MM-10232, MM-10259: Improve error handling from invalid json (#8668)

* MM-10232: improve error handling from malformed slash command responses

Switch to json.Unmarshal, which doesn't obscure JSON parse failures like
json.Decode. The latter is primarily designed for streams of JSON, not
necessarily unmarshalling just a single object.

* rework HumanizedJsonError to expose Line and Character discretely

* MM-10259: pinpoint line and character where json config error occurs

* tweak HumanizeJsonError to accept err first

utils/jsonutils/json.go

@@ -0,0 +1,56 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See License.txt for license information.
+
+package jsonutils
+
+import (
+	"bytes"
+	"encoding/json"
+
+	"github.com/pkg/errors"
+)
+
+type HumanizedJsonError struct {
+	Err       error
+	Line      int
+	Character int
+}
+
+func (e *HumanizedJsonError) Error() string {
+	return e.Err.Error()
+}
+
+// HumanizeJsonError extracts error offsets and annotates the error with useful context
+func HumanizeJsonError(err error, data []byte) error {
+	if syntaxError, ok := err.(*json.SyntaxError); ok {
+		return NewHumanizedJsonError(syntaxError, data, syntaxError.Offset)
+	} else if unmarshalError, ok := err.(*json.UnmarshalTypeError); ok {
+		return NewHumanizedJsonError(unmarshalError, data, unmarshalError.Offset)
+	} else {
+		return err
+	}
+}
+
+func NewHumanizedJsonError(err error, data []byte, offset int64) *HumanizedJsonError {
+	if err == nil {
+		return nil
+	}
+
+	if offset < 0 || offset > int64(len(data)) {
+		return &HumanizedJsonError{
+			Err: errors.Wrapf(err, "invalid offset %d", offset),
+		}
+	}
+
+	lineSep := []byte{'\n'}
+
+	line := bytes.Count(data[:offset], lineSep) + 1
+	lastLineOffset := bytes.LastIndex(data[:offset], lineSep)
+	character := int(offset) - (lastLineOffset + 1) + 1
+
+	return &HumanizedJsonError{
+		Line:      line,
+		Character: character,
+		Err:       errors.Wrapf(err, "parsing error at line %d, character %d", line, character),
+	}
+}