internal/http3: make Server response include headers that can be inferred

This CL makes Server automatically set certain headers in a response
when they can be inferred and have not been explicitly set in a handler.

For now, "Date" and "Content-Type" headers are supported. Note that
unlike HTTP/1 and HTTP/2, "Content-Length" header is not automatically
set.

For golang/go#70914

Change-Id: Ie8d64ee0efb10118dfcba7a156f6f6886f565046
Reviewed-on: https://go-review.googlesource.com/c/net/+/746760
Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-by: Damien Neil <dneil@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

internal/http3/body.go

@@ -44,18 +44,34 @@ type bodyWriter struct {
 	enc     *qpackEncoder // QPACK encoder used by the connection.
 }
 
-func (w *bodyWriter) Write(p []byte) (n int, err error) {
-	if w.remain >= 0 && int64(len(p)) > w.remain {
+func (w *bodyWriter) write(ps ...[]byte) (n int, err error) {
+	var size int64
+	for _, p := range ps {
+		size += int64(len(p))
+	}
+	// If write is called with empty byte slices, just return instead of
+	// sending out a DATA frame containing nothing.
+	if size == 0 {
+		return 0, nil
+	}
+	if w.remain >= 0 && size > w.remain {
 		return 0, &streamError{
 			code:    errH3InternalError,
 			message: w.name + " body longer than specified content length",
 		}
 	}
 	w.st.writeVarint(int64(frameTypeData))
-	w.st.writeVarint(int64(len(p)))
-	n, err = w.st.Write(p)
-	if w.remain >= 0 {
-		w.remain -= int64(n)
+	w.st.writeVarint(size)
+	for _, p := range ps {
+		var n2 int
+		n2, err = w.st.Write(p)
+		n += n2
+		if w.remain >= 0 {
+			w.remain -= int64(n)
+		}
+		if err != nil {
+			break
+		}
 	}
 	if w.flush && err == nil {
 		err = w.st.Flush()
@@ -66,6 +82,10 @@ func (w *bodyWriter) Write(p []byte) (n int, err error) {
 	return n, err
 }
 
+func (w *bodyWriter) Write(p []byte) (n int, err error) {
+	return w.write(p)
+}
+
 func (w *bodyWriter) Close() error {
 	if w.remain > 0 {
 		return errors.New(w.name + " body shorter than specified content length")

internal/http3/server.go

@@ -13,6 +13,7 @@ import (
 	"strconv"
 	"strings"
 	"sync"
+	"time"
 
 	"golang.org/x/net/http/httpguts"
 	"golang.org/x/net/internal/httpcommon"
@@ -253,10 +254,11 @@ func (sc *serverConn) handleRequestStream(st *stream) error {
 	defer req.Body.Close()
 
 	rw := &responseWriter{
-		st:         st,
-		headers:    make(http.Header),
-		trailer:    make(http.Header),
-		isHeadResp: req.Method == "HEAD",
+		st:             st,
+		headers:        make(http.Header),
+		trailer:        make(http.Header),
+		bb:             make(bodyBuffer, 0, defaultBodyBufferCap),
+		cannotHaveBody: req.Method == "HEAD",
 		bw: &bodyWriter{
 			st:     st,
 			remain: -1,
@@ -268,8 +270,18 @@ func (sc *serverConn) handleRequestStream(st *stream) error {
 	defer rw.close()
 	if reqInfo.NeedsContinue {
 		req.Body.(*bodyReader).send100Continue = func() {
-			rw.WriteHeader(http.StatusContinue)
-			rw.Flush()
+			rw.mu.Lock()
+			defer rw.mu.Unlock()
+			if rw.wroteHeader {
+				return
+			}
+			encHeaders := rw.bw.enc.encode(func(f func(itype indexType, name, value string)) {
+				f(mayIndex, ":status", strconv.Itoa(http.StatusContinue))
+			})
+			rw.st.writeVarint(int64(frameTypeHeaders))
+			rw.st.writeVarint(int64(len(encHeaders)))
+			rw.st.Write(encHeaders)
+			rw.st.Flush()
 		}
 	}
 
