Check if (r *Response).Writer is http.Flusher before calling Flush

[MarkRosemaker]

There is no guarantee that the writer is a flusher. In fact, I noticed this issue because a service of a company I work for has paniced because it wasn't:

[PANIC RECOVER] interface conversion: *http.timeoutWriter is not http.Flusher: missing method Flush goroutine 4811 [running]:
github.com/labstack/echo/v4/middleware.RecoverWithConfig.func1.1.1()
	/go/src/service/vendor/github.com/labstack/echo/v4/middleware/recover.go:77 +0x10d
panic({0xec1320, 0xc000266b10})
	/usr/local/go/src/runtime/panic.go:1038 +0x215
github.com/getsentry/sentry-go/echo.(*handler).recoverWithSentry(0xc000481350, 0x7fa2a6, 0xc00074ca00)
	/go/src/service/vendor/github.com/getsentry/sentry-go/echo/sentryecho.go:69 +0xf0
panic({0xec1320, 0xc000266b10})
	/usr/local/go/src/runtime/panic.go:1038 +0x215
github.com/labstack/echo/v4.(*Response).Flush(0x40ac5d)
	/go/src/service/vendor/github.com/labstack/echo/v4/response.go:87 +0x2d
github.com/labstack/echo/v4.(*Response).Flush(0x40ac5d)
	/go/src/service/vendor/github.com/labstack/echo/v4/response.go:87 +0x38
github.com/labstack/echo/v4.(*Response).Flush(0x7e51)
	/go/src/service/vendor/github.com/labstack/echo/v4/response.go:87 +0x38
github.com/labstack/echo/v4.(*Response).Flush(0xc00062c8a0)
	/go/src/service/vendor/github.com/labstack/echo/v4/response.go:87 +0x38
net/http/httputil.(*maxLatencyWriter).Write(0xc000266ae0, {0xc000312000, 0x8000, 0x8000})
	/usr/local/go/src/net/http/httputil/reverseproxy.go:513 +0x1dc
net/http/httputil.(*ReverseProxy).copyBuffer(0x9d5f82, {0x1241da0, 0xc000266ae0}, {0x1241b40, 0xc0002b78c0}, {0x0, 0x0, 0x600000000000000})
	/usr/local/go/src/net/http/httputil/reverseproxy.go:466 +0x217
net/http/httputil.(*ReverseProxy).copyResponse(0xc00026d348, {0x1240aa0, 0xc00062c8a0}, {0x1241b40, 0xc0002b78c0}, 0xffffffffffffffff)
	/usr/local/go/src/net/http/httputil/reverseproxy.go:449 +0x2d0
net/http/httputil.(*ReverseProxy).ServeHTTP(0xc00026d348, {0x12501d0, 0xc00062c8a0}, 0xc00074ca00)
	/usr/local/go/src/net/http/httputil/reverseproxy.go:338 +0xc65

[aldas]

@MarkRosemaker could you describe a little how this Response is created. I assume it is not standard http.response or writer instance, something is doing some wrapping to Writer before Response is passed to ReverseProxy? You do not happen to have similar ResponseWriter like BodyDump has

echo/middleware/body_dump.go

Line 97 in 8b4cce5

func (w *bodyDumpResponseWriter) Write(b []byte) (int, error) {

but without Flush method?

[MarkRosemaker]

@MarkRosemaker could you describe a little how this Response is created. I assume it is not standard http.response or writer instance, something is doing some wrapping to Writer before Response is passed to ReverseProxy? You do not happen to have similar ResponseWriter like BodyDump has

echo/middleware/body_dump.go

Line 97 in 8b4cce5

func (w *bodyDumpResponseWriter) Write(b []byte) (int, error) {

but without Flush method?

Thanks for the reply, @aldas ! It seems to me the response was set via SetResponse in one of our middlewares. The response was wrapped in another object that updates our metrics as well as doing what the initial response does.

[aldas]

Could you check in debugger view what type *echo.Response.Writer field value is (struct type at the moment not interface) and check if that type originates from your application or some some 3rd party library?

[MarkRosemaker]

Could you check in debugger view what type *echo.Response.Writer field value is (struct type at the moment not interface) and check if that type originates from your application or some some 3rd party library?

Most likely, from what I could find out, it is a struct of our own (the one mentioned above). I've made another PR in one of our repos to implement the Flush method in that one.

I think we just did not consider that echo expects the writer to be a http.Flusher because it's a http.ResponseWriter, not both.