plumbing: Do not swallow http message coming from VCS providers.

For diagnostics reasons we want to surface error messages coming from VCS providers.
That's why we introduce the reason field to Err struct in http package.
This field can be used by an end user of the library in order to better understand failures.

plumbing/transport/http/common.go

@@ -406,14 +406,28 @@ func (a *TokenAuth) String() string {
 // Err is a dedicated error to return errors based on status code
 type Err struct {
 	Response *http.Response
+	Reason   string
 }
 
-// NewErr returns a new Err based on a http response
+// NewErr returns a new Err based on a http response and closes response body
+// if needed
 func NewErr(r *http.Response) error {
 	if r.StatusCode >= http.StatusOK && r.StatusCode < http.StatusMultipleChoices {
 		return nil
 	}
 
+	var reason string
+
+	// If a response message is present, add it to error
+	var messageBuffer bytes.Buffer
+	if r.Body != nil {
+		messageLength, _ := messageBuffer.ReadFrom(r.Body)
+		if messageLength > 0 {
+			reason = messageBuffer.String()
+		}
+		_ = r.Body.Close()
+	}
+
 	switch r.StatusCode {
 	case http.StatusUnauthorized:
 		return transport.ErrAuthenticationRequired
@@ -423,7 +437,7 @@ func NewErr(r *http.Response) error {
 		return transport.ErrRepositoryNotFound
 	}
 
-	return plumbing.NewUnexpectedError(&Err{r})
+	return plumbing.NewUnexpectedError(&Err{r, reason})
 }
 
 // StatusCode returns the status code of the response

plumbing/transport/http/common_test.go

@@ -3,6 +3,7 @@ package http
 import (
 	"crypto/tls"
 	"fmt"
+	"io"
 	"log"
 	"net"
 	"net/http"
@@ -14,6 +15,7 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/go-git/go-git/v5/plumbing"
 	"github.com/go-git/go-git/v5/plumbing/transport"
 
 	fixtures "github.com/go-git/go-git-fixtures/v4"
@@ -90,6 +92,23 @@ func (s *ClientSuite) TestNewHTTPError40x(c *C) {
 		"unexpected client error.*")
 }
 
+func (s *ClientSuite) TestNewUnexpectedError(c *C) {
+	res := &http.Response{
+		StatusCode: 500,
+		Body:       io.NopCloser(strings.NewReader("Unexpected error")),
+	}
+
+	err := NewErr(res)
+	c.Assert(err, NotNil)
+	c.Assert(err, FitsTypeOf, &plumbing.UnexpectedError{})
+
+	unexpectedError, _ := err.(*plumbing.UnexpectedError)
+	c.Assert(unexpectedError.Err, FitsTypeOf, &Err{})
+
+	httpError, _ := unexpectedError.Err.(*Err)
+	c.Assert(httpError.Reason, Equals, "Unexpected error")
+}
+
 func (s *ClientSuite) Test_newSession(c *C) {
 	cl := NewClientWithOptions(nil, &ClientOptions{
 		CacheMaxEntries: 2,

plumbing/transport/http/receive_pack.go

@@ -102,7 +102,6 @@ func (s *rpSession) doRequest(
 	}
 
 	if err := NewErr(res); err != nil {
-		_ = res.Body.Close()
 		return nil, err
 	}
 

plumbing/transport/http/upload_pack.go

@@ -100,7 +100,6 @@ func (s *upSession) doRequest(
 	}
 
 	if err := NewErr(res); err != nil {
-		_ = res.Body.Close()
 		return nil, err
 	}