net/http: make Transport.RoundTrip check context.Done earlier

Fixes #25852

Change-Id: I35c630367c8f1934dcffc0b0e08891d55a903518
Reviewed-on: https://go-review.googlesource.com/118560
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Andrew Bonventre <andybons@golang.org>

src/net/http/transport.go

@@ -370,6 +370,13 @@ func (t *Transport) roundTrip(req *Request) (*Response, error) {
 	}
 
 	for {
+		select {
+		case <-ctx.Done():
+			req.closeBody()
+			return nil, ctx.Err()
+		default:
+		}
+
 		// treq gets modified by roundTrip, so we need to recreate for each retry.
 		treq := &transportRequest{Request: req, trace: trace}
 		cm, err := t.connectMethodForRequest(treq)

src/net/http/transport_test.go

@@ -4544,3 +4544,28 @@ func TestNoBodyOnChunked304Response(t *testing.T) {
 type funcWriter func([]byte) (int, error)
 
 func (f funcWriter) Write(p []byte) (int, error) { return f(p) }
+
+type doneContext struct {
+	context.Context
+	err error
+}
+
+func (doneContext) Done() <-chan struct{} {
+	c := make(chan struct{})
+	close(c)
+	return c
+}
+
+func (d doneContext) Err() error { return d.err }
+
+// Issue 25852: Transport should check whether Context is done early.
+func TestTransportCheckContextDoneEarly(t *testing.T) {
+	tr := &Transport{}
+	req, _ := NewRequest("GET", "http://fake.example/", nil)
+	wantErr := errors.New("some error")
+	req = req.WithContext(doneContext{context.Background(), wantErr})
+	_, err := tr.RoundTrip(req)
+	if err != wantErr {
+		t.Errorf("error = %v; want %v", err, wantErr)
+	}
+}