WrapError: wrap an error with fields to be logged by zap.Error

Related to uber-go/guide#179

Callsites that receive an error should either log, or return an error.

However, if the callsite has additioanl context, the simplest option is
to add it to the error, but it's then flattened into a string, losing
the benefit of structured logging. This often results in callsites
logging with additional fields, and returning an error that is likely
to be logged again.

`WrapError` provides a way for callsites to return an error that
includes fields to be logged, which will be added to an `errorFields`
key.

fields_error.go

@@ -0,0 +1,54 @@
+// Copyright (c) 2023 Uber Technologies, Inc.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy
+// of this software and associated documentation files (the "Software"), to deal
+// in the Software without restriction, including without limitation the rights
+// to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+// copies of the Software, and to permit persons to whom the Software is
+// furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+// IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+// OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+// THE SOFTWARE.
+
+package zap
+
+import (
+	"go.uber.org/zap/zapcore"
+)
+
+type errorWithFields struct {
+	err    error
+	fields []Field
+}
+
+// WrapError returns an error that will log the provided fields if the error
+// is logged using `Error`.
+func WrapError(err error, fields ...Field) error {
+	return &errorWithFields{
+		err:    err,
+		fields: fields,
+	}
+}
+
+func (e errorWithFields) Unwrap() error {
+	return e.err
+}
+
+func (e errorWithFields) Error() string {
+	return e.err.Error()
+}
+
+func (e errorWithFields) MarshalLogObject(oe zapcore.ObjectEncoder) error {
+	for _, f := range e.fields {
+		f.AddTo(oe)
+	}
+	return nil
+}

zapcore/error.go

@@ -75,6 +75,13 @@ func encodeError(key string, err error, enc ObjectEncoder) (retErr error) {
 			enc.AddString(key+"Verbose", verbose)
 		}
 	}
+
+	if errObj, ok := err.(ObjectMarshaler); ok {
+		if err := enc.AddObject(key+"Fields", errObj); err != nil {
+			return err
+		}
+	}
+
 	return nil
 }