Bug fix for default error handling with gzip

middleware/gzip/gzip.go

@@ -4,6 +4,7 @@ package gzip
 
 import (
 	"compress/gzip"
+	"fmt"
 	"io"
 	"net/http"
 	"strings"
@@ -11,7 +12,10 @@ import (
 	"github.com/mholt/caddy/middleware"
 )
 
-// Gzip is a middleware type which gzips HTTP responses.
+// Gzip is a middleware type which gzips HTTP responses. It is
+// imperative that any handler which writes to a gzipped response
+// specifies the Content-Type, otherwise some clients will assume
+// application/x-gzip and try to download a file.
 type Gzip struct {
 	Next middleware.Handler
 }
@@ -28,11 +32,26 @@ func (g Gzip) ServeHTTP(w http.ResponseWriter, r *http.Request) (int, error) {
 	if !strings.Contains(r.Header.Get("Accept-Encoding"), "gzip") {
 		return g.Next.ServeHTTP(w, r)
 	}
+
 	w.Header().Set("Content-Encoding", "gzip")
 	gzipWriter := gzip.NewWriter(w)
 	defer gzipWriter.Close()
 	gz := gzipResponseWriter{Writer: gzipWriter, ResponseWriter: w}
-	return g.Next.ServeHTTP(gz, r)
+
+	// Any response in forward middleware will now be compressed
+	status, err := g.Next.ServeHTTP(gz, r)
+
+	// If there was an error that remained unhandled, we need
+	// to send something back before gzipWriter gets closed at
+	// the return of this method!
+	if status >= 400 {
+		gz.Header().Set("Content-Type", "text/plain") // very necessary
+		gz.WriteHeader(status)
+		fmt.Fprintf(gz, "%d %s", status, http.StatusText(status))
+		return 0, err
+	} else {
+		return status, err
+	}
 }
 
 // gzipResponeWriter wraps the underlying Write method