http2: add I/O timeouts

Addresses hanging transport when on blocking I/O. There are many scenario where
the roundtrip hangs on write or read and won't be unlocked by current
cancelation systems (context, Request.Cancel, ...).

This adds read and write deadlines support.
The writer disables the read deadline and enables the write deadline, then after
the write is successful, it disables the write deadline and re-enables the read
deadline.
The read loop also sets its read deadline after a successful read since the next
frame is not predictable.
It guarantees that an I/O will not timeout before IOTimeout and will timeout
after a complete block before at least IOTimeout.

See issue: golang/go#23559

http2/transport.go

@@ -94,6 +94,10 @@ type Transport struct {
 	// to mean no limit.
 	MaxHeaderListSize uint32
 
+	// IOTimeout, if non-zero, enables timeouts on read and write to the
+	// connection.
+	IOTimeout time.Duration
+
 	// t1, if non-nil, is the standard library Transport using
 	// this transport. Its settings are used (but not its
 	// RoundTrip method, etc).
@@ -321,7 +325,9 @@ func (noCachedConnError) Error() string             { return "http2: no cached c
 // or its equivalent renamed type in net/http2's h2_bundle.go. Both types
 // may coexist in the same running program.
 func isNoCachedConnError(err error) bool {
-	_, ok := err.(interface{ IsHTTP2NoCachedConnError() })
+	_, ok := err.(interface {
+		IsHTTP2NoCachedConnError()
+	})
 	return ok
 }
 
@@ -529,6 +535,38 @@ func (t *Transport) NewClientConn(c net.Conn) (*ClientConn, error) {
 	return t.newClientConn(c, false)
 }
 
+type timeoutWriter struct {
+	c       net.Conn
+	timeout time.Duration
+}
+
+// Write writes to the underlying connection and manages both read and write
+// deadlines.
+func (w timeoutWriter) Write(p []byte) (int, error) {
+	// The read deadline is disabled to allow so the reader doesn't timeout
+	// while there are no pending requests.
+	// The write deadline is set.
+	// The write occurs and the connection is closed on timeout interrupting the
+	// read with an error.
+	// If the write was successful it sets the deadline for the current read.
+	now := time.Now()
+	w.c.SetReadDeadline(time.Time{})
+	w.c.SetWriteDeadline(now.Add(w.timeout))
+	n, err := w.c.Write(p)
+	if err != nil {
+		if ne, ok := err.(net.Error); ok && ne.Timeout() {
+			// The write end of the connection is no longer in a known
+			// consistent state, unlock the read loop by closing the connection
+			// and force a cleanup.
+			w.c.Close()
+			return n, err
+		}
+	}
+	w.c.SetWriteDeadline(time.Time{})
+	w.c.SetReadDeadline(time.Now().Add(w.timeout))
+	return n, err
+}
+
 func (t *Transport) newClientConn(c net.Conn, singleUse bool) (*ClientConn, error) {
 	cc := &ClientConn{
 		t:                     t,
@@ -555,9 +593,14 @@ func (t *Transport) newClientConn(c net.Conn, singleUse bool) (*ClientConn, erro
 	cc.cond = sync.NewCond(&cc.mu)
 	cc.flow.add(int32(initialWindowSize))
 
+	var w io.Writer = cc.tconn
+	if d := cc.ioTimeout(); d > 0 {
+		w = timeoutWriter{c: cc.tconn, timeout: d}
+	}
+
 	// TODO: adjust this writer size to account for frame size +
 	// MTU + crypto/tls record padding.
-	cc.bw = bufio.NewWriter(stickyErrWriter{c, &cc.werr})
+	cc.bw = bufio.NewWriter(stickyErrWriter{w, &cc.werr})
 	cc.br = bufio.NewReader(c)
 	cc.fr = NewFramer(cc.bw, cc.br)
 	cc.fr.ReadMetaHeaders = hpack.NewDecoder(initialHeaderTableSize, nil)
@@ -733,6 +776,10 @@ func (cc *ClientConn) responseHeaderTimeout() time.Duration {
 	return 0
 }
 
+func (cc *ClientConn) ioTimeout() time.Duration {
+	return cc.t.IOTimeout
+}
+
 // checkConnHeaders checks whether req has any invalid connection-level headers.
 // per RFC 7540 section 8.1.2.2: Connection-Specific Header Fields.
 // Certain headers are special-cased as okay but not transmitted later.
@@ -1473,12 +1520,23 @@ func (rl *clientConnReadLoop) run() error {
 	rl.closeWhenIdle = cc.t.disableKeepAlives() || cc.singleUse
 	gotReply := false // ever saw a HEADERS reply
 	gotSettings := false
+	timeout := cc.ioTimeout()
 	for {
 		f, err := cc.fr.ReadFrame()
 		if err != nil {
 			cc.vlogf("http2: Transport readFrame error on conn %p: (%T) %v", cc, err, err)
 		}
-		if se, ok := err.(StreamError); ok {
+		if ne, ok := err.(net.Error); ok && ne.Timeout() {
+			cc.mu.Lock()
+			idle := len(cc.streams) == 0
+			if idle {
+				// let the idle timer handle the timeout.
+				cc.tconn.SetReadDeadline(time.Time{})
+				continue
+			}
+			cc.mu.Unlock()
+			return err
+		} else if se, ok := err.(StreamError); ok {
 			if cs := cc.streamByID(se.StreamID, false); cs != nil {
 				cs.cc.writeStreamReset(cs.ID, se.Code, err)
 				cs.cc.forgetStreamID(cs.ID)
@@ -1537,6 +1595,10 @@ func (rl *clientConnReadLoop) run() error {
 		if rl.closeWhenIdle && gotReply && maybeIdle {
 			cc.closeIfIdle()
 		}
+		if timeout > 0 {
+			// Upon successful read, set timeout on next read.
+			cc.tconn.SetReadDeadline(time.Now().Add(timeout))
+		}
 	}
 }