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// Copyright (c) 2017 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 multierr allows combining one or more errors together.
//
// Overview
//
// Errors can be combined with the use of the Combine function.
//
// multierr.Combine(
// reader.Close(),
// writer.Close(),
// conn.Close(),
// )
//
// If only two errors are being combined, the Append function may be used
// instead.
//
// err = multierr.Append(reader.Close(), writer.Close())
//
// This makes it possible to record resource cleanup failures from deferred
// blocks with the help of named return values.
//
// func sendRequest(req Request) (err error) {
// conn, err := openConnection()
// if err != nil {
// return err
// }
// defer func() {
// err = multierr.Append(err, conn.Close())
// }()
// // ...
// }
//
// The underlying list of errors for a returned error object may be retrieved
// with the Errors function.
//
// errors := multierr.Errors(err)
// if len(errors) > 0 {
// fmt.Println("The following errors occurred:")
// }
//
// Advanced Usage
//
// Errors returned by Combine and Append MAY implement the following
// interface.
//
// type errorGroup interface {
// // Returns a slice containing the underlying list of errors.
// //
// // This slice MUST NOT be modified by the caller.
// Errors() []error
// }
//
// Note that if you need access to list of errors behind a multierr error, you
// should prefer using the Errors function. That said, if you need cheap
// read-only access to the underlying errors slice, you can attempt to cast
// the error to this interface. You MUST handle the failure case gracefully
// because errors returned by Combine and Append are not guaranteed to
// implement this interface.
//
// var errors []error
// group, ok := err.(errorGroup)
// if ok {
// errors = group.Errors()
// } else {
// errors = []error{err}
// }
package multierr // import "go.uber.org/multierr"
import (
"bytes"
"fmt"
"io"
"strings"
"sync"
"go.uber.org/atomic"
)
var (
// Separator for single-line error messages.
_singlelineSeparator = []byte("; ")
// Prefix for multi-line messages
_multilinePrefix = []byte("the following errors occurred:")
// Prefix for the first and following lines of an item in a list of
// multi-line error messages.
//
// For example, if a single item is:
//
// foo
// bar
//
// It will become,
//
// - foo
// bar
_multilineSeparator = []byte("\n - ")
_multilineIndent = []byte(" ")
)
// _bufferPool is a pool of bytes.Buffers.
var _bufferPool = sync.Pool{
New: func() interface{} {
return &bytes.Buffer{}
},
}
type errorGroup interface {
Errors() []error
}
// Errors returns a slice containing zero or more errors that the supplied
// error is composed of. If the error is nil, the returned slice is empty.
//
// err := multierr.Append(r.Close(), w.Close())
// errors := multierr.Errors(err)
//
// If the error is not composed of other errors, the returned slice contains
// just the error that was passed in.
//
// Callers of this function are free to modify the returned slice.
func Errors(err error) []error {
if err == nil {
return nil
}
// Note that we're casting to multiError, not errorGroup. Our contract is
// that returned errors MAY implement errorGroup. Errors, however, only
// has special behavior for multierr-specific error objects.
//
// This behavior can be expanded in the future but I think it's prudent to
// start with as little as possible in terms of contract and possibility
// of misuse.
eg, ok := err.(*multiError)
if !ok {
return []error{err}
}
errors := eg.Errors()
result := make([]error, len(errors))
copy(result, errors)
return result
}
// multiError is an error that holds one or more errors.
//
// An instance of this is guaranteed to be non-empty and flattened. That is,
// none of the errors inside multiError are other multiErrors.
//
// multiError formats to a semi-colon delimited list of error messages with
// %v and with a more readable multi-line format with %+v.
type multiError struct {
copyNeeded atomic.Bool
errors []error
}
var _ errorGroup = (*multiError)(nil)
// Errors returns the list of underlying errors.
