Takahiro Suzuki | d7bf820 | 2020-12-17 20:21:59 +0900 | [diff] [blame] | 1 | // Package errors provides simple error handling primitives. |
| 2 | // |
| 3 | // The traditional error handling idiom in Go is roughly akin to |
| 4 | // |
| 5 | // if err != nil { |
| 6 | // return err |
| 7 | // } |
| 8 | // |
| 9 | // which when applied recursively up the call stack results in error reports |
| 10 | // without context or debugging information. The errors package allows |
| 11 | // programmers to add context to the failure path in their code in a way |
| 12 | // that does not destroy the original value of the error. |
| 13 | // |
| 14 | // Adding context to an error |
| 15 | // |
| 16 | // The errors.Wrap function returns a new error that adds context to the |
| 17 | // original error by recording a stack trace at the point Wrap is called, |
| 18 | // together with the supplied message. For example |
| 19 | // |
| 20 | // _, err := ioutil.ReadAll(r) |
| 21 | // if err != nil { |
| 22 | // return errors.Wrap(err, "read failed") |
| 23 | // } |
| 24 | // |
| 25 | // If additional control is required, the errors.WithStack and |
| 26 | // errors.WithMessage functions destructure errors.Wrap into its component |
| 27 | // operations: annotating an error with a stack trace and with a message, |
| 28 | // respectively. |
| 29 | // |
| 30 | // Retrieving the cause of an error |
| 31 | // |
| 32 | // Using errors.Wrap constructs a stack of errors, adding context to the |
| 33 | // preceding error. Depending on the nature of the error it may be necessary |
| 34 | // to reverse the operation of errors.Wrap to retrieve the original error |
| 35 | // for inspection. Any error value which implements this interface |
| 36 | // |
| 37 | // type causer interface { |
| 38 | // Cause() error |
| 39 | // } |
| 40 | // |
| 41 | // can be inspected by errors.Cause. errors.Cause will recursively retrieve |
| 42 | // the topmost error that does not implement causer, which is assumed to be |
| 43 | // the original cause. For example: |
| 44 | // |
| 45 | // switch err := errors.Cause(err).(type) { |
| 46 | // case *MyError: |
| 47 | // // handle specifically |
| 48 | // default: |
| 49 | // // unknown error |
| 50 | // } |
| 51 | // |
| 52 | // Although the causer interface is not exported by this package, it is |
| 53 | // considered a part of its stable public interface. |
| 54 | // |
| 55 | // Formatted printing of errors |
| 56 | // |
| 57 | // All error values returned from this package implement fmt.Formatter and can |
| 58 | // be formatted by the fmt package. The following verbs are supported: |
| 59 | // |
| 60 | // %s print the error. If the error has a Cause it will be |
| 61 | // printed recursively. |
| 62 | // %v see %s |
| 63 | // %+v extended format. Each Frame of the error's StackTrace will |
| 64 | // be printed in detail. |
| 65 | // |
| 66 | // Retrieving the stack trace of an error or wrapper |
| 67 | // |
| 68 | // New, Errorf, Wrap, and Wrapf record a stack trace at the point they are |
| 69 | // invoked. This information can be retrieved with the following interface: |
| 70 | // |
| 71 | // type stackTracer interface { |
| 72 | // StackTrace() errors.StackTrace |
| 73 | // } |
| 74 | // |
| 75 | // The returned errors.StackTrace type is defined as |
| 76 | // |
| 77 | // type StackTrace []Frame |
| 78 | // |
| 79 | // The Frame type represents a call site in the stack trace. Frame supports |
| 80 | // the fmt.Formatter interface that can be used for printing information about |
| 81 | // the stack trace of this error. For example: |
| 82 | // |
| 83 | // if err, ok := err.(stackTracer); ok { |
| 84 | // for _, f := range err.StackTrace() { |
| 85 | // fmt.Printf("%+s:%d", f) |
| 86 | // } |
| 87 | // } |
| 88 | // |
| 89 | // Although the stackTracer interface is not exported by this package, it is |
| 90 | // considered a part of its stable public interface. |
| 91 | // |
| 92 | // See the documentation for Frame.Format for more details. |
| 93 | package errors |
| 94 | |
| 95 | import ( |
| 96 | "fmt" |
| 97 | "io" |
| 98 | ) |
| 99 | |
| 100 | // New returns an error with the supplied message. |
| 101 | // New also records the stack trace at the point it was called. |
| 102 | func New(message string) error { |
| 103 | return &fundamental{ |
| 104 | msg: message, |
| 105 | stack: callers(), |
| 106 | } |
| 107 | } |
| 108 | |
| 109 | // Errorf formats according to a format specifier and returns the string |
| 110 | // as a value that satisfies error. |
| 111 | // Errorf also records the stack trace at the point it was called. |
| 112 | func Errorf(format string, args ...interface{}) error { |
| 113 | return &fundamental{ |
| 114 | msg: fmt.Sprintf(format, args...), |
| 115 | stack: callers(), |