@@ -290,14 +302,31 @@ func (sc *serverConn) abort(err error) {
 	}
 }
 
+// responseCanHaveBody reports whether a given response status code permits a
+// body. See RFC 7230, section 3.3.
+func responseCanHaveBody(status int) bool {
+	switch {
+	case status >= 100 && status <= 199:
+		return false
+	case status == 204:
+		return false
+	case status == 304:
+		return false
+	}
+	return true
+}
+
 type responseWriter struct {
-	st          *stream
-	bw          *bodyWriter
-	mu          sync.Mutex
-	headers     http.Header
-	trailer     http.Header
-	wroteHeader bool // Non-1xx header has been (logically) written.
-	isHeadResp  bool // response is for a HEAD request.
+	st             *stream
+	bw             *bodyWriter
+	mu             sync.Mutex
+	headers        http.Header
+	trailer        http.Header
+	bb             bodyBuffer
+	wroteHeader    bool // Non-1xx header has been (logically) written.
+	statusCode     int  // Status of the response that will be sent in HEADERS frame.
+	statusCodeSet  bool // Status of the response has been set via a call to WriteHeader.
+	cannotHaveBody bool // Response should not have a body (e.g. response to a HEAD request).
 }
 
 func (rw *responseWriter) Header() http.Header {
@@ -322,11 +351,13 @@ func (rw *responseWriter) prepareTrailerForWriteLocked() {
 
 // Caller must hold rw.mu. If rw.wroteHeader is true, calling this method is a
 // no-op.
-func (rw *responseWriter) writeHeaderLockedOnce(statusCode int) {
+func (rw *responseWriter) writeHeaderLockedOnce() {
 	if rw.wroteHeader {
 		return
 	}
-
+	if !responseCanHaveBody(rw.statusCode) {
+		rw.cannotHaveBody = true
+	}
 	// If there is any Trailer declared in headers, save them so we know which
 	// trailers have been pre-declared. Also, write back the extracted value,
 	// which is canonicalized, to rw.Header for consistency.
@@ -335,10 +366,9 @@ func (rw *responseWriter) writeHeaderLockedOnce(statusCode int) {
 		rw.headers.Set("Trailer", strings.Join(slices.Sorted(maps.Keys(rw.trailer)), ", "))
 	}
 
-	enc := &qpackEncoder{}
-	enc.init()
-	encHeaders := enc.encode(func(f func(itype indexType, name, value string)) {
-		f(mayIndex, ":status", strconv.Itoa(statusCode))
+	rw.bb.inferHeader(rw.headers, rw.statusCode)
+	encHeaders := rw.bw.enc.encode(func(f func(itype indexType, name, value string)) {
+		f(mayIndex, ":status", strconv.Itoa(rw.statusCode))
 		for name, values := range rw.headers {
 			if !httpguts.ValidHeaderFieldName(name) {
 				continue
@@ -352,45 +382,128 @@ func (rw *responseWriter) writeHeaderLockedOnce(statusCode int) {
 			}
 		}
 	})
+
 	rw.st.writeVarint(int64(frameTypeHeaders))
 	rw.st.writeVarint(int64(len(encHeaders)))
 	rw.st.Write(encHeaders)
-	if statusCode >= http.StatusOK {
+	if rw.statusCode >= http.StatusOK {
 		rw.wroteHeader = true
 	}
 }
 
 func (rw *responseWriter) WriteHeader(statusCode int) {
+	// TODO: handle sending informational status headers (e.g. 103).
 	rw.mu.Lock()
 	defer rw.mu.Unlock()
-	rw.writeHeaderLockedOnce(statusCode)
+	if rw.statusCodeSet {
+		return
+	}
+	rw.statusCodeSet = true
+	rw.statusCode = statusCode
 }
 
