Highlight where the error occurred in the JSON syntax error

Author: Tonkpils

config/config.go

@@ -4,6 +4,8 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"io"
+	"log"
 	"os"
 	"path/filepath"
 	"runtime"
@@ -143,15 +145,28 @@ func (c *Config) read() error {
 
 	if err := json.NewDecoder(f).Decode(&c); err != nil {
 		var extra string
-		if _, ok := err.(*json.SyntaxError); ok {
-			extra = ":\nThe file contains invalid JSON syntax"
+		if serr, ok := err.(*json.SyntaxError); ok {
+			if _, serr := f.Seek(0, os.SEEK_SET); serr != nil {
+				log.Fatalf("seek error: %v", serr)
+			}
+			extra = fmt.Sprintf(":\nThe file contains invalid JSON syntax at '%s' <~",
+				findInvalidJSON(f, serr.Offset))
 		}
-		return fmt.Errorf("error parsing JSON in the config file %s%s: %s", f.Name(), extra, err)
+		return fmt.Errorf("error parsing JSON in the config file %s%s\n%s", f.Name(), extra, err)
 	}
 
 	return nil
 }
 
+func findInvalidJSON(f io.ReaderAt, pos int64) string {
+	buf := make([]byte, 13)
+	if _, err := f.ReadAt(buf, pos-13); err != nil {
+		log.Fatalf("read error: %v", err)
+	}
+
+	return string(buf)
+}
+
 // IsAuthenticated returns true if the config contains an API key.
 // This does not check whether or not that key is valid.
 func (c *Config) IsAuthenticated() bool {