A better way to print prettified json syntax error msg

pkg/quick/errorutil.go

@@ -0,0 +1,96 @@
+/*
+ * Quick - Quick key value store for config files and persistent state files
+ *
+ * Minio Client (C) 2015 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package quick
+
+import (
+	"bufio"
+	"bytes"
+	"encoding/json"
+	"errors"
+	"fmt"
+	"io"
+
+	"github.com/olekukonko/ts"
+)
+
+const errorFmt = "%5d: %s <--  "
+
+// FormatJSONSyntaxError generates a pretty printed json syntax error since
+// golang doesn't provide an easy way to report the location of the error
+func FormatJSONSyntaxError(data io.Reader, sErr *json.SyntaxError) error {
+	if sErr == nil {
+		return nil
+	}
+
+	var readLine bytes.Buffer
+
+	bio := bufio.NewReader(data)
+	errLine := int64(1)
+	readBytes := int64(0)
+
+	// termWidth is set to a default one to use when we are
+	// not able to calculate terminal width via OS syscalls
+	termWidth := 25
+
+	// errorShift is the length of the minimum needed place for
+	// error msg accessoires, like <--, etc.. We calculate it
+	// dynamically to avoid an eventual bug after modifying errorFmt
+	errorShift := len(fmt.Sprintf(errorFmt, 1, ""))
+
+	if termSize, err := ts.GetSize(); err == nil {
+		termWidth = termSize.Col()
+	}
+
+	for {
+		b, err := bio.ReadByte()
+		if err != nil {
+			if err != io.EOF {
+				return err
+			}
+			break
+		}
+		readBytes++
+		if readBytes > sErr.Offset {
+			break
+		}
+		switch b {
+		case '\n':
+			readLine.Reset()
+			errLine++
+		case '\t':
+			readLine.WriteByte(' ')
+		case '\r':
+			break
+		default:
+			readLine.WriteByte(b)
+		}
+	}
+
+	lineLen := readLine.Len()
+	idx := lineLen - termWidth + errorShift
+	if idx < 0 || idx > lineLen-1 {
+		idx = 0
+	}
+
+	errorStr := fmt.Sprintf("JSON syntax error at line %d, col %d : %s.\n",
+		errLine, readLine.Len(), sErr)
+	errorStr += fmt.Sprintf(errorFmt, errLine, readLine.String()[idx:])
+
+	return errors.New(errorStr)
+}

pkg/quick/quick.go

@@ -19,6 +19,7 @@
 package quick
 
 import (
+	"bytes"
 	"encoding/json"
 	"fmt"
 	"io/ioutil"
@@ -144,7 +145,12 @@ func Load(filename string, data interface{}) (Config, *probe.Error) {
 
 	err = json.Unmarshal(fileData, &data)
 	if err != nil {
-		return nil, probe.NewError(err)
+		switch err := err.(type) {
+		case *json.SyntaxError:
+			return nil, probe.NewError(FormatJSONSyntaxError(bytes.NewReader(fileData), err))
+		default:
+			return nil, probe.NewError(err)
+		}
 	}
 
 	config, perr := New(data)
@@ -182,7 +188,12 @@ func (d *config) Load(filename string) *probe.Error {
 
 	err = json.Unmarshal(fileData, d.data)
 	if err != nil {
-		return probe.NewError(err)
+		switch err := err.(type) {
+		case *json.SyntaxError:
+			return probe.NewError(FormatJSONSyntaxError(bytes.NewReader(fileData), err))
+		default:
+			return probe.NewError(err)
+		}
 	}
 
 	if err := CheckData(d.data); err != nil {