 func (rw *responseWriter) Write(b []byte) (int, error) {
+	// Calling Write implicitly calls WriteHeader(200) if WriteHeader has not
+	// been called before.
+	rw.WriteHeader(http.StatusOK)
 	rw.mu.Lock()
 	defer rw.mu.Unlock()
-	rw.writeHeaderLockedOnce(http.StatusOK)
-	if rw.isHeadResp {
-		return 0, nil
+
+	// If b fits entirely in our body buffer, save it to the buffer and return
+	// early so we can coalesce small writes.
+	// As a special case, we always want to save b to the buffer even when b is
+	// big if we had yet to write our header, so we can infer headers like
+	// "Content-Type" with as much information as possible.
+	initialBufLen := len(rw.bb)
+	if !rw.wroteHeader || len(b) <= cap(rw.bb)-len(rw.bb) {
+		b = rw.bb.write(b)
+		if len(b) == 0 {
+			return len(b), nil
+		}
 	}
-	return rw.bw.Write(b)
+
+	// Reaching this point means that our buffer has been sufficiently filled.
+	// Therefore, we now want to:
+	// 1. Infer and write response headers based on our body buffer, if not
+	// done yet.
+	// 2. Write our body buffer and the rest of b (if any).
+	// 3. Reset the current body buffer so it can be used again.
+	rw.writeHeaderLockedOnce()
+	if rw.cannotHaveBody {
+		return len(b), nil
+	}
+	if n, err := rw.bw.write(rw.bb, b); err != nil {
+		return max(0, n-initialBufLen), err
+	}
+	rw.bb.discard()
+	return len(b), nil
 }
 
 func (rw *responseWriter) Flush() {
+	// Calling Flush implicitly calls WriteHeader(200) if WriteHeader has not
+	// been called before.
+	rw.WriteHeader(http.StatusOK)
 	rw.mu.Lock()
-	rw.writeHeaderLockedOnce(http.StatusOK)
+	rw.writeHeaderLockedOnce()
+	if !rw.cannotHaveBody {
+		rw.bw.Write(rw.bb)
+		rw.bb.discard()
+	}
 	rw.mu.Unlock()
-	rw.bw.st.Flush()
+	rw.st.Flush()
 }
 
 func (rw *responseWriter) close() error {
+	rw.Flush()
 	rw.mu.Lock()
 	defer rw.mu.Unlock()
-	rw.writeHeaderLockedOnce(http.StatusOK)
 	rw.prepareTrailerForWriteLocked()
-
 	if err := rw.bw.Close(); err != nil {
 		return err
 	}
 	return rw.st.stream.Close()
 }
+
+// defaultBodyBufferCap is the default number of bytes of body that we are
+// willing to save in a buffer for the sake of inferring headers and coalescing
+// small writes. 512 was chosen to be consistent with how much
+// http.DetectContentType is willing to read.
+const defaultBodyBufferCap = 512
+
+// bodyBuffer is a buffer used to store body content of a response.
+type bodyBuffer []byte
+
+// write writes b to the buffer. It returns a new slice of b, which contains
+// any remaining data that could not be written to the buffer, if any.
+func (bb *bodyBuffer) write(b []byte) []byte {
+	n := min(len(b), cap(*bb)-len(*bb))
+	*bb = append(*bb, b[:n]...)
+	return b[n:]
+}
+
+// discard resets the buffer so it can be used again.
+func (bb *bodyBuffer) discard() {
+	*bb = (*bb)[:0]
+}
+
+// inferHeader populates h with the header values that we can infer from our
+// current buffer content, if not already explicitly set. This method should be
+// called only once with as much body content as possible in the buffer, before
+// a HEADERS frame is sent, and before discard has been called. Doing so
+// properly is the responsibility of the caller.
+func (bb *bodyBuffer) inferHeader(h http.Header, status int) {
+	if _, ok := h["Date"]; !ok {
+		h.Set("Date", time.Now().UTC().Format(http.TimeFormat))
+	}
+	// If the Content-Encoding is non-blank, we shouldn't
+	// sniff the body. See Issue golang.org/issue/31753.
+	_, hasCE := h["Content-Encoding"]
+	_, hasCT := h["Content-Type"]
+	if !hasCE && !hasCT && responseCanHaveBody(status) && len(*bb) > 0 {
+		h.Set("Content-Type", http.DetectContentType(*bb))
+	}
+	// We can technically infer Content-Length too here, as long as the entire
+	// response body fits within hi.buf and does not require flushing. However,
+	// we have chosen not to do so for now as Content-Length is not very
+	// important for HTTP/3, and such inconsistent behavior might be confusing.
+}

