sendfile.patch

diff --git a/modules/caddyhttp/responsewriter.go b/modules/caddyhttp/responsewriter.go
index 374bbfbac6..9820b41bc6 100644
--- a/modules/caddyhttp/responsewriter.go
+++ b/modules/caddyhttp/responsewriter.go
@@ -62,6 +62,16 @@ func (rww *ResponseWriterWrapper) Push(target string, opts *http.PushOptions) er
 	return ErrNotImplemented
 }
 
+// ReadFrom implements io.ReaderFrom. It simply calls the underlying
+// ResponseWriter's ReadFrom method if there is one, otherwise it defaults
+// to io.Copy.
+func (rww *ResponseWriterWrapper) ReadFrom(r io.Reader) (n int64, err error) {
+	if rf, ok := rww.ResponseWriter.(io.ReaderFrom); ok {
+		return rf.ReadFrom(r)
+	}
+	return io.Copy(rww.ResponseWriter, r)
+}
+
 // HTTPInterfaces mix all the interfaces that middleware ResponseWriters need to support.
 type HTTPInterfaces interface {
 	http.ResponseWriter
@@ -188,9 +198,26 @@ func (rr *responseRecorder) Write(data []byte) (int, error) {
 	} else {
 		n, err = rr.buf.Write(data)
 	}
-	if err == nil {
-		rr.size += n
+
+	rr.size += n
+	return n, err
+}
+
+func (rr *responseRecorder) ReadFrom(r io.Reader) (int64, error) {
+	rr.WriteHeader(http.StatusOK)
+	var n int64
+	var err error
+	if rr.stream {
+		if rf, ok := rr.ResponseWriter.(io.ReaderFrom); ok {
+			n, err = rf.ReadFrom(r)
+		} else {
+			n, err = io.Copy(rr.ResponseWriter, r)
+		}
+	} else {
+		n, err = rr.buf.ReadFrom(r)
 	}
+
+	rr.size += int(n)
 	return n, err
 }
 
