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[agent] Fix a race condition when writing a response to a cancelled request. #135

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merged 1 commit into from
Mar 21, 2024
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[agent] Fix a race condition when writing a response to a cancelled request. #135

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9 changes: 7 additions & 2 deletions agent/utils/utils.go
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Original file line number Diff line number Diff line change
Expand Up @@ -487,7 +487,7 @@ func (w *streamingResponseWriter) WriteHeader(status int) {
protoMajor = w.r.ProtoMajor
protoMinor = w.r.ProtoMinor
}
w.respChan <- &http.Response{
resp := &http.Response{
Proto: proto,
ProtoMajor: protoMajor,
ProtoMinor: protoMinor,
Expand All @@ -497,6 +497,11 @@ func (w *streamingResponseWriter) WriteHeader(status int) {
Body: w.bodyReader,
Trailer: w.trailer,
}
select {
case w.respChan <- resp:
case <-w.r.Context().Done():
w.bodyReader.Close()
}
}

func (w *streamingResponseWriter) Write(bs []byte) (int, error) {
Expand Down Expand Up @@ -543,7 +548,7 @@ func (w *streamingResponseWriter) CloseWithError(err error) error {
func NewResponseForwarder(client *http.Client, proxyHost, backendID, requestID string, r *http.Request, metricHandler *metrics.MetricHandler) (ResponseWriteCloser, error) {
// Construct a streaming response writer so we can process the written response
// in separate goroutines as it is written.
respChan := make(chan *http.Response, 1)
respChan := make(chan *http.Response)
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Just to clarify, is this change needed for the fix to work, or is it more of a defensive change?

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This is needed for the fix to work.

The issue was that on one end the response was being buffered, but on the other end it was never being read and consumed.

The fact that it wasn't consumed (written to the proxy request) meant that the response body was never being closed, and thus subsequent writes to the response body would block indefinitely.

To fix this, we have to make the goroutine reading the response and the one writing the response be synchronized through this channel.

NOTE: This reasoning is subtle, but no one has to remember it; if anyone tries to change this in the future the newly changed test will fail.

rw := NewStreamingResponseWriter(respChan, r)

// Construct two ends of a pipe that we will use for concurrently writing the response
Expand Down
7 changes: 6 additions & 1 deletion agent/utils/utils_test.go
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Original file line number Diff line number Diff line change
Expand Up @@ -512,11 +512,16 @@ func TestResponseForwarderWithRequestCancellation(t *testing.T) {
defer proxyServer.Close()
defer proxyServer.CloseClientConnections()
proxyClient := proxyServer.Client()
_, err := NewResponseForwarder(proxyClient, proxyServer.URL+"/", backendID, requestID, endUserRequest, nil)
rf, err := NewResponseForwarder(proxyClient, proxyServer.URL+"/", backendID, requestID, endUserRequest, nil)
if err != nil {
t.Fatal(err)
}
cancel()
rf.WriteHeader(http.StatusOK)
_, err = rf.Write([]byte(http.StatusText(http.StatusOK)))
if got, want := err, io.ErrClosedPipe; got != want {
t.Errorf("Unexepcted error writing a response for a cancelled request: got %v, want %v", got, want)
}
select {
case err := <-errsChan:
if err != nil {
Expand Down
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