internal/http3/server_test.go

@@ -11,6 +11,8 @@ import (
 	"net/netip"
 	"net/url"
 	"reflect"
+	"slices"
+	"strconv"
 	"testing"
 	"testing/synctest"
 	"time"
@@ -82,7 +84,7 @@ func TestServerHeader(t *testing.T) {
 			"header-from-client": {"that", "should", "be", "echoed"},
 		}))
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{
+		reqStream.wantSomeHeaders(http.Header{
 			":status":            {"204"},
 			"Header-From-Client": {"that", "should", "be", "echoed"},
 		})
@@ -129,7 +131,7 @@ func TestServerPseudoHeader(t *testing.T) {
 			":path":      {"/some/path?query=value&query2=value2#fragment"},
 		})
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{":status": {"321"}})
+		reqStream.wantSomeHeaders(http.Header{":status": {"321"}})
 		reqStream.wantClosed("request is complete")
 
 		reqStream = tc.newStream(streamTypeRequest)
@@ -155,7 +157,7 @@ func TestServerInvalidHeader(t *testing.T) {
 		reqStream := tc.newStream(streamTypeRequest)
 		reqStream.writeHeaders(requestHeader(nil))
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{
+		reqStream.wantSomeHeaders(http.Header{
 			":status":      {"200"},
 			"Valid-Name":   {"valid value"},
 			"Valid-Name-2": {"valid value 2"},
@@ -183,9 +185,9 @@ func TestServerBody(t *testing.T) {
 		reqStream.writeData(bodyContent)
 		reqStream.stream.stream.CloseWrite()
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{":status": {"200"}})
-		reqStream.wantData([]byte("/"))
-		reqStream.wantData(bodyContent)
+		reqStream.wantSomeHeaders(http.Header{":status": {"200"}})
+		// Small multiple calls to Write will be coalesced into one DATA frame.
+		reqStream.wantData(append([]byte("/"), bodyContent...))
 		reqStream.wantClosed("request is complete")
 	})
 }
@@ -202,14 +204,14 @@ func TestServerHeadResponseNoBody(t *testing.T) {
 		reqStream := tc.newStream(streamTypeRequest)
 		reqStream.writeHeaders(requestHeader(nil))
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{":status": {"200"}})
+		reqStream.wantSomeHeaders(http.Header{":status": {"200"}})
 		reqStream.wantData(bodyContent)
 		reqStream.wantClosed("request is complete")
 
 		reqStream = tc.newStream(streamTypeRequest)
 		reqStream.writeHeaders(requestHeader(http.Header{":method": {http.MethodHead}}))
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{":status": {"200"}})
+		reqStream.wantSomeHeaders(http.Header{":status": {"200"}})
 		reqStream.wantClosed("request is complete")
 	})
 }
@@ -225,7 +227,7 @@ func TestServerHandlerEmpty(t *testing.T) {
 		reqStream := tc.newStream(streamTypeRequest)
 		reqStream.writeHeaders(requestHeader(nil))
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{":status": {"200"}})
+		reqStream.wantSomeHeaders(http.Header{":status": {"200"}})
 		reqStream.wantClosed("request is complete")
 	})
 }
@@ -291,7 +293,7 @@ func TestServerHandlerStreaming(t *testing.T) {
 		reqStream := tc.newStream(streamTypeRequest)
 		reqStream.writeHeaders(requestHeader(nil))
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{":status": {"200"}})
+		reqStream.wantSomeHeaders(http.Header{":status": {"200"}})
 
 		for _, data := range []string{"a", "bunch", "of", "things", "to", "stream"} {
 			stream <- data
@@ -335,13 +337,14 @@ func TestServerExpect100Continue(t *testing.T) {
 		// Wait until server responds with HTTP status 100 before sending the
 		// body.
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{":status": {"100"}})
+		reqStream.wantSomeHeaders(http.Header{":status": {"100"}})
 		body := []byte("body that will be echoed back if we get status 100")
 		reqStream.writeData(body)
 		reqStream.stream.stream.CloseWrite()
 
 		// Receive the server's response after sending the body.
-		reqStream.wantHeaders(http.Header{":status": {"200"}})
+		synctest.Wait()
+		reqStream.wantSomeHeaders(http.Header{":status": {"200"}})
 		reqStream.wantData(body)
 		reqStream.wantClosed("request is complete")
 	})
@@ -365,12 +368,43 @@ func TestServerExpect100ContinueRejected(t *testing.T) {
 
 		// Server rejects it.
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{":status": {"403"}})
+		reqStream.wantSomeHeaders(http.Header{":status": {"403"}})
 		reqStream.wantData(rejectBody)
 		reqStream.wantClosed("request is complete")
 	})
 }
 