//
// This slice MUST NOT be modified.
func (merr *multiError) Errors() []error {
if merr == nil {
return nil
}
return merr.errors
}
func (merr *multiError) Error() string {
if merr == nil {
return ""
}
buff := _bufferPool.Get().(*bytes.Buffer)
buff.Reset()
merr.writeSingleline(buff)
result := buff.String()
_bufferPool.Put(buff)
return result
}
func (merr *multiError) Format(f fmt.State, c rune) {
if c == 'v' && f.Flag('+') {
merr.writeMultiline(f)
} else {
merr.writeSingleline(f)
}
}
func (merr *multiError) writeSingleline(w io.Writer) {
first := true
for _, item := range merr.errors {
if first {
first = false
} else {
w.Write(_singlelineSeparator)
}
io.WriteString(w, item.Error())
}
}
func (merr *multiError) writeMultiline(w io.Writer) {
w.Write(_multilinePrefix)
for _, item := range merr.errors {
w.Write(_multilineSeparator)
writePrefixLine(w, _multilineIndent, fmt.Sprintf("%+v", item))
}
}
// Writes s to the writer with the given prefix added before each line after
// the first.
func writePrefixLine(w io.Writer, prefix []byte, s string) {
first := true
for len(s) > 0 {
if first {
first = false
} else {
w.Write(prefix)
}
idx := strings.IndexByte(s, '\n')
if idx < 0 {
idx = len(s) - 1
}
io.WriteString(w, s[:idx+1])
s = s[idx+1:]
}
}
type inspectResult struct {
// Number of top-level non-nil errors
Count int
// Total number of errors including multiErrors
Capacity int
// Index of the first non-nil error in the list. Value is meaningless if
// Count is zero.
FirstErrorIdx int
// Whether the list contains at least one multiError
ContainsMultiError bool
}
// Inspects the given slice of errors so that we can efficiently allocate
// space for it.
func inspect(errors []error) (res inspectResult) {
first := true
for i, err := range errors {
if err == nil {
continue
}
res.Count++
if first {
first = false
res.FirstErrorIdx = i
}
if merr, ok := err.(*multiError); ok {
res.Capacity += len(merr.errors)
res.ContainsMultiError = true
} else {
res.Capacity++
}
}
return
}
// fromSlice converts the given list of errors into a single error.
func fromSlice(errors []error) error {
res := inspect(errors)
switch res.Count {
case 0:
return nil
case 1:
// only one non-nil entry
return errors[res.FirstErrorIdx]
case len(errors):
if !res.ContainsMultiError {
// already flat
return &multiError{errors: errors}
}
}
nonNilErrs := make([]error, 0, res.Capacity)
for _, err := range errors[res.FirstErrorIdx:] {
if err == nil {
continue
}
if nested, ok := err.(*multiError); ok {
nonNilErrs = append(nonNilErrs, nested.errors...)
} else {
nonNilErrs = append(nonNilErrs, err)
}
}
return &multiError{errors: nonNilErrs}
}
// Combine combines the passed errors into a single error.
//
// If zero arguments were passed or if all items are nil, a nil error is
// returned.
//
// Combine(nil, nil) // == nil
//
// If only a single error was passed, it is returned as-is.
//
// Combine(err) // == err
//
// Combine skips over nil arguments so this function may be used to combine
// together errors from operations that fail independently of each other.
//
// multierr.Combine(
// reader.Close(),
// writer.Close(),
// pipe.Close(),
// )
//
// If any of the passed errors is a multierr error, it will be flattened along
// with the other errors.
//
// multierr.Combine(multierr.Combine(err1, err2), err3)
// // is the same as
// multierr.Combine(err1, err2, err3)
//
// The returned error formats into a readable multi-line error message if
// formatted with %+v.
//
// fmt.Sprintf("%+v", multierr.Combine(err1, err2))
func Combine(errors ...error) error {
return fromSlice(errors)
}
// Append appends the given errors together. Either value may be nil.
//
// This function is a specialization of Combine for the common case where
// there are only two errors.
//
// err = multierr.Append(reader.Close(), writer.Close())
//
// The following pattern may also be used to record failure of deferred
// operations without losing information about the original error.
//
// func doSomething(..) (err error) {
// f := acquireResource()
// defer func() {
// err = multierr.Append(err, f.Close())
// }()
func Append(left error, right error) error {
switch {
case left == nil:
return right
case right == nil:
return left
}
if _, ok := right.(*multiError); !ok {
if l, ok := left.(*multiError); ok && !l.copyNeeded.Swap(true) {
// Common case where the error on the left is constantly being
// appended to.
errs := append(l.errors, right)
return &multiError{errors: errs}
} else if !ok {
// Both errors are single errors.
return &multiError{errors: []error{left, right}}
}
}
// Either right or both, left and right, are multiErrors. Rely on usual
// expensive logic.
errors := [2]error{left, right}
return fromSlice(errors[0:])
}
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