gzip: avoid unnecessary allocations when not compressing (#2396)

caddyhttp/gzip/gzip.go

@@ -17,6 +17,7 @@
 package gzip
 
 import (
+	"compress/gzip"
 	"io"
 	"net/http"
 	"strings"
@@ -65,21 +66,31 @@ outer:
 			}
 		}
 
+		// In order to avoid unused memory allocation, gzip.putWriter only be called when gzip compression happened.
+		// see https://github.com/mholt/caddy/issues/2395
+		gz := &gzipResponseWriter{
+			ResponseWriterWrapper: &httpserver.ResponseWriterWrapper{ResponseWriter: w},
+			newWriter: func() io.Writer {
 				// gzipWriter modifies underlying writer at init,
 				// use a discard writer instead to leave ResponseWriter in
 				// original form.
-		gzipWriter := getWriter(c.Level)
-		defer putWriter(c.Level, gzipWriter)
-		gz := &gzipResponseWriter{
-			Writer:                gzipWriter,
-			ResponseWriterWrapper: &httpserver.ResponseWriterWrapper{ResponseWriter: w},
+				return getWriter(c.Level)
+			},
+		}
+
+		defer func() {
+			if gzWriter, ok := gz.internalWriter.(*gzip.Writer); ok {
+				putWriter(c.Level, gzWriter)
 			}
+		}()
 
 		var rw http.ResponseWriter
 		// if no response filter is used
 		if len(c.ResponseFilters) == 0 {
 			// replace discard writer with ResponseWriter
-			gzipWriter.Reset(w)
+			if gzWriter, ok := gz.Writer().(*gzip.Writer); ok {
+				gzWriter.Reset(w)
+			}
 			rw = gz
 		} else {
 			// wrap gzip writer with ResponseFilterWriter
@@ -106,9 +117,10 @@ outer:
 // gzipResponeWriter wraps the underlying Write method
 // with a gzip.Writer to compress the output.
 type gzipResponseWriter struct {
-	io.Writer
+	internalWriter io.Writer
 	*httpserver.ResponseWriterWrapper
 	statusCodeWritten bool
+	newWriter         func() io.Writer
 }
 
 // WriteHeader wraps the underlying WriteHeader method to prevent
@@ -135,9 +147,17 @@ func (w *gzipResponseWriter) Write(b []byte) (int, error) {
 	if !w.statusCodeWritten {
 		w.WriteHeader(http.StatusOK)
 	}
-	n, err := w.Writer.Write(b)
+	n, err := w.Writer().Write(b)
 	return n, err
 }
 
+//Writer use a lazy way to initialize Writer
+func (w *gzipResponseWriter) Writer() io.Writer {
+	if w.internalWriter == nil {
+		w.internalWriter = w.newWriter()
+	}
+	return w.internalWriter
+}
+
 // Interface guards
 var _ httpserver.HTTPInterfaces = (*gzipResponseWriter)(nil)

caddyhttp/gzip/responsefilter.go

@@ -82,7 +82,7 @@ func (r *ResponseFilterWriter) WriteHeader(code int) {
 
 	if r.shouldCompress {
 		// replace discard writer with ResponseWriter
-		if gzWriter, ok := r.gzipResponseWriter.Writer.(*gzip.Writer); ok {
+		if gzWriter, ok := r.gzipResponseWriter.Writer().(*gzip.Writer); ok {
 			gzWriter.Reset(r.ResponseWriter)
 		}
 		// use gzip WriteHeader to include and delete

caddyhttp/gzip/responsefilter_test.go

@@ -47,7 +47,7 @@ func TestLengthFilter(t *testing.T) {
 		for j, filter := range filters {
 			r := httptest.NewRecorder()
 			r.Header().Set("Content-Length", fmt.Sprint(ts.length))
-			wWriter := NewResponseFilterWriter([]ResponseFilter{filter}, &gzipResponseWriter{gzip.NewWriter(r), &httpserver.ResponseWriterWrapper{ResponseWriter: r}, false})
+			wWriter := NewResponseFilterWriter([]ResponseFilter{filter}, &gzipResponseWriter{gzip.NewWriter(r), &httpserver.ResponseWriterWrapper{ResponseWriter: r}, false, nil})
 			if filter.ShouldCompress(wWriter) != ts.shouldCompress[j] {
 				t.Errorf("Test %v: Expected %v found %v", i, ts.shouldCompress[j], filter.ShouldCompress(r))
 			}