diff --git a/vendor/golang.org/x/net/http2/client_conn_pool.go b/vendor/golang.org/x/net/http2/client_conn_pool.go
new file mode 100644
index 0000000..652bc11
--- /dev/null
+++ b/vendor/golang.org/x/net/http2/client_conn_pool.go
@@ -0,0 +1,313 @@
+// Copyright 2015 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.
+
+// Transport code's client connection pooling.
+
+package http2
+
+import (
+	"context"
+	"crypto/tls"
+	"errors"
+	"net/http"
+	"sync"
+)
+
+// ClientConnPool manages a pool of HTTP/2 client connections.
+type ClientConnPool interface {
+	GetClientConn(req *http.Request, addr string) (*ClientConn, error)
+	MarkDead(*ClientConn)
+}
+
+// clientConnPoolIdleCloser is the interface implemented by ClientConnPool
+// implementations which can close their idle connections.
+type clientConnPoolIdleCloser interface {
+	ClientConnPool
+	closeIdleConnections()
+}
+
+var (
+	_ clientConnPoolIdleCloser = (*clientConnPool)(nil)
+	_ clientConnPoolIdleCloser = noDialClientConnPool{}
+)
+
+// TODO: use singleflight for dialing and addConnCalls?
+type clientConnPool struct {
+	t *Transport
+
+	mu sync.Mutex // TODO: maybe switch to RWMutex
+	// TODO: add support for sharing conns based on cert names
+	// (e.g. share conn for googleapis.com and appspot.com)
+	conns        map[string][]*ClientConn // key is host:port
+	dialing      map[string]*dialCall     // currently in-flight dials
+	keys         map[*ClientConn][]string
+	addConnCalls map[string]*addConnCall // in-flight addConnIfNeede calls
+}
+
+func (p *clientConnPool) GetClientConn(req *http.Request, addr string) (*ClientConn, error) {
+	return p.getClientConn(req, addr, dialOnMiss)
+}
+
+const (
+	dialOnMiss   = true
+	noDialOnMiss = false
+)
+
+// shouldTraceGetConn reports whether getClientConn should call any
+// ClientTrace.GetConn hook associated with the http.Request.
+//
+// This complexity is needed to avoid double calls of the GetConn hook
+// during the back-and-forth between net/http and x/net/http2 (when the
+// net/http.Transport is upgraded to also speak http2), as well as support
+// the case where x/net/http2 is being used directly.
+func (p *clientConnPool) shouldTraceGetConn(st clientConnIdleState) bool {
+	// If our Transport wasn't made via ConfigureTransport, always
+	// trace the GetConn hook if provided, because that means the
+	// http2 package is being used directly and it's the one
+	// dialing, as opposed to net/http.
+	if _, ok := p.t.ConnPool.(noDialClientConnPool); !ok {
+		return true
+	}
+	// Otherwise, only use the GetConn hook if this connection has
+	// been used previously for other requests. For fresh
+	// connections, the net/http package does the dialing.
+	return !st.freshConn
+}
+
+func (p *clientConnPool) getClientConn(req *http.Request, addr string, dialOnMiss bool) (*ClientConn, error) {
+	if isConnectionCloseRequest(req) && dialOnMiss {
+		// It gets its own connection.
+		traceGetConn(req, addr)
+		const singleUse = true
+		cc, err := p.t.dialClientConn(req.Context(), addr, singleUse)
+		if err != nil {
+			return nil, err
+		}
+		return cc, nil
+	}
+	for {
+		p.mu.Lock()
+		for _, cc := range p.conns[addr] {
+			if st := cc.idleState(); st.canTakeNewRequest {
+				if p.shouldTraceGetConn(st) {
+					traceGetConn(req, addr)
+				}
+				p.mu.Unlock()
+				return cc, nil
+			}
+		}
+		if !dialOnMiss {
+			p.mu.Unlock()
+			return nil, ErrNoCachedConn
+		}
+		traceGetConn(req, addr)
+		call := p.getStartDialLocked(req.Context(), addr)
+		p.mu.Unlock()
+		<-call.done
+		if shouldRetryDial(call, req) {
+			continue
+		}
+		return call.res, call.err
+	}
+}
+
+// dialCall is an in-flight Transport dial call to a host.
+type dialCall struct {
+	_ incomparable
+	p *clientConnPool
+	// the context associated with the request
+	// that created this dialCall
+	ctx  context.Context
+	done chan struct{} // closed when done
+	res  *ClientConn   // valid after done is closed
+	err  error         // valid after done is closed
+}
+
+// requires p.mu is held.
+func (p *clientConnPool) getStartDialLocked(ctx context.Context, addr string) *dialCall {
+	if call, ok := p.dialing[addr]; ok {
+		// A dial is already in-flight. Don't start another.
+		return call
+	}
+	call := &dialCall{p: p, done: make(chan struct{}), ctx: ctx}
+	if p.dialing == nil {
+		p.dialing = make(map[string]*dialCall)
+	}
+	p.dialing[addr] = call
+	go call.dial(call.ctx, addr)
+	return call
+}
+
+// run in its own goroutine.
