Make request body readable and changeable in interceptFunc and beforeFunc (#81)

Hi! 

This PR gives users the ability to access and change `request.Body` in
functions registered with `RegisterInterceptFunc` and
`RegisterBeforeFunc`. I've already described it in issue #80.
Also, I've added couple of simple tests to check ability to change
request data in these functions.

**Changes:**

1. In **rpc/v2** close `request.Body` after the execution of
`beforeFunc`'s and `interceptFunc`'s. Update codec request info after
calls to functions above.
2. Read request body bytes, decode it to codec format and provide
`bytes.Buffer` and `request.Body` for underlying functions in
**v2/json**, **v2/json2** and **v2/protorpc** codecs.

Of course, exists a better way to do that, but it will require changes
in the signature of `RegisterInterceptFunc` and `RegisterBeforeFunc` and
it would be breaking changes. If both of these methods will have an
original `*http.Request` as input parameter, they could be executed
before the creation of codec. In this case, users can access and alter
request data, and only after that it would be read by the codec and
marshaled to service request params. But since these changes are
breaking, it's not an option at the moment, probably it can fit the next
version or release.

Would be nice to know your opinion, thanks!

Author: groovili

v2/json/server.go

@@ -6,9 +6,11 @@
 package json
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
 	"log"
 	"net/http"
 
@@ -79,9 +81,22 @@ func (c *Codec) NewRequest(r *http.Request) rpc.CodecRequest {
 
 // newCodecRequest returns a new CodecRequest.
 func newCodecRequest(r *http.Request) rpc.CodecRequest {
-	// Decode the request body and check if RPC method is valid.
 	req := new(serverRequest)
-	err := json.NewDecoder(r.Body).Decode(req)
+
+	// Copy request body for decoding and access of underlying methods
+	b, err := io.ReadAll(r.Body)
+	if err != nil {
+		return &CodecRequest{request: req, err: err}
+	}
+	// Close original body
+	r.Body.Close()
+
+	// Decode the request body and check if RPC method is valid.
+	err = json.Unmarshal(b, req)
+
+	// Add close method to buffer and pass as request body
+	r.Body = io.NopCloser(bytes.NewBuffer(b))
+
 	return &CodecRequest{request: req, err: err}
 }
 

v2/json2/server.go

@@ -6,7 +6,9 @@
 package json2
 
 import (
+	"bytes"
 	"encoding/json"
+	"io"
 	"net/http"
 
 	"github.com/gorilla/rpc/v2"
@@ -99,10 +101,24 @@ func (c *Codec) NewRequest(r *http.Request) rpc.CodecRequest {
 
 // newCodecRequest returns a new CodecRequest.
 func newCodecRequest(r *http.Request, encoder rpc.Encoder, errorMapper func(error) error) rpc.CodecRequest {
-	// Decode the request body and check if RPC method is valid.
 	req := new(serverRequest)
-	err := json.NewDecoder(r.Body).Decode(req)
 
+	// Copy request body for decoding and access of underlying methods
+	b, err := io.ReadAll(r.Body)
+	if err != nil {
+		err = &Error{
+			Code:    E_PARSE,
+			Message: err.Error(),
+			Data:    req,
+		}
+
+		return &CodecRequest{request: req, err: err, encoder: encoder, errorMapper: errorMapper}
+	}
+	// Close original body
+	r.Body.Close()
+
+	// Decode the request body and check if RPC method is valid.
+	err = json.Unmarshal(b, req)
 	if err != nil {
 		err = &Error{
 			Code:    E_PARSE,
@@ -117,6 +133,9 @@ func newCodecRequest(r *http.Request, encoder rpc.Encoder, errorMapper func(erro
 		}
 	}
 
+	// Add close method to buffer and pass as request body
+	r.Body = io.NopCloser(bytes.NewBuffer(b))
+
 	return &CodecRequest{request: req, err: err, encoder: encoder, errorMapper: errorMapper}
 }
 

v2/protorpc/server.go

@@ -6,6 +6,7 @@
 package protorpc
 
 import (
+	"bytes"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -78,11 +79,24 @@ func newCodecRequest(r *http.Request) rpc.CodecRequest {
 		return &CodecRequest{request: req, err: fmt.Errorf("rpc: no method: %s", path)}
 	}
 	req.Method = path[index+1:]
-	err := json.NewDecoder(r.Body).Decode(&req.Params)
+
+	// Copy request body for decoding and access of underlying methods
+	b, err := io.ReadAll(r.Body)
+	if err != nil {
+		return &CodecRequest{request: req, err: err}
+	}
+	// Close original body
+	r.Body.Close()
+
+	err = json.Unmarshal(b, &req.Params)
 	var codecErr error
 	if err != io.EOF {
 		codecErr = err
 	}
+
+	// Add close method to buffer and pass as request body
+	r.Body = io.NopCloser(bytes.NewBuffer(b))
+
 	return &CodecRequest{request: req, err: codecErr}
 }
 

v2/server.go

@@ -178,12 +178,6 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		codecReq.WriteError(w, http.StatusBadRequest, errGet)
 		return
 	}
-	// Decode the args.
-	args := reflect.New(methodSpec.argsType)
-	if errRead := codecReq.ReadRequest(args.Interface()); errRead != nil {
-		codecReq.WriteError(w, http.StatusBadRequest, errRead)
-		return
-	}
 