| 116 | } |
| 117 | } |
| 118 | |
| 119 | // fundamental is an error that has a message and a stack, but no caller. |
| 120 | type fundamental struct { |
| 121 | msg string |
| 122 | *stack |
| 123 | } |
| 124 | |
| 125 | func (f *fundamental) Error() string { return f.msg } |
| 126 | |
| 127 | func (f *fundamental) Format(s fmt.State, verb rune) { |
| 128 | switch verb { |
| 129 | case 'v': |
| 130 | if s.Flag('+') { |
| 131 | io.WriteString(s, f.msg) |
| 132 | f.stack.Format(s, verb) |
| 133 | return |
| 134 | } |
| 135 | fallthrough |
| 136 | case 's': |
| 137 | io.WriteString(s, f.msg) |
| 138 | case 'q': |
| 139 | fmt.Fprintf(s, "%q", f.msg) |
| 140 | } |
| 141 | } |
| 142 | |
| 143 | // WithStack annotates err with a stack trace at the point WithStack was called. |
| 144 | // If err is nil, WithStack returns nil. |
| 145 | func WithStack(err error) error { |
| 146 | if err == nil { |
| 147 | return nil |
| 148 | } |
| 149 | return &withStack{ |
| 150 | err, |
| 151 | callers(), |
| 152 | } |
| 153 | } |
| 154 | |
| 155 | type withStack struct { |
| 156 | error |
| 157 | *stack |
| 158 | } |
| 159 | |
| 160 | func (w *withStack) Cause() error { return w.error } |
| 161 | |
| 162 | func (w *withStack) Format(s fmt.State, verb rune) { |
| 163 | switch verb { |
| 164 | case 'v': |
| 165 | if s.Flag('+') { |
| 166 | fmt.Fprintf(s, "%+v", w.Cause()) |
| 167 | w.stack.Format(s, verb) |
| 168 | return |
| 169 | } |
| 170 | fallthrough |
| 171 | case 's': |
| 172 | io.WriteString(s, w.Error()) |
| 173 | case 'q': |
| 174 | fmt.Fprintf(s, "%q", w.Error()) |
| 175 | } |
| 176 | } |
| 177 | |
| 178 | // Wrap returns an error annotating err with a stack trace |
| 179 | // at the point Wrap is called, and the supplied message. |
| 180 | // If err is nil, Wrap returns nil. |
| 181 | func Wrap(err error, message string) error { |
| 182 | if err == nil { |
| 183 | return nil |
| 184 | } |
| 185 | err = &withMessage{ |
| 186 | cause: err, |
| 187 | msg: message, |
| 188 | } |
| 189 | return &withStack{ |
| 190 | err, |
| 191 | callers(), |
| 192 | } |
| 193 | } |
| 194 | |
| 195 | // Wrapf returns an error annotating err with a stack trace |
| 196 | // at the point Wrapf is called, and the format specifier. |
| 197 | // If err is nil, Wrapf returns nil. |
| 198 | func Wrapf(err error, format string, args ...interface{}) error { |
| 199 | if err == nil { |
| 200 | return nil |
| 201 | } |
| 202 | err = &withMessage{ |
| 203 | cause: err, |
| 204 | msg: fmt.Sprintf(format, args...), |
| 205 | } |
| 206 | return &withStack{ |
| 207 | err, |
| 208 | callers(), |
| 209 | } |
| 210 | } |
| 211 | |
| 212 | // WithMessage annotates err with a new message. |
| 213 | // If err is nil, WithMessage returns nil. |
| 214 | func WithMessage(err error, message string) error { |
| 215 | if err == nil { |
| 216 | return nil |
| 217 | } |
| 218 | return &withMessage{ |
| 219 | cause: err, |
| 220 | msg: message, |
| 221 | } |
| 222 | } |
| 223 | |
| 224 | // WithMessagef annotates err with the format specifier. |
| 225 | // If err is nil, WithMessagef returns nil. |
| 226 | func WithMessagef(err error, format string, args ...interface{}) error { |
| 227 | if err == nil { |
| 228 | return nil |
| 229 | } |
| 230 | return &withMessage{ |
| 231 | cause: err, |
| 232 | msg: fmt.Sprintf(format, args...), |
| 233 | } |
| 234 | } |
| 235 | |
| 236 | type withMessage struct { |
| 237 | cause error |
| 238 | msg string |
| 239 | } |
| 240 | |
| 241 | func (w *withMessage) Error() string { return w.msg + ": " + w.cause.Error() } |
| 242 | func (w *withMessage) Cause() error { return w.cause } |
| 243 | |
| 244 | func (w *withMessage) Format(s fmt.State, verb rune) { |
| 245 | switch verb { |
| 246 | case 'v': |
| 247 | if s.Flag('+') { |
| 248 | fmt.Fprintf(s, "%+v\n", w.Cause()) |
| 249 | io.WriteString(s, w.msg) |
| 250 | return |
| 251 | } |
| 252 | fallthrough |
| 253 | case 's', 'q': |
| 254 | io.WriteString(s, w.Error()) |
| 255 | } |
| 256 | } |
| 257 | |
| 258 | // Cause returns the underlying cause of the error, if possible. |
| 259 | // An error value has a cause if it implements the following |
| 260 | // interface: |
| 261 | // |
| 262 | // type causer interface { |
| 263 | // Cause() error |
| 264 | // } |
| 265 | // |
| 266 | // If the error does not implement Cause, the original error will |
| 267 | // be returned. If the error is nil, nil will be returned without further |
| 268 | // investigation. |
| 269 | func Cause(err error) error { |
| 270 | type causer interface { |
| 271 | Cause() error |
| 272 | } |
| 273 | |
| 274 | for err != nil { |
| 275 | cause, ok := err.(causer) |
| 276 | if !ok { |
| 277 | break |
| 278 | } |
| 279 | err = cause.Cause() |
| 280 | } |
| 281 | return err |
| 282 | } |