appengine/gaemiddleware/standard/urlfetch.go

// Copyright 2018 The LUCI Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package standard

import (
	"context"
	"net/http"

	"go.chromium.org/luci/gae/service/urlfetch"
)

// contextAwareURLFetch implements http.RoundTripper by instantiating GAE's
// urlfetch.Transport for each request.
//
// Requests done through GAE's urlfetch.Transport inherit the deadline of
// a context used to create the transport, totally ignoring the deadline in the
// request's context. This leads to surprising bugs.
//
// contextAwareURLFetch works around this problem by instantiating a new
// urlfetch.Transport for each request, using request's context as a basis
// (if possible), and falling back to the context provided during the creation
// otherwise.
type contextAwareURLFetch struct {
	ctx context.Context
}

// RoundTrip is part of http.RoundTripper interface.
func (c *contextAwareURLFetch) RoundTrip(r *http.Request) (*http.Response, error) {
	rt := urlfetch.Get(r.Context())
	if rt == nil {
		rt = urlfetch.Get(c.ctx)
		if rt == nil {
			panic("no http.RoundTripper is set in context")
		}
	}
	return rt.RoundTrip(r)
}