 	// Call the registered Intercept Function
 	if s.interceptFunc != nil {
@@ -206,6 +200,24 @@ func (s *Server) ServeHTTP(w http.ResponseWriter, r *http.Request) {
 		s.beforeFunc(requestInfo)
 	}
 
+	// Close request body after Intercept and Before Function if it exists
+	// if it's already closed, error still would be nil
+	if r.Body != nil {
+		r.Body.Close()
+	}
+
+	// Update codec request with request values after Intercept and Before functions if they exist
+	if s.interceptFunc != nil || s.beforeFunc != nil {
+		codecReq = codec.NewRequest(r)
+	}
+
+	// Decode the args.
+	args := reflect.New(methodSpec.argsType)
+	if errRead := codecReq.ReadRequest(args.Interface()); errRead != nil {
+		codecReq.WriteError(w, http.StatusBadRequest, errRead)
+		return
+	}
+
 	// Prepare the reply, we need it even if validation fails
 	reply := reflect.New(methodSpec.replyType)
 	errValue := []reflect.Value{nilErrorValue}

v2/server_test.go

@@ -6,7 +6,10 @@
 package rpc
 
 import (
+	"bytes"
+	"encoding/json"
 	"errors"
+	"io"
 	"log"
 	"net/http"
 	"strconv"
@@ -94,6 +97,30 @@ func (r MockCodecRequest) WriteError(w http.ResponseWriter, status int, err erro
 	}
 }
 
+type MockCodecJson struct {
+}
+
+func (c MockCodecJson) NewRequest(r *http.Request) CodecRequest {
+	if r.Body == nil {
+		return MockCodecRequest{}
+	}
+
+	inp := new(Service1Request)
+	b, err := io.ReadAll(r.Body)
+	if err != nil {
+		return MockCodecRequest{}
+	}
+	r.Body.Close()
+
+	if err := json.Unmarshal(b, inp); err != nil {
+		return MockCodecRequest{}
+	}
+
+	r.Body = io.NopCloser(bytes.NewBuffer(b))
+
+	return MockCodecRequest{inp.A, inp.B}
+}
+
 type MockResponseWriter struct {
 	header http.Header
 	Status int
@@ -211,6 +238,98 @@ func TestInterception(t *testing.T) {
 		t.Errorf("Response body was %s, should be %s.", w.Body, strconv.Itoa(expected))
 	}
 }
+
+func TestInterceptionWithChange(t *testing.T) {
+	const (
+		A = 2
+		B = 3
+		C = 5
+	)
+	expectedBeforeChange := A * B
+	expectedAfterChange := A * C
+
+	r2, err := http.NewRequest("POST", "mocked/request", bytes.NewBuffer([]byte(`{"A": 2, "B":5}`)))
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	s := NewServer()
+	s.RegisterService(new(Service1), "")
+	s.RegisterCodec(MockCodecJson{}, "mock")
+	s.RegisterInterceptFunc(func(i *RequestInfo) *http.Request {
+		return r2
+	})
+
+	r, err := http.NewRequest("POST", "", bytes.NewBuffer([]byte(`{A: 2, B:3}`)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	r.Header.Set("Content-Type", "mock; dummy")
+	w := NewMockResponseWriter()
+	s.ServeHTTP(w, r)
+	if w.Status != 200 {
+		t.Errorf("Status was %d, should be 200.", w.Status)
+	}
+
+	if w.Body != strconv.Itoa(expectedBeforeChange) && w.Body == strconv.Itoa(expectedAfterChange) {
+		return
+	}
+
+	t.Errorf("Response body was %s, should be %s.", w.Body, strconv.Itoa(expectedAfterChange))
+}
+
+func TestBeforeFunc(t *testing.T) {
+	const (
+		A = 2
+		B = 3
+		C = 5
+	)
+	expectedBeforeChange := A * B
+	expectedAfterChange := A * C
+
+	s := NewServer()
+	s.RegisterService(new(Service1), "")
+	s.RegisterCodec(MockCodecJson{}, "mock")
+	s.RegisterBeforeFunc(func(i *RequestInfo) {
+		r := i.Request
+
+		inp := new(Service1Request)
+		err := json.NewDecoder(r.Body).Decode(inp)
+		if err != nil {
+			t.Error(err)
+			t.Fail()
+		}
+
+		inp.B = C
+
+		b, err := json.Marshal(inp)
+		if err != nil {
+			t.Error(err)
+			t.Fail()
+		}
+
+		r.Body = io.NopCloser(bytes.NewBuffer(b))
+		i.Request = r
+	})
+
+	r, err := http.NewRequest("POST", "", bytes.NewBuffer([]byte(`{"A":2, "B":10}`)))
+	if err != nil {
+		t.Fatal(err)
+	}
+	r.Header.Set("Content-Type", "mock; dummy")
+	w := NewMockResponseWriter()
+	s.ServeHTTP(w, r)
+	if w.Status != 200 {
+		t.Errorf("Status was %d, should be 200.", w.Status)
+	}
+
+	if w.Body != strconv.Itoa(expectedBeforeChange) && w.Body == strconv.Itoa(expectedAfterChange) {
+		return
+	}
+
+	t.Errorf("Response body was %s, should be %s.", w.Body, strconv.Itoa(expectedAfterChange))
+}
+
 func TestValidationSuccessful(t *testing.T) {
 	const (
 		A = 2