Improve error handling and ordering of JWT code

The if statements have been changed in order, to check for
an error first, then fall back to normal functionality. This is
to make the code more Go-idomatic, and easier to read.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alexellis2@gmail.com>

auth/jwt_auth.go

@@ -13,6 +13,55 @@ import (
 	jwt "github.com/dgrijalva/jwt-go"
 )
 
+// JWTAuth token issued by Github in response to signed JWT Token
+type JWTAuth struct {
+	Token     string    `json:"token"`
+	ExpiresAt time.Time `json:"expires_at"`
+}
+
+// MakeAccessTokenForInstallation makes an access token for an installation / private key
+func MakeAccessTokenForInstallation(appID string, installation int, privateKey string) (string, error) {
+	signed, err := GetSignedJwtToken(appID, privateKey)
+
+	if err != nil {
+		msg := fmt.Sprintf("can't run GetSignedJwtToken for app_id: %s and installation_id: %d, error: %v", appID, installation, err)
+
+		fmt.Printf("Error %s\n", msg)
+		return "", err
+	}
+
+	req, err := http.NewRequest(http.MethodPost,
+		fmt.Sprintf("https://api.github.com/app/installations/%d/access_tokens", installation), nil)
+	if err != nil {
+		return "", err
+	}
+
+	req.Header.Add("Authorization", fmt.Sprintf("Bearer %s", signed))
+	req.Header.Add("Accept", "application/vnd.github.machine-man-preview+json")
+
+	res, err := http.DefaultClient.Do(req)
+
+	if err != nil {
+		msg := fmt.Sprintf("can't get access_token for app_id: %s and installation_id: %d error: %v", appID, installation, err)
+		fmt.Printf("Error: %s\n", msg)
+		return "", fmt.Errorf("%s", msg)
+	}
+
+	defer res.Body.Close()
+
+	bytesOut, readErr := ioutil.ReadAll(res.Body)
+	if readErr != nil {
+		return "", readErr
+	}
+
+	jwtAuth := JWTAuth{}
+	jsonErr := json.Unmarshal(bytesOut, &jwtAuth)
+	if jsonErr != nil {
+		return "", jsonErr
+	}
+	return jwtAuth.Token, nil
+}
+
 // GetSignedJwtToken get a tokens signed with private key
 func GetSignedJwtToken(appID string, privateKey string) (string, error) {
 
@@ -39,43 +88,3 @@ func GetSignedJwtToken(appID string, privateKey string) (string, error) {
 
 	return string(signedVal), nil
 }
-
-// JWTAuth token issued by Github in response to signed JWT Token
-type JWTAuth struct {
-	Token     string    `json:"token"`
-	ExpiresAt time.Time `json:"expires_at"`
-}
-
-// MakeAccessTokenForInstallation makes an access token for an installation / private key
-func MakeAccessTokenForInstallation(appID string, installation int, privateKey string) (string, error) {
-	signed, err := GetSignedJwtToken(appID, privateKey)
-
-	if err == nil {
-		c := http.Client{}
-		req, _ := http.NewRequest(http.MethodPost, fmt.Sprintf("https://api.github.com/app/installations/%d/access_tokens", installation), nil)
-
-		req.Header.Add("Authorization", "Bearer "+signed)
-		req.Header.Add("Accept", "application/vnd.github.machine-man-preview+json")
-
-		res, err := c.Do(req)
-
-		if err == nil {
-			defer res.Body.Close()
-			bytesOut, readErr := ioutil.ReadAll(res.Body)
-			if readErr != nil {
-				return "", readErr
-			}
-
-			jwtAuth := JWTAuth{}
-			jsonErr := json.Unmarshal(bytesOut, &jwtAuth)
-			if jsonErr != nil {
-				return "", jsonErr
-			}
-
-			return jwtAuth.Token, nil
-		}
-	}
-	fmt.Println(fmt.Sprintf("Clound not get access token, %v", err))
-
-	return "", err
-}