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

common/errors/tags_test.go

Index: common/errors/tags_test.go
diff --git a/common/errors/tags_test.go b/common/errors/tags_test.go
new file mode 100644
index 0000000000000000000000000000000000000000..b0e47d5e96aa7a9cfd630b5997ef93e735151560
--- /dev/null
+++ b/common/errors/tags_test.go
@@ -0,0 +1,134 @@
+// Copyright 2017 The LUCI Authors. All rights reserved.
+// Use of this source code is governed under the Apache License, Version 2.0
+// that can be found in the LICENSE file.
+
+package errors
+
+import (
+	stdErr "errors"
+	"testing"
+
+	. "github.com/smartystreets/goconvey/convey"
+)
+
+type customInt int
+type customIntTag struct{ Key TagKey }
+
+func (t customIntTag) With(i customInt) TagValue { return TagValue{t.Key, i} }
+func (t customIntTag) In(err error) (v customInt, ok bool) {
+	d, ok := TagValueIn(t.Key, err)
+	if ok {
+		v = d.(customInt)
+	}
+	return
+}
+
+var aCustomIntTag = customIntTag{NewTagKey("errors.testing custom int tag")}
+
+type stringTag struct{ Key TagKey }
+
+func (t stringTag) With(s string) TagValue { return TagValue{t.Key, s} }
+func (t stringTag) In(err error) (v string, ok bool) {
+	d, ok := TagValueIn(t.Key, err)
+	if ok {
+		v = d.(string)
+	}
+	return
+}
+
+var aStringTag = stringTag{NewTagKey("errors.testing string tag")}
+
+func TestTags(t *testing.T) {
+	t.Parallel()
+
+	aBoolTag := BoolTag{NewTagKey("errors.testing tag")}
+
+	Convey("Tags", t, func() {
+		Convey(`have unique tagKey values`, func() {
+			tagSet := map[TagKey]struct{}{}
+			tagSet[aBoolTag.Key] = struct{}{}
+			tagSet[aStringTag.Key] = struct{}{}
+			tagSet[aCustomIntTag.Key] = struct{}{}
+			So(tagSet, ShouldHaveLength, 3)
+		})
+
+		Convey(`can be applied to errors`, func() {
+			Convey(`at creation time`, func() {
+				err := New("I am an error", aBoolTag, aStringTag.With("hi"))
+				So(aBoolTag.In(err), ShouldBeTrue)
+				d, ok := aStringTag.In(err)
+				So(ok, ShouldBeTrue)
+				So(d, ShouldEqual, "hi")
+
+				_, ok = aCustomIntTag.In(err)
+				So(ok, ShouldBeFalse)
+			})
+
+			Convey(`added to existing errors`, func() {
+				err := New("I am an error")
+				err2 := aCustomIntTag.With(236).Apply(err)
+				err2 = aBoolTag.Apply(err2)
+
+				d, ok := aCustomIntTag.In(err2)
+				So(ok, ShouldBeTrue)
+				So(d, ShouldEqual, customInt(236))
+				So(aBoolTag.In(err2), ShouldBeTrue)
+
+				_, ok = aCustomIntTag.In(err)
+				So(ok, ShouldBeFalse)
+				So(aBoolTag.In(err), ShouldBeFalse)
+			})
+
+			Convey(`added to stdlib errors`, func() {
+				err := stdErr.New("I am an error")
+				err2 := aStringTag.With("hi").Apply(err)
+
+				d, ok := aStringTag.In(err2)
+				So(ok, ShouldBeTrue)
+				So(d, ShouldEqual, "hi")
+
+				_, ok = aStringTag.In(err)
+				So(ok, ShouldBeFalse)
+			})
+
+			Convey(`multiple applications has the last one win`, func() {
+				err := New("I am an error")
+				err = aStringTag.With("hi").Apply(err)
+				err = aStringTag.With("there").Apply(err)
+				err = aStringTag.With("winner").Apply(err)
+
+				d, ok := aStringTag.In(err)
+				So(ok, ShouldBeTrue)
+				So(d, ShouldEqual, "winner")
+
+				Convey(`muliterrors are first to last`, func() {
+					err = NewMultiError(
+						New("a", aStringTag.With("hi"), aBoolTag),
+						New("b", aCustomIntTag.With(10), aStringTag.With("no")),
+						New("c", aCustomIntTag.With(20), aStringTag.With("nopers")),
+					)
+
+					So(aBoolTag.In(err), ShouldBeTrue)
+
+					d, ok := aStringTag.In(err)
+					So(ok, ShouldBeTrue)
+					So(d, ShouldEqual, "hi")
+
+					ci, ok := aCustomIntTag.In(err)
+					So(ok, ShouldBeTrue)
+					So(ci, ShouldEqual, customInt(10))
+
+					Convey(`and all the correct values show up with GetTags`, func() {
+						tags := GetTags(err)
+						So(tags, ShouldContainKey, aStringTag.Key)
+						So(tags, ShouldContainKey, aBoolTag.Key)
+
+						So(tags[aCustomIntTag.Key], ShouldEqual, 10)
+						So(tags[aStringTag.Key], ShouldEqual, "hi")
+						So(tags[aBoolTag.Key], ShouldEqual, true)
+					})
+				})
+			})
+		})
+	})
+}