Don't use sync.Pool for json.NewEncoder target buffers (#421)

* Don't release buf to sync.Pool to early

The slice returned by `noescapeJSONMarshal` continues to be accessed
even after `pool.Put` was called on the buffer. This might lead to
incorrect data being sent in request body if the buffer is acquired and
re-written by a concurrently running goroutine.

So make sure we release the buffer once the request completes.

* Release request body buf back to sync.Pool only after body is closed

net/http.RoundTripper may access request body in a separate goroutine,
so we need to wait release the buf back to sync.Pool only after the
body is closed. From the docs:

  	// RoundTrip must always close the body, including on errors,
	// but depending on the implementation may do so in a separate
	// goroutine even after RoundTrip returns. This means that
	// callers wanting to reuse the body for subsequent requests
	// must arrange to wait for the Close call before doing so.

client.go

@@ -869,7 +869,6 @@ func (c *Client) GetClient() *http.Client {
 // Executes method executes the given `Request` object and returns response
 // error.
 func (c *Client) execute(req *Request) (*Response, error) {
-	defer releaseBuffer(req.bodyBuf)
 	// Apply Request middleware
 	var err error
 
@@ -903,6 +902,8 @@ func (c *Client) execute(req *Request) (*Response, error) {
 		return nil, wrapNoRetryErr(err)
 	}
 
+	req.RawRequest.Body = newRequestBodyReleaser(req.RawRequest.Body, req.bodyBuf)
+
 	req.Time = time.Now()
 	resp, err := c.httpClient.Do(req.RawRequest)
 

middleware.go

@@ -458,7 +458,7 @@ func handleRequestBody(c *Client, r *Request) (err error) {
 		bodyBytes = []byte(s)
 	} else if IsJSONType(contentType) &&
 		(kind == reflect.Struct || kind == reflect.Map || kind == reflect.Slice) {
-		bodyBytes, err = jsonMarshal(c, r, r.Body)
+		r.bodyBuf, err = jsonMarshal(c, r, r.Body)
 		if err != nil {
 			return
 		}

request.go

@@ -870,11 +870,14 @@ func (r *Request) initValuesMap() {
 	}
 }
 
-var noescapeJSONMarshal = func(v interface{}) ([]byte, error) {
+var noescapeJSONMarshal = func(v interface{}) (*bytes.Buffer, error) {
 	buf := acquireBuffer()
-	defer releaseBuffer(buf)
 	encoder := json.NewEncoder(buf)
 	encoder.SetEscapeHTML(false)
-	err := encoder.Encode(v)
-	return buf.Bytes(), err
+	if err := encoder.Encode(v); err != nil {
+		releaseBuffer(buf)
+		return nil, err
+	}
+
+	return buf, nil
 }

util.go

@@ -19,6 +19,7 @@ import (
 	"runtime"
 	"sort"
 	"strings"
+	"sync"
 )
 
 //‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾‾
@@ -139,13 +140,19 @@ type ResponseLog struct {
 //_______________________________________________________________________
 
 // way to disable the HTML escape as opt-in
-func jsonMarshal(c *Client, r *Request, d interface{}) ([]byte, error) {
-	if !r.jsonEscapeHTML {
-		return noescapeJSONMarshal(d)
-	} else if !c.jsonEscapeHTML {
+func jsonMarshal(c *Client, r *Request, d interface{}) (*bytes.Buffer, error) {
+	if !r.jsonEscapeHTML || !c.jsonEscapeHTML {
 		return noescapeJSONMarshal(d)
 	}
-	return c.JSONMarshal(d)
+
+	data, err := c.JSONMarshal(d)
+	if err != nil {
+		return nil, err
+	}
+
+	buf := acquireBuffer()
+	_, _ = buf.Write(data)
+	return buf, nil
 }
 
 func firstNonEmpty(v ...string) string {
@@ -283,6 +290,34 @@ func releaseBuffer(buf *bytes.Buffer) {
 	}
 }
 
+// requestBodyReleaser wraps requests's body and implements custom Close for it.
+// The Close method closes original body and releases request body back to sync.Pool.
+type requestBodyReleaser struct {
+	releaseOnce sync.Once
+	reqBuf      *bytes.Buffer
+	io.ReadCloser
+}
+
+func newRequestBodyReleaser(respBody io.ReadCloser, reqBuf *bytes.Buffer) io.ReadCloser {
+	if reqBuf == nil {
+		return respBody
+	}
+
+	return &requestBodyReleaser{
+		reqBuf:     reqBuf,
+		ReadCloser: respBody,
+	}
+}
+
+func (rr *requestBodyReleaser) Close() error {
+	err := rr.ReadCloser.Close()
+	rr.releaseOnce.Do(func() {
+		releaseBuffer(rr.reqBuf)
+	})
+
+	return err
+}
+
 func closeq(v interface{}) {
 	if c, ok := v.(io.Closer); ok {
 		silently(c.Close())