@@ -251,4 +278,10 @@ type ShouldBufferFunc func(status int, header http.Header) bool
 var (
 	_ HTTPInterfaces   = (*ResponseWriterWrapper)(nil)
 	_ ResponseRecorder = (*responseRecorder)(nil)
+
+	// Implementing ReaderFrom can be such a significant
+	// optimization that it should probably be required!
+	// see PR #5022 (25%-50% speedup)
+	_ io.ReaderFrom = (*ResponseWriterWrapper)(nil)
+	_ io.ReaderFrom = (*responseRecorder)(nil)
 )
diff --git a/modules/caddyhttp/responsewriter_test.go b/modules/caddyhttp/responsewriter_test.go
new file mode 100644
index 0000000000..1913932003
--- /dev/null
+++ b/modules/caddyhttp/responsewriter_test.go
@@ -0,0 +1,165 @@
+package caddyhttp
+
+import (
+	"bytes"
+	"fmt"
+	"io"
+	"net/http"
+	"strings"
+	"testing"
+)
+
+type responseWriterSpy interface {
+	http.ResponseWriter
+	Written() string
+	CalledReadFrom() bool
+}
+
+var (
+	_ responseWriterSpy = (*baseRespWriter)(nil)
+	_ responseWriterSpy = (*readFromRespWriter)(nil)
+)
+
+// a barebones http.ResponseWriter mock
+type baseRespWriter []byte
+
+func (brw *baseRespWriter) Write(d []byte) (int, error) {
+	*brw = append(*brw, d...)
+	return len(d), nil
+}
+func (brw *baseRespWriter) Header() http.Header        { return nil }
+func (brw *baseRespWriter) WriteHeader(statusCode int) {}
+func (brw *baseRespWriter) Written() string            { return string(*brw) }
+func (brw *baseRespWriter) CalledReadFrom() bool       { return false }
+
+// an http.ResponseWriter mock that supports ReadFrom
+type readFromRespWriter struct {
+	baseRespWriter
+	called bool
+}
+
+func (rf *readFromRespWriter) ReadFrom(r io.Reader) (int64, error) {
+	rf.called = true
+	return io.Copy(&rf.baseRespWriter, r)
+}
+
+func (rf *readFromRespWriter) CalledReadFrom() bool { return rf.called }
+
+func TestResponseWriterWrapperReadFrom(t *testing.T) {
+	tests := map[string]struct {
+		responseWriter responseWriterSpy
+		wantReadFrom   bool
+	}{
+		"no ReadFrom": {
+			responseWriter: &baseRespWriter{},
+			wantReadFrom:   false,
+		},
+		"has ReadFrom": {
+			responseWriter: &readFromRespWriter{},
+			wantReadFrom:   true,
+		},
+	}
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			// what we expect middlewares to do:
+			type myWrapper struct {
+				*ResponseWriterWrapper
+			}
+
+			wrapped := myWrapper{
+				ResponseWriterWrapper: &ResponseWriterWrapper{ResponseWriter: tt.responseWriter},
+			}
+
+			const srcData = "boo!"
+			// hides everything but Read, since strings.Reader implements WriteTo it would
+			// take precedence over our ReadFrom.
+			src := struct{ io.Reader }{strings.NewReader(srcData)}
+
+			fmt.Println(name)
+			if _, err := io.Copy(wrapped, src); err != nil {
+				t.Errorf("Copy() err = %v", err)
+			}
+
+			if got := tt.responseWriter.Written(); got != srcData {
+				t.Errorf("data = %q, want %q", got, srcData)
+			}
+
+			if tt.responseWriter.CalledReadFrom() != tt.wantReadFrom {
+				if tt.wantReadFrom {
+					t.Errorf("ReadFrom() should have been called")
+				} else {
+					t.Errorf("ReadFrom() should not have been called")
+				}
+			}
+		})
+	}
+}
+
+func TestResponseRecorderReadFrom(t *testing.T) {
+	tests := map[string]struct {
+		responseWriter responseWriterSpy
+		shouldBuffer   bool
+		wantReadFrom   bool
+	}{
+		"buffered plain": {
+			responseWriter: &baseRespWriter{},
+			shouldBuffer:   true,
+			wantReadFrom:   false,
+		},
+		"streamed plain": {
+			responseWriter: &baseRespWriter{},
+			shouldBuffer:   false,
+			wantReadFrom:   false,
+		},
+		"buffered ReadFrom": {
+			responseWriter: &readFromRespWriter{},
+			shouldBuffer:   true,
+			wantReadFrom:   false,
+		},
+		"streamed ReadFrom": {
+			responseWriter: &readFromRespWriter{},
+			shouldBuffer:   false,
+			wantReadFrom:   true,
+		},
+	}
+	for name, tt := range tests {
+		t.Run(name, func(t *testing.T) {
+			var buf bytes.Buffer
+
+			rr := NewResponseRecorder(tt.responseWriter, &buf, func(status int, header http.Header) bool {
+				return tt.shouldBuffer
+			})
+
+			const srcData = "boo!"
+			// hides everything but Read, since strings.Reader implements WriteTo it would
+			// take precedence over our ReadFrom.
+			src := struct{ io.Reader }{strings.NewReader(srcData)}
+
+			if _, err := io.Copy(rr, src); err != nil {
+				t.Errorf("Copy() err = %v", err)
+			}
+
+			wantStreamed := srcData
+			wantBuffered := ""
+			if tt.shouldBuffer {
+				wantStreamed = ""
+				wantBuffered = srcData
+			}
+
+			if got := tt.responseWriter.Written(); got != wantStreamed {
+				t.Errorf("streamed data = %q, want %q", got, wantStreamed)
+			}
+			if got := buf.String(); got != wantBuffered {
+				t.Errorf("buffered data = %q, want %q", got, wantBuffered)
+			}
+
+			if tt.responseWriter.CalledReadFrom() != tt.wantReadFrom {
+				if tt.wantReadFrom {
+					t.Errorf("ReadFrom() should have been called")
+				} else {
+					t.Errorf("ReadFrom() should not have been called")
+				}
+			}
+		})
+	}
+}