responseBodyEnd Callback Reports Zero Byte Count for Failed Requests (4xx/5xx), Even When Backend Returns Error Body

[ddas09]

Description:
When handling failed requests (such as HTTP 4xx or 5xx responses), the responseBodyEnd callback is invoked with a byteCount of 0, even if the server includes a response body containing error details. This happens regardless of whether the response body is empty or contains information like error messages or stack traces.

I have set up a custom retry interceptor that retries the request up to two times. The backend API returns an HTTP 500 error for the first two attempts and a successful HTTP 200 response for the final retry. However, I noticed that when the backend returns a 500 error, the responseBodyEnd callback always reports a byteCount of 0, even though the body contains error details. On successful (HTTP 200) responses, the byteCount is non-zero, as expected.

Is this expected behavior? As per my understanding the byteCount should reflect the size of the body, even for error responses.

Below is the sample code snippets -
Backend API (Express):

const internalError = catchAsync(async (req: Request, res: Response) => {
  const retryCount = parseInt(req.headers["x-retry-count"] as string) || 0;

  if (retryCount >= 3) {
    res.status(200).json({ message: "Success after 3 retries!" });
  } else {
    res.status(500).json({ message: "Something went wrong on the server." });
  }
});

Here is the custom retry interceptor I am using to retry the request up to two times:

class RetryInterceptor : Interceptor {
    private val maxRetryCount = 3
    private val retryDelayMillis = 500L

    @Throws(IOException::class)
    override fun intercept(chain: Interceptor.Chain): Response {
        var attempt = 0
        var lastException: IOException? = null
        var response: Response? = null

        do {
            attempt++

            try {
                val modifiedRequest = chain.request().newBuilder()
                    .addHeader("X-Retry-Count", attempt.toString())
                    .build()

                response = chain.proceed(modifiedRequest)

                if (response.isSuccessful) {
                    return response
                }
            } catch (e: IOException) {
                lastException = e
            }

            // we need to close the response before retrying the request again
            response?.close()

            if (attempt < maxRetryCount) {
                try {
                    TimeUnit.MILLISECONDS.sleep(attempt * retryDelayMillis)
                } catch (e: InterruptedException) {
                    Thread.currentThread().interrupt()
                }
            }
        } while (attempt < maxRetryCount)

        throw lastException ?: IOException("Unknown error after $maxRetryCount retries")
    }

[swankjesse]

Looks like a bug!

[ddas09]

@swankjesse, is there any other way to determine the response body size using the Events listener API?