net/http: don't reuse a server connection after any Write errors

Fixes #8534

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/149340044

src/net/http/server.go

@@ -114,6 +114,8 @@ type conn struct {
 	remoteAddr string               // network address of remote side
 	server     *Server              // the Server on which the connection arrived
 	rwc        net.Conn             // i/o connection
+	w          io.Writer            // checkConnErrorWriter's copy of wrc, not zeroed on Hijack
+	werr       error                // any errors writing to w
 	sr         liveSwitchReader     // where the LimitReader reads from; usually the rwc
 	lr         *io.LimitedReader    // io.LimitReader(sr)
 	buf        *bufio.ReadWriter    // buffered(lr,rwc), reading from bufio->limitReader->sr->rwc
@@ -432,13 +434,14 @@ func (srv *Server) newConn(rwc net.Conn) (c *conn, err error) {
 	c.remoteAddr = rwc.RemoteAddr().String()
 	c.server = srv
 	c.rwc = rwc
+	c.w = rwc
 	if debugServerConnections {
 		c.rwc = newLoggingConn("server", c.rwc)
 	}
 	c.sr = liveSwitchReader{r: c.rwc}
 	c.lr = io.LimitReader(&c.sr, noLimit).(*io.LimitedReader)
 	br := newBufioReader(c.lr)
-	bw := newBufioWriterSize(c.rwc, 4<<10)
+	bw := newBufioWriterSize(checkConnErrorWriter{c}, 4<<10)
 	c.buf = bufio.NewReadWriter(br, bw)
 	return c, nil
 }
@@ -956,8 +959,10 @@ func (w *response) bodyAllowed() bool {
 // 2. (*response).w, a *bufio.Writer of bufferBeforeChunkingSize bytes
 // 3. chunkWriter.Writer (whose writeHeader finalizes Content-Length/Type)
 //    and which writes the chunk headers, if needed.
-// 4. conn.buf, a bufio.Writer of default (4kB) bytes
-// 5. the rwc, the net.Conn.
+// 4. conn.buf, a bufio.Writer of default (4kB) bytes, writing to ->
+// 5. checkConnErrorWriter{c}, which notes any non-nil error on Write
+//    and populates c.werr with it if so. but otherwise writes to:
+// 6. the rwc, the net.Conn.
 //
 // TODO(bradfitz): short-circuit some of the buffering when the
 // initial header contains both a Content-Type and Content-Length.
@@ -1027,6 +1032,12 @@ func (w *response) finishRequest() {
 		// Did not write enough. Avoid getting out of sync.
 		w.closeAfterReply = true
 	}
+
+	// There was some error writing to the underlying connection
+	// during the request, so don't re-use this conn.
+	if w.conn.werr != nil {
+		w.closeAfterReply = true
+	}
 }
 
 func (w *response) Flush() {
@@ -2068,3 +2079,18 @@ func (c *loggingConn) Close() (err error) {
 	log.Printf("%s.Close() = %v", c.name, err)
 	return
 }
+
+// checkConnErrorWriter writes to c.rwc and records any write errors to c.werr.
+// It only contains one field (and a pointer field at that), so it
+// fits in an interface value without an extra allocation.
+type checkConnErrorWriter struct {
+	c *conn
+}
+
+func (w checkConnErrorWriter) Write(p []byte) (n int, err error) {
+	n, err = w.c.w.Write(p) // c.w == c.rwc, except after a hijack, when rwc is nil.
+	if err != nil && w.c.werr == nil {
+		w.c.werr = err
+	}
+	return
+}