n")
-		if err != nil {
-			return
+		if _, err := io.WriteString(w, "Transfer-Encoding: chunked\r\n"); err != nil {
+			return err
 		}
 	}
 
 	// Write Trailer header
 	if t.Trailer != nil {
-		// TODO: At some point, there should be a generic mechanism for
-		// writing long headers, using HTTP line splitting
-		io.WriteString(w, "Trailer: ")
-		needComma := false
+		keys := make([]string, 0, len(t.Trailer))
 		for k := range t.Trailer {
 			k = CanonicalHeaderKey(k)
 			switch k {
 			case "Transfer-Encoding", "Trailer", "Content-Length":
 				return &badStringError{"invalid Trailer key", k}
 			}
-			if needComma {
-				io.WriteString(w, ",")
+			keys = append(keys, k)
+		}
+		if len(keys) > 0 {
+			sort.Strings(keys)
+			// TODO: could do better allocation-wise here, but trailers are rare,
+			// so being lazy for now.
+			if _, err := io.WriteString(w, "Trailer: "+strings.Join(keys, ",")+"\r\n"); err != nil {
+				return err
 			}
-			io.WriteString(w, k)
-			needComma = true
 		}
-		_, err = io.WriteString(w, "\r\n")
 	}
 
-	return
+	return nil
 }
 
-func (t *transferWriter) WriteBody(w io.Writer) (err error) {
+func (t *transferWriter) WriteBody(w io.Writer) error {
+	var err error
 	var ncopy int64
 
 	// Write body
@@ -228,11 +227,16 @@ func (t *transferWriter) WriteBody(w io.Writer) (err error) {
 
 	// TODO(petar): Place trailer writer code here.
 	if chunked(t.TransferEncoding) {
+		// Write Trailer header
+		if t.Trailer != nil {
+			if err := t.Trailer.Write(w); err != nil {
+				return err
+			}
+		}
 		// Last chunk, empty trailer
 		_, err = io.WriteString(w, "\r\n")
 	}
-
-	return
+	return err
 }
 
 type transferReader struct {
@@ -510,7 +514,7 @@ func fixTrailer(header Header, te []string) (Header, error) {
 		case "Transfer-Encoding", "Trailer", "Content-Length":
 			return nil, &badStringError{"bad trailer key", key}
 		}
-		trailer.Del(key)
+		trailer[key] = nil
 	}
 	if len(trailer) == 0 {
 		return nil, nil
@@ -642,13 +646,23 @@ func (b *body) readTrailer() error {
 	}
 	switch rr := b.hdr.(type) {
 	case *Request:
-		rr.Trailer = Header(hdr)
+		mergeSetHeader(&rr.Trailer, Header(hdr))
 	case *Response:
-		rr.Trailer = Header(hdr)
+		mergeSetHeader(&rr.Trailer, Header(hdr))
 	}
 	return nil
 }
 
+func mergeSetHeader(dst *Header, src Header) {
+	if *dst == nil {
+		*dst = src
+		return
+	}
+	for k, vv := range src {
+		(*dst)[k] = vv
+	}
+}
+
 func (b *body) Close() error {
 	b.mu.Lock()
 	defer b.mu.Unlock()