commit 585415ced4abef0b0ceff6c495690df98db83abb
author iannucci <iannucci@chromium.org> Wed Jun 28 18:13:14 2017
committer Commit Bot <commit-bot@chromium.org> Wed Jun 28 18:13:14 2017
tree 5d51e928ecb18f1853f998b1ace5a9f37109d257
parent 2ae56cd555233728c302e9a67d229fc35aaf007f

[errors] de-specialize Transient in favor of Tags.

Previously 'transience' was a hard-coded attribute of the errors package. This
change adds 'tags' as a way to put attributes (potentially with some data) onto an
error as it goes up the stack.

This removes multiple symbols from the errors package (the stack context stuff
is now an implementation detail, tags are better than ExtractData).

The 'transience' bit has now moved to the 'common/retry/transient' package.
Marking an error as transient is now `err = transient.Tag.Apply(err)`, or
`errors.Annotate(err).Tag(transient.Tag).Err()` (as part of another batch
of annotations).

API Review in a Nutshell:
  // for simple 'boolean' tags, where presence is the only important piece of
  // information.
  var FatalTag = errors.BoolTag{errors.NewTagKey("this is a fatal error!")}

  func (...) error {
    return FatalTag.Apply(err)
  }

  func (...) {
    if FatalTag.In(err) {
      os.Exit(-1)
    }
  }

  // tags may also have associated typesafe values
  type Severity int
  type severityTagType struct {Key errors.TagKey}
  func (t severityTagType) With(s Severity) errors.TagValue { return errors.MkTagValue(t.Key, s) }
  func (t severityTagType) In(err error) (s Severity, ok bool) {
    d, ok := errors.TagValueIn(t.Key, err)
    if ok {
      s = d.(Severity)
    }
    return
  }
  var SeverityTag = severityTagType{errors.NewTagKey("holds a Severity value")}

  func (...) error {
    return SeverityTag.With(Severity(10)).Apply(err)
  }

  func (...) {
    if s, hasSeverity := SeverityTag.In(err); hasSeverity {
      if s.(Severity) > 9000 {
        fmt.Println("it's pretty severe")
      }
    }
  }

BUG=

Review-Url: https://codereview.chromium.org/2951393002