common/json: add Unmarshal with method with syntax errors

common/command/template.go

 package command
 
 import (
-	"encoding/json"
 	"errors"
 	"fmt"
+	jsonutil "github.com/mitchellh/packer/common/json"
 	"github.com/mitchellh/packer/packer"
 	"io/ioutil"
 	"log"
@@ -116,7 +116,7 @@ func readFileVars(path string) (map[string]string, error) {
 	}
 
 	vars := make(map[string]string)
-	err = json.Unmarshal(bytes, &vars)
+	err = jsonutil.Unmarshal(bytes, &vars)
 	if err != nil {
 		return nil, err
 	}

common/json/unmarshal.go

+package json
+
+import (
+	"bytes"
+	"encoding/json"
+	"fmt"
+)
+
+// Unmarshal is wrapper around json.Unmarshal that returns user-friendly
+// errors when there are syntax errors.
+func Unmarshal(data []byte, i interface{}) error {
+	err := json.Unmarshal(data, i)
+	if err != nil {
+		syntaxErr, ok := err.(*json.SyntaxError)
+		if !ok {
+			return err
+		}
+
+		// We have a syntax error. Extract out the line number and friends.
+		// https://groups.google.com/forum/#!topic/golang-nuts/fizimmXtVfc
+		newline := []byte{'\x0a'}
+
+		// Calculate the start/end position of the line where the error is
+		start := bytes.LastIndex(data[:syntaxErr.Offset], newline) + 1
+		end := len(data)
+		if idx := bytes.Index(data[start:], newline); idx >= 0 {
+			end = start + idx
+		}
+
+		// Count the line number we're on plus the offset in the line
+		line := bytes.Count(data[:start], newline) + 1
+		pos := int(syntaxErr.Offset) - start - 1
+
+		err = fmt.Errorf("Error in line %d, char %d: %s\n%s",
+			line, pos, syntaxErr, data[start:end])
+		return err
+	}
+
+	return nil
+}

common/json/unmarshal_test.go

+package json

packer/template.go

 package packer
 
 import (
-	"bytes"
-	"encoding/json"
 	"fmt"
 	"github.com/mitchellh/mapstructure"
+	jsonutil "github.com/mitchellh/packer/common/json"
 	"io/ioutil"
 	"sort"
 )
@@ -68,31 +67,8 @@ type rawProvisionerConfig struct {
 // way.
 func ParseTemplate(data []byte) (t *Template, err error) {
 	var rawTplInterface interface{}
-	err = json.Unmarshal(data, &rawTplInterface)
+	err = jsonutil.Unmarshal(data, &rawTplInterface)
 	if err != nil {
-		syntaxErr, ok := err.(*json.SyntaxError)
-		if !ok {
-			return
-		}
-
-		// We have a syntax error. Extract out the line number and friends.
-		// https://groups.google.com/forum/#!topic/golang-nuts/fizimmXtVfc
-		newline := []byte{'\x0a'}
-
-		// Calculate the start/end position of the line where the error is
-		start := bytes.LastIndex(data[:syntaxErr.Offset], newline) + 1
-		end := len(data)
-		if idx := bytes.Index(data[start:], newline); idx >= 0 {
-			end = start + idx
-		}
-
-		// Count the line number we're on plus the offset in the line
-		line := bytes.Count(data[:start], newline) + 1
-		pos := int(syntaxErr.Offset) - start - 1
-
-		err = fmt.Errorf("Error in line %d, char %d: %s\n%s",
-			line, pos, syntaxErr, data[start:end])
-
 		return
 	}