caddyhttp: Use sync.Pool to reduce lengthReader allocations (#5848)

* Use sync.Pool to reduce lengthReader allocations

Signed-off-by: Harish Shan <140232061+perhapsmaple@users.noreply.github.com>

* Add defer putLengthReader to prevent leak

Signed-off-by: Harish Shan <140232061+perhapsmaple@users.noreply.github.com>

* Cleanup in putLengthReader

Co-authored-by: Francis Lavoie <lavofr@gmail.com>

---------

Signed-off-by: Harish Shan <140232061+perhapsmaple@users.noreply.github.com>
Co-authored-by: Francis Lavoie <lavofr@gmail.com>

modules/caddyhttp/server.go

@@ -318,7 +318,8 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		// so we can track the number of bytes read from it
 		var bodyReader *lengthReader
 		if r.Body != nil {
-			bodyReader = &lengthReader{Source: r.Body}
+			bodyReader = getLengthReader(r.Body)
+			defer putLengthReader(bodyReader)
 			r.Body = bodyReader
 		}
 
@@ -902,6 +903,24 @@ type lengthReader struct {
 	Length int
 }
 
+var lengthReaderPool = sync.Pool{
+	New: func() interface{} {
+		return &lengthReader{}
+	},
+}
+
+func getLengthReader(source io.ReadCloser) *lengthReader {
+	reader := lengthReaderPool.Get().(*lengthReader)
+	reader.Source = source
+	return reader
+}
+
+func putLengthReader(reader *lengthReader) {
+	reader.Source = nil
+	reader.Length = 0
+	lengthReaderPool.Put(reader)
+}
+
 func (r *lengthReader) Read(b []byte) (int, error) {
 	n, err := r.Source.Read(b)
 	r.Length += n