+func TestServerNoExpect100ContinueAfterNormalResponse(t *testing.T) {
+	synctest.Test(t, func(t *testing.T) {
+		ts := newTestServer(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+			w.WriteHeader(200)
+			w.(http.Flusher).Flush()
+			// This should not cause an HTTP 100 status to be sent since we
+			// have sent an HTTP 200 response already.
+			io.ReadAll(r.Body)
+		}))
+		tc := ts.connect()
+		tc.greet()
+
+		// Client sends an Expect: 100-continue request.
+		reqStream := tc.newStream(streamTypeRequest)
+		reqStream.writeHeaders(requestHeader(http.Header{
+			"Expect": {"100-continue"},
+		}))
+		// Client sends a body prematurely. This should not happen, unless a
+		// client misbehaves. We do so here anyways so the server handler can
+		// read the request body without hanging, which would normally cause an
+		// HTTP 100 to be sent.
+		reqStream.writeData([]byte("some body"))
+		reqStream.stream.stream.CloseWrite()
+
+		// Verify that no HTTP 100 was sent.
+		synctest.Wait()
+		reqStream.wantSomeHeaders(http.Header{":status": {"200"}})
+		reqStream.wantClosed("request is complete")
+	})
+}
+
 func TestServerHandlerReadReqWithNoBody(t *testing.T) {
 	synctest.Test(t, func(t *testing.T) {
 		serverBody := []byte("hello from server!")
@@ -389,7 +423,7 @@ func TestServerHandlerReadReqWithNoBody(t *testing.T) {
 		reqStream.writeHeaders(requestHeader(nil))
 		reqStream.stream.stream.CloseWrite()
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{":status": {"200"}})
+		reqStream.wantSomeHeaders(http.Header{":status": {"200"}})
 		reqStream.wantData(serverBody)
 		reqStream.wantClosed("request is complete")
 
@@ -400,7 +434,7 @@ func TestServerHandlerReadReqWithNoBody(t *testing.T) {
 			"Content-Length": {"0"},
 		}))
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{":status": {"200"}})
+		reqStream.wantSomeHeaders(http.Header{":status": {"200"}})
 		reqStream.wantData(serverBody)
 		reqStream.wantClosed("request is complete")
 	})
@@ -508,12 +542,12 @@ func TestServerHandlerWriteTrailer(t *testing.T) {
 		reqStream := tc.newStream(streamTypeRequest)
 		reqStream.writeHeaders(requestHeader(nil))
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{
+		reqStream.wantSomeHeaders(http.Header{
 			":status": {"200"},
 			"Trailer": {"Server-Trailer-A, Server-Trailer-B, Server-Trailer-C"},
 		})
 		reqStream.wantData(body)
-		reqStream.wantHeaders(http.Header{
+		reqStream.wantSomeHeaders(http.Header{
 			"Server-Trailer-A": {"valuea"},
 			"Server-Trailer-C": {"valuec"},
 		})
@@ -539,11 +573,11 @@ func TestServerHandlerWriteTrailerNoBody(t *testing.T) {
 		reqStream := tc.newStream(streamTypeRequest)
 		reqStream.writeHeaders(requestHeader(nil))
 		synctest.Wait()
-		reqStream.wantHeaders(http.Header{
+		reqStream.wantSomeHeaders(http.Header{
 			":status": {"200"},
 			"Trailer": {"Server-Trailer-A, Server-Trailer-B, Server-Trailer-C"},
 		})
-		reqStream.wantHeaders(http.Header{
+		reqStream.wantSomeHeaders(http.Header{
 			"Server-Trailer-A": {"valuea"},
 			"Server-Trailer-C": {"valuec"},
 		})
@@ -551,6 +585,211 @@ func TestServerHandlerWriteTrailerNoBody(t *testing.T) {
 	})
 }
 