+func (c *dialCall) dial(ctx context.Context, addr string) {
+	const singleUse = false // shared conn
+	c.res, c.err = c.p.t.dialClientConn(ctx, addr, singleUse)
+	close(c.done)
+
+	c.p.mu.Lock()
+	delete(c.p.dialing, addr)
+	if c.err == nil {
+		c.p.addConnLocked(addr, c.res)
+	}
+	c.p.mu.Unlock()
+}
+
+// addConnIfNeeded makes a NewClientConn out of c if a connection for key doesn't
+// already exist. It coalesces concurrent calls with the same key.
+// This is used by the http1 Transport code when it creates a new connection. Because
+// the http1 Transport doesn't de-dup TCP dials to outbound hosts (because it doesn't know
+// the protocol), it can get into a situation where it has multiple TLS connections.
+// This code decides which ones live or die.
+// The return value used is whether c was used.
+// c is never closed.
+func (p *clientConnPool) addConnIfNeeded(key string, t *Transport, c *tls.Conn) (used bool, err error) {
+	p.mu.Lock()
+	for _, cc := range p.conns[key] {
+		if cc.CanTakeNewRequest() {
+			p.mu.Unlock()
+			return false, nil
+		}
+	}
+	call, dup := p.addConnCalls[key]
+	if !dup {
+		if p.addConnCalls == nil {
+			p.addConnCalls = make(map[string]*addConnCall)
+		}
+		call = &addConnCall{
+			p:    p,
+			done: make(chan struct{}),
+		}
+		p.addConnCalls[key] = call
+		go call.run(t, key, c)
+	}
+	p.mu.Unlock()
+
+	<-call.done
+	if call.err != nil {
+		return false, call.err
+	}
+	return !dup, nil
+}
+
+type addConnCall struct {
+	_    incomparable
+	p    *clientConnPool
+	done chan struct{} // closed when done
+	err  error
+}
+
+func (c *addConnCall) run(t *Transport, key string, tc *tls.Conn) {
+	cc, err := t.NewClientConn(tc)
+
+	p := c.p
+	p.mu.Lock()
+	if err != nil {
+		c.err = err
+	} else {
+		p.addConnLocked(key, cc)
+	}
+	delete(p.addConnCalls, key)
+	p.mu.Unlock()
+	close(c.done)
+}
+
+// p.mu must be held
+func (p *clientConnPool) addConnLocked(key string, cc *ClientConn) {
+	for _, v := range p.conns[key] {
+		if v == cc {
+			return
+		}
+	}
+	if p.conns == nil {
+		p.conns = make(map[string][]*ClientConn)
+	}
+	if p.keys == nil {
+		p.keys = make(map[*ClientConn][]string)
+	}
+	p.conns[key] = append(p.conns[key], cc)
+	p.keys[cc] = append(p.keys[cc], key)
+}
+
+func (p *clientConnPool) MarkDead(cc *ClientConn) {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	for _, key := range p.keys[cc] {
+		vv, ok := p.conns[key]
+		if !ok {
+			continue
+		}
+		newList := filterOutClientConn(vv, cc)
+		if len(newList) > 0 {
+			p.conns[key] = newList
+		} else {
+			delete(p.conns, key)
+		}
+	}
+	delete(p.keys, cc)
+}
+
+func (p *clientConnPool) closeIdleConnections() {
+	p.mu.Lock()
+	defer p.mu.Unlock()
+	// TODO: don't close a cc if it was just added to the pool
+	// milliseconds ago and has never been used. There's currently
+	// a small race window with the HTTP/1 Transport's integration
+	// where it can add an idle conn just before using it, and
+	// somebody else can concurrently call CloseIdleConns and
+	// break some caller's RoundTrip.
+	for _, vv := range p.conns {
+		for _, cc := range vv {
+			cc.closeIfIdle()
+		}
+	}
+}
+
+func filterOutClientConn(in []*ClientConn, exclude *ClientConn) []*ClientConn {
+	out := in[:0]
+	for _, v := range in {
+		if v != exclude {
+			out = append(out, v)
+		}
+	}
+	// If we filtered it out, zero out the last item to prevent
+	// the GC from seeing it.
+	if len(in) != len(out) {
+		in[len(in)-1] = nil
+	}
+	return out
+}
+
+// noDialClientConnPool is an implementation of http2.ClientConnPool
+// which never dials. We let the HTTP/1.1 client dial and use its TLS
+// connection instead.
+type noDialClientConnPool struct{ *clientConnPool }
+
+func (p noDialClientConnPool) GetClientConn(req *http.Request, addr string) (*ClientConn, error) {
+	return p.getClientConn(req, addr, noDialOnMiss)
+}
+
+// shouldRetryDial reports whether the current request should
+// retry dialing after the call finished unsuccessfully, for example
+// if the dial was canceled because of a context cancellation or
+// deadline expiry.
+func shouldRetryDial(call *dialCall, req *http.Request) bool {
+	if call.err == nil {
+		// No error, no need to retry
+		return false
+	}
+	if call.ctx == req.Context() {
+		// If the call has the same context as the request, the dial
+		// should not be retried, since any cancellation will have come
+		// from this request.
+		return false
+	}
+	if !errors.Is(call.err, context.Canceled) && !errors.Is(call.err, context.DeadlineExceeded) {
+		// If the call error is not because of a context cancellation or a deadline expiry,
+		// the dial should not be retried.
+		return false
+	}
+	// Only retry if the error is a context cancellation error or deadline expiry
+	// and the context associated with the call was canceled or expired.
+	return call.ctx.Err() != nil
+}