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// Copyright 2015 Google Inc. All Rights Reserved.
//
// 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 httputil
import (
"fmt"
"io"
"net/http"
"reflect"
"time"
"golang.org/x/net/context"
)
// Wait for the allDone channel to be closed. If the context is cancelled
// before then, cancel the HTTP request via the canceller repeatedly until
// allDone is closed.
//
// We must cancel repeatedly because the http package's design for cancellation
// is flawed: the canceller must naturally race with the transport receiving
// the request (CancelRequest may be called before RoundTrip, and in this case
// the cancellation will be lost). This is especially a problem with wrapper
// transports like the oauth2 one, which may do extra work before calling
// through to http.Transport.
func propagateCancellation(
ctx context.Context,
allDone chan struct{},
c CancellableRoundTripper,
req *http.Request) {
// If the context is not cancellable, there is nothing interesting we can do.
cancelChan := ctx.Done()
if cancelChan == nil {
return
}
// If the user closes allDone before the context is cancelled, we can return
// immediately.
select {
case <-allDone:
return
case <-cancelChan:
}
// The context has been cancelled. Repeatedly cancel the HTTP request as
// described above until the user closes allDone.
for {
c.CancelRequest(req)
select {
case <-allDone:
return
case <-time.After(10 * time.Millisecond):
}
}
}
// An io.ReadCloser that closes a channel when it is closed. Used to clean up
// the propagateCancellation helper.
type closeChannelReadCloser struct {
wrapped io.ReadCloser
c chan struct{}
}
func (rc *closeChannelReadCloser) Read(p []byte) (n int, err error) {
n, err = rc.wrapped.Read(p)
return
}
func (rc *closeChannelReadCloser) Close() (err error) {
if rc.c != nil {
close(rc.c)
rc.c = nil
}
err = rc.wrapped.Close()
return
}
// Call client.Do with the supplied request, cancelling the request if the
// context is cancelled. Return an error if the client does not support
// cancellation.
func Do(
ctx context.Context,
client *http.Client,
req *http.Request) (resp *http.Response, err error) {
// Make sure the transport supports cancellation.
c, ok := client.Transport.(CancellableRoundTripper)
if !ok {
err = fmt.Errorf(
"Transport of type %v doesn't support cancellation",
reflect.TypeOf(client.Transport))
return
}
// Set up a goroutine that will watch the context for cancellation, and a
// channel that tells it that it can return.
allDone := make(chan struct{})
go propagateCancellation(ctx, allDone, c, req)
// Call through. On error, clean up the helper function.
resp, err = client.Do(req)
if err != nil {
close(allDone)
return
}
// Otherwise, wrap the body in a layer that will tell the helper function
// that it can return when it is closed.
resp.Body = &closeChannelReadCloser{
wrapped: resp.Body,
c: allDone,
}
return
}
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