+func TestServerInfersHeaders(t *testing.T) {
+	tests := []struct {
+		name            string
+		flushedEarly    bool
+		responseStatus  int
+		does100Continue bool
+		declaredHeader  http.Header
+		want            http.Header
+	}{
+		{
+			name:           "infers undeclared headers",
+			responseStatus: 200,
+			declaredHeader: http.Header{
+				"Some-Other-Header": {"some value"},
+			},
+			want: http.Header{
+				"Date":              {"Sat, 01 Jan 2000 00:00:00 GMT"}, // Synctest starting time.
+				"Content-Type":      {"text/html; charset=utf-8"},
+				"Some-Other-Header": {"some value"},
+			},
+		},
+		{
+			name:           "does not write over declared header",
+			responseStatus: 200,
+			declaredHeader: http.Header{
+				"Date":              {"some date"},
+				"Content-Type":      {"some content type"},
+				"Some-Other-Header": {"some value"},
+			},
+			want: http.Header{
+				"Date":              {"some date"},
+				"Content-Type":      {"some content type"},
+				"Some-Other-Header": {"some value"},
+			},
+		},
+		{
+			name:           "does not infer content type for response with no body",
+			responseStatus: 304, // 304 status response has no body.
+			declaredHeader: http.Header{
+				"Some-Other-Header": {"some value"},
+			},
+			want: http.Header{
+				"Date":              {"Sat, 01 Jan 2000 00:00:00 GMT"}, // Synctest starting time.
+				"Some-Other-Header": {"some value"},
+			},
+		},
+		{
+			// See golang.org/issue/31753.
+			name:           "does not infer content type for response with declared content encoding",
+			responseStatus: 200,
+			declaredHeader: http.Header{
+				"Content-Encoding":  {"some encoding"},
+				"Some-Other-Header": {"some value"},
+			},
+			want: http.Header{
+				"Date":              {"Sat, 01 Jan 2000 00:00:00 GMT"}, // Synctest starting time.
+				"Content-Encoding":  {"some encoding"},
+				"Some-Other-Header": {"some value"},
+			},
+		},
+		{
+			name:           "does not infer content type when header is flushed before body is written",
+			responseStatus: 200,
+			flushedEarly:   true,
+			declaredHeader: http.Header{
+				"Some-Other-Header": {"some value"},
+			},
+			want: http.Header{
+				"Date":              {"Sat, 01 Jan 2000 00:00:00 GMT"}, // Synctest starting time.
+				"Some-Other-Header": {"some value"},
+			},
+		},
+		{
+			name:            "infers header for the header that comes after 100 continue",
+			responseStatus:  200,
+			does100Continue: true,
+			declaredHeader: http.Header{
+				"Some-Other-Header": {"some value"},
+			},
+			want: http.Header{
+				"Date":              {"Sat, 01 Jan 2000 00:00:00 GMT"}, // Synctest starting time.
+				"Content-Type":      {"text/html; charset=utf-8"},
+				"Some-Other-Header": {"some value"},
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		synctestSubtest(t, tt.name, func(t *testing.T) {
+			body := []byte("<html>some html content</html>")
+			streamIdle := make(chan bool)
+			ts := newTestServer(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				if tt.does100Continue {
+					<-streamIdle
+					io.ReadAll(r.Body)
+				}
+				for name, values := range tt.declaredHeader {
+					for _, value := range values {
+						w.Header().Add(name, value)
+					}
+				}
+				w.WriteHeader(tt.responseStatus)
+				if tt.flushedEarly {
+					w.(http.Flusher).Flush()
+				}
+				// Write the body one byte at a time. To confirm that body
+				// writes are buffered and that Content-Type will not be
+				// wrongly identified as text/plain rather than text/html.
+				for _, b := range body {
+					w.Write([]byte{b})
+				}
+			}))
+			tc := ts.connect()
+			tc.greet()
+
+			reqStream := tc.newStream(streamTypeRequest)
+
+			if tt.does100Continue {
+				reqStream.writeHeaders(requestHeader(http.Header{
+					"Expect": {"100-continue"},
+				}))
+				reqStream.wantIdle("stream is idle until server sends an HTTP 100 status")
+				streamIdle <- true
+				synctest.Wait()
+				reqStream.wantHeaders(http.Header{":status": {"100"}})
+			}
+
+			reqStream.writeHeaders(requestHeader(nil))
+			synctest.Wait()
+			tt.want.Add(":status", strconv.Itoa(tt.responseStatus))
+			reqStream.wantHeaders(tt.want)
+			if responseCanHaveBody(tt.responseStatus) {
+				reqStream.wantData(body)
+			}
+			reqStream.wantClosed("request is complete")
+		})
+	}
+}
+
+func TestServerBuffersBodyWrite(t *testing.T) {
+	tests := []struct {
+		name      string
+		bodyLen   int
+		writeSize int
+		flushes   bool
+	}{
+		{
+			name:      "buffers small body content",
+			bodyLen:   defaultBodyBufferCap * 10,
+			writeSize: 5,
+			flushes:   false,
+		},
+		{
+			name:      "does not buffer large body content",
+			bodyLen:   defaultBodyBufferCap * 10,
+			writeSize: defaultBodyBufferCap * 2,
+			flushes:   false,
+		},
+		{
+			name:      "does not buffer flushed body content",
+			bodyLen:   defaultBodyBufferCap * 10,
+			writeSize: 10,
+			flushes:   true,
+		},
+	}
+	for _, tt := range tests {
+		synctestSubtest(t, tt.name, func(t *testing.T) {
+			ts := newTestServer(t, http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+				for n := 0; n < tt.bodyLen; n += tt.writeSize {
+					data := slices.Repeat([]byte("a"), min(tt.writeSize, tt.bodyLen-n))
+					w.Write(data)
+					if tt.flushes {
+						w.(http.Flusher).Flush()
+					}
+				}
+			}))
+			tc := ts.connect()
+			tc.greet()
+
+			reqStream := tc.newStream(streamTypeRequest)
+			reqStream.writeHeaders(requestHeader(nil))
+			synctest.Wait()
+			reqStream.wantHeaders(nil)
+			switch {
+			case tt.writeSize > defaultBodyBufferCap:
+				// After using the buffer once, it is no longer used since the
+				// writeSize is larger than the buffer.
+				for n := 0; n < tt.bodyLen; n += tt.writeSize {
+					reqStream.wantData(slices.Repeat([]byte("a"), min(tt.writeSize, tt.bodyLen-n)))
+				}
+			case tt.flushes:
+				for n := 0; n < tt.bodyLen; n += tt.writeSize {
+					reqStream.wantData(slices.Repeat([]byte("a"), min(tt.writeSize, tt.bodyLen-n)))
+				}
+			case tt.writeSize <= defaultBodyBufferCap:
+				dataLen := defaultBodyBufferCap + tt.writeSize - (defaultBodyBufferCap % tt.writeSize)
+				for n := 0; n < tt.bodyLen; n += dataLen {
+					reqStream.wantData(slices.Repeat([]byte("a"), min(dataLen, tt.bodyLen-n)))
+				}
+			}
+			reqStream.wantClosed("request is complete")
+		})
+	}
+}
+
 type testServer struct {
 	t  testing.TB
 	s  *Server

