How to modify the response from middleware?

[sazzer]

I'm writing some middleware to do simple cache responses. Specifically, it will return an HTTP 304, and will return no body and no content-type header if the if-none-match or if-modified-since request headers match the corresponding etag or last-modified response headers.

All of this is easy enough, except changing the response body. Specifically, what I've got is this:

func cachingMiddleware() echo.MiddlewareFunc {
	return func(next echo.HandlerFunc) echo.HandlerFunc {
		return func(c echo.Context) error {
			ifNoneMatch := c.Request().Header.Get("if-none-match")
			ifModifiedSince := c.Request().Header.Get("if-modified-since")

			c.Response().Before(func() {
				etag := c.Response().Header().Get("etag")
				lastModified := c.Response().Header().Get("last-modified")

				cacheHit := true

				switch {
				case ifNoneMatch == "" && ifModifiedSince == "":
					// No cache headers!
					cacheHit = false
				case ifNoneMatch != "" && ifNoneMatch != etag:
					// If-None-Match header provided and differs from ETag.
					cacheHit = false
				case ifModifiedSince != "" && ifModifiedSince != lastModified:
					// If-Modified-Since header provided and differs from Last-Modified.
					cacheHit = false
				}

				slog.Debug("Caching result",
					"if-none-match", ifNoneMatch,
					"if-modified-since", ifModifiedSince,
					"etag", etag,
					"last-modified", lastModified,
					"cacheHit", cacheHit,
				)

				if cacheHit {
					c.Response().Status = http.StatusNotModified
					c.Response().Header().Del("content-type")
					// TODO: DON'T SEND THE RESPONSE BODY
				}
			})

			result := next(c)

			return result
		}
	}
}

The bit that I can't work out is the "TODO: DON'T SEND THE RESPONSE BODY" bit.

What's the recommended way to achieve this?

Cheers