Gzip header forces file download

I am trying to gzip all responses.
In main.go

mux := mux.NewRouter()
mux.Use(middlewareHeaders)
mux.Use(gzipHandler)

Then I have the middlewares:

func gzipHandler(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		gz := gzip.NewWriter(w)
		defer gz.Close()
		gzr := gzipResponseWriter{Writer: gz, ResponseWriter: w}
		next.ServeHTTP(gzr, r)
	})
}

func middlewareHeaders(next http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		w.Header().Set("Cache-Control", "max-age=2592000") // 30 days
		w.Header().Set("Content-Encoding", "gzip")
		w.Header().Set("Strict-Transport-Security", "max-age=63072000; includeSubDomains; preload")
		w.Header().Set("Access-Control-Allow-Headers", "Content-Type,X-Amz-Date,Authorization,X-Api-Key,X-Amz-Security-Token")
		w.Header().Set("Access-Control-Allow-Methods", "POST")
		w.Header().Set("Access-Control-Allow-Origin", "origin")
		w.Header().Set("Access-Control-Allow-Credentials", "true")
		w.Header().Set("Access-Control-Expose-Headers", "AMP-Access-Control-Allow-Source-Origin")
		w.Header().Set("AMP-Access-Control-Allow-Source-Origin", os.Getenv("DOMAIN"))
		next.ServeHTTP(w, r)
	})
}

When I curl the site I get

curl -v https://example.com
*   Trying 44.234.222.27:443...
* TCP_NODELAY set
* Connected to example.com (XX.XXX.XXX.XX) port 443 (#0)
* ALPN, offering h2
* ALPN, offering http/1.1
* successfully set certificate verify locations:
*   CAfile: /etc/ssl/certs/ca-certificates.crt
  CApath: /etc/ssl/certs
* TLSv1.3 (OUT), TLS handshake, Client hello (1):
* TLSv1.3 (IN), TLS handshake, Server hello (2):
* TLSv1.2 (IN), TLS handshake, Certificate (11):
* TLSv1.2 (IN), TLS handshake, Server key exchange (12):
* TLSv1.2 (IN), TLS handshake, Server finished (14):
* TLSv1.2 (OUT), TLS handshake, Client key exchange (16):
* TLSv1.2 (OUT), TLS change cipher, Change cipher spec (1):
* TLSv1.2 (OUT), TLS handshake, Finished (20):
* TLSv1.2 (IN), TLS handshake, Finished (20):
* SSL connection using TLSv1.2 / ECDHE-RSA-AES128-GCM-SHA256
* ALPN, server accepted to use h2
* Server certificate:
*  subject: CN=example.com
*  start date: Mar 16 00:00:00 2021 GMT
*  expire date: Apr 16 23:59:59 2022 GMT
*  subjectAltName: host "example.com" matched cert's "example.com"
*  issuer: C=GB; ST=Greater Manchester; L=Salford; O=Sectigo Limited; CN=Sectigo RSA Domain Validation Secure Server CA
*  SSL certificate verify ok.
* Using HTTP2, server supports multi-use
* Connection state changed (HTTP/2 confirmed)
* Copying HTTP/2 data in stream buffer to connection buffer after upgrade: len=0
* Using Stream ID: 1 (easy handle 0x55cadcebfe10)
> GET / HTTP/2
> Host: example.com
> user-agent: curl/7.68.0
> accept: */*
> 
* Connection state changed (MAX_CONCURRENT_STREAMS == 128)!
< HTTP/2 200 
< date: Mon, 07 Jun 2021 20:13:19 GMT
< access-control-allow-credentials: true
< access-control-allow-headers: Content-Type,X-Amz-Date,Authorization,X-Api-Key,X-Amz-Security-Token
< access-control-allow-methods: POST
< access-control-allow-origin: origin
< access-control-expose-headers: AMP-Access-Control-Allow-Source-Origin
< amp-access-control-allow-source-origin: example.com
< cache-control: max-age=2592000
< content-encoding: gzip
< strict-transport-security: max-age=63072000; includeSubDomains; preload
< vary: Accept-Encoding
< 
Warning: Binary output can mess up your terminal. Use "--output -" to tell 
Warning: curl to output it to your terminal anyway, or consider "--output 
Warning: <FILE>" to save to a file.
* Failed writing body (0 != 3506)
* stopped the pause stream!
* Connection #0 to host example.com left intact

When enabling the gzip handler and gzip header the browser wants to download a file.

Can anyone spot my error?

1. You should only gzip when it's requested by the client.

Accept-Encoding: gzip is never requested, but you gzip the response anyway.

So curl gives it back to you as-is.

2. Given the behavior of your browser, it sounds like double-compression. Maybe you have some HTTP reverse proxy in place which already handles compression to the browser, but doesn't compress backend traffic. So you may not need any gzipping at the backend at all - try curl --compressed to confirm this.

3. You should filter out Content-Length from the response. Content-Length is the final size of the compressed HTTP response, so the value changes during compression.

4. You should not blindly apply compression to all URI's. Some handlers perform gzipping already (e.g. prometheus /metrics), and some are pointless to compress (e.g. .png, .zip, .gz). At the very least strip Accept-Encoding: gzip from the request before passing it down the handler chain, to avoid double-gzipping.

5. Transparent gzipping in Go has been implemented before. A quick search reveals this gist (adjusted for point #4 above):

package main

import (
    "compress/gzip"
    "io"
    "io/ioutil"
    "net/http"
    "strings"
    "sync"
)

var gzPool = sync.Pool{
    New: func() interface{} {
        w := gzip.NewWriter(ioutil.Discard)
        return w
    },
}

type gzipResponseWriter struct {
    io.Writer
    http.ResponseWriter
}

func (w *gzipResponseWriter) WriteHeader(status int) {
    w.Header().Del("Content-Length")
    w.ResponseWriter.WriteHeader(status)
}

func (w *gzipResponseWriter) Write(b []byte) (int, error) {
    return w.Writer.Write(b)
}

func Gzip(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        if !strings.Contains(r.Header.Get("Accept-Encoding"), "gzip") {
            next.ServeHTTP(w, r)
            return
        }

        w.Header().Set("Content-Encoding", "gzip")

        gz := gzPool.Get().(*gzip.Writer)
        defer gzPool.Put(gz)

        gz.Reset(w)
        defer gz.Close()

        r.Header.Del("Accept-Encoding")
        next.ServeHTTP(&gzipResponseWriter{ResponseWriter: w, Writer: gz}, r)
    })
}

Note - the above doesn't support chunked encoding and trailers. So there's still opportunity for improvement.