Echo response object calls flush on unflushable objects

Issue Description

The Echo Response object causes a panic when calling the Flush method with an unflushable parent writer.

For example, if you are using TimeoutMiddleware, which creates TimeoutHandler (github.com/labstack/echo/v4/middleware/timeout.go:124) and then creates a timeout writer (src/net/http/server.go:3584), and you try to call the Flush method in the echo.Context object, it will result in a panic. This happens because it doesn't check if the Writer implements the Flush method before calling it (github.com/labstack/echo/v4/response.go:87). This issue can be handled similarly to how the standard http library does it (httputil/reverseproxy.go:524).

I've noticed duplicated issues, but in my humble opinion, you should try to unpack writers and search for flushable ones, similar to how it's done in the standard library. What do you think?

Checklist

Dependencies installed
No typos
Searched existing issues and docs

Expected behaviour

Calling Flush method with response writers that doesn't implements Flusher interface should not panicking

Actual behaviour

Calling Flush method with response writers that doesn't implements Flusher interface is panicking

Steps to reproduce

Use timeout middleware
Implement endpoint that call c.(echo.Context).Response().Flush() method
Curl that endpoint

Working code to debug

func main() {
	e := echo.New()
	e.Use(middleware.TimeoutWithConfig(middleware.TimeoutConfig{
		Timeout: 3 * time.Second,
	}))

	e.GET("/ping", func(c echo.Context) error {
		c.Response().Flush()
		return c.String(http.StatusOK, "pong")
	})

	if err := e.Start(":8080"); err != nil {
		log.Fatalln(err.Error())
	}
}

Version/commit

github.com/labstack/echo/v4 v4.11.4

	e.GET("/ping", func(c echo.Context) error {
		c.Response().Flush()
		return c.String(http.StatusOK, "pong")
	})

Is not very good example. Or lets rephrase that - this shows that Timeout middleware has problems with handling requests that want to flush the response.

We could change Response.Flush() to support unwrapping but I think we still should panic if unwrapping ends up with http.ErrNotSupported. Hiding the fact that Flush did not do anything hides potential problems. In that example we have explicit call for Flush which means that there is some requirement why we need to flush. No-oping that could be a bug.

echo/response.go

Line 86 in 29aab27

func (r *Response) Flush() {

to

func (r *Response) Flush() {
	err := http.NewResponseController(r.Writer).Flush()
	if err != nil && errors.Is(err, http.ErrNotSupported) {
		panic(fmt.Errorf("response writer flushing is not supported"))
	}
}

and make Timeout middleware support unwrapping (and other custom writers we have created)

NB: you should avoid using Timeout middleware. In its basic form it just sends the response to the client and potentially does not end your handler goroutine if you have not properly implemented context cancellation checks.

If you want to properly handle timeout you need to implement context checks and this renders Timeout MW useless. As then you check for context.DeadlineExceeded/context.Canceled and know it is reached you will can send the response to client.

I agree that it's supposed to cause panic if flushing is not supported, but at least, it should have a chance to unwrap parent writers to check if there is a flusher one.

About timeout middleware, I don't fully understand you, It does just what I want it to do - cancel context, if timeout is reached

I'll create PR for #2592 (comment) changes

@qerdcv in you example you are using middleware.TimeoutWithConfig this uses goroutine to serve your request. Maybe you wanted to use middleware.ContextTimeout() ?

ContextTimeout just replaces context for request

echo/middleware/context_timeout.go

Line 61 in ea529bb

timeoutContext, cancel := context.WithTimeout(c.Request().Context(), config.Timeout)
TimeoutWithConfig runs your request in separate goroutine

echo/middleware/timeout.go

Line 124 in ea529bb

handler := http.TimeoutHandler(handlerWrapper, config.Timeout, config.ErrorMessage)

https://github.com/golang/go/blob/e17e5308fd5a26da5702d16cc837ee77cdb30ab6/src/net/http/server.go#L3590-L3598

There are recommendations for timeout handling at the start of https://github.com/labstack/echo/blob/ea529bbab6602db8bd9fc0746405a3687ffbd885/middleware/timeout.go

It seems that you are right.
I'll recheck what behaviour exactly I need.
Thanks!

done in #2595