internal/http3/transport_test.go

@@ -215,6 +215,43 @@ func (ts *testQUICStream) wantHeaders(want http.Header) {
 	}
 }
 
+// wantSomeHeaders reads a HEADERS frame and asserts that want is a subset of
+// the read HEADERS frame.
+// This is like wantHeaders, but headers that are in the HEADERS frame but not
+// in want are ignored.
+func (ts *testQUICStream) wantSomeHeaders(want http.Header) {
+	ts.t.Helper()
+	ftype, err := ts.readFrameHeader()
+	if err != nil {
+		ts.t.Fatalf("want HEADERS frame, got error: %v", err)
+	}
+	if ftype != frameTypeHeaders {
+		ts.t.Fatalf("want HEADERS frame, got: %v", ftype)
+	}
+
+	if want == nil {
+		panic("use wantHeaders(nil) instead to ignore the content of the frame")
+	}
+
+	got := make(http.Header)
+	var dec qpackDecoder
+	err = dec.decode(ts.stream, func(_ indexType, name, value string) error {
+		got.Add(name, value)
+		return nil
+	})
+	for name := range got {
+		if _, ok := want[name]; !ok {
+			delete(got, name)
+		}
+	}
+	if diff := diffHeaders(got, want); diff != "" {
+		ts.t.Fatalf("unexpected response headers:\n%v", diff)
+	}
+	if err := ts.endFrame(); err != nil {
+		ts.t.Fatalf("endFrame: %v", err)
+	}
+}
+
 func (ts *testQUICStream) encodeHeaders(h http.Header) []byte {
 	ts.t.Helper()
 	var enc qpackEncoder