error.Is testing checker

The following makes it easier to compare errors. Currently, if
we attempt to use satisfies checker, we'll more thank likely endup
with a deprecation warning that Is prefix predicate variants from
the errors package is being used.

The solution is to make a new checker that we can move to, which has
the semantics that we want moving forward.

checkers/error.go

@@ -0,0 +1,52 @@
+// Copyright 2023 Canonical Ltd.
+// Licensed under the LGPLv3, see LICENCE file for details.
+
+package checkers
+
+import (
+	"errors"
+	"fmt"
+	"reflect"
+
+	gc "gopkg.in/check.v1"
+)
+
+type errorIsChecker struct {
+	*gc.CheckerInfo
+}
+
+// ErrorIs checks whether a value is an error that matches the other
+// argument.
+var ErrorIs gc.Checker = &errorIsChecker{
+	CheckerInfo: &gc.CheckerInfo{
+		Name:   "ErrorIs",
+		Params: []string{"obtained", "error"},
+	},
+}
+
+var (
+	errType = reflect.TypeOf((*error)(nil)).Elem()
+)
+
+func (checker *errorIsChecker) Check(params []interface{}, names []string) (result bool, err string) {
+	if params[1] == nil || params[0] == nil {
+		return params[1] == params[0], ""
+	}
+
+	f := reflect.ValueOf(params[1])
+	ft := f.Type()
+	if !ft.Implements(errType) {
+		return false, fmt.Sprintf("wrong error target type, got: %s", ft)
+	}
+
+	v := reflect.ValueOf(params[0])
+	vt := v.Type()
+	if !v.IsValid() {
+		return false, fmt.Sprintf("wrong argument type %s for %s", vt, ft)
+	}
+	if !vt.Implements(errType) {
+		return false, fmt.Sprintf("wrong argument type %s for %s", vt, ft)
+	}
+
+	return errors.Is(v.Interface().(error), f.Interface().(error)), ""
+}

checkers/error_test.go

@@ -0,0 +1,69 @@
+package checkers_test
+
+import (
+	"fmt"
+
+	"github.com/juju/errors"
+	jc "github.com/juju/testing/checkers"
+	gc "gopkg.in/check.v1"
+)
+
+type ErrorSuite struct{}
+
+var _ = gc.Suite(&ErrorSuite{})
+
+var errorIsTests = []struct {
+	arg    interface{}
+	target interface{}
+	result bool
+	msg    string
+}{{
+	arg:    fmt.Errorf("bar"),
+	target: nil,
+	result: false,
+}, {
+	arg:    nil,
+	target: fmt.Errorf("bar"),
+	result: false,
+}, {
+	arg:    nil,
+	target: nil,
+	result: true,
+}, {
+	arg:    fmt.Errorf("bar"),
+	target: fmt.Errorf("foo"),
+	result: false,
+}, {
+	arg:    errors.ConstError("bar"),
+	target: errors.ConstError("foo"),
+	result: false,
+}, {
+	arg:    errors.ConstError("foo"),
+	target: errors.ConstError("foo"),
+	result: true,
+}, {
+	arg:    errors.Trace(errors.ConstError("foo")),
+	target: errors.ConstError("foo"),
+	result: true,
+}, {
+	arg:    errors.ConstError("foo"),
+	target: "blah",
+	msg:    "wrong error target type, got: string",
+}, {
+	arg:    "blah",
+	target: errors.ConstError("foo"),
+	msg:    "wrong argument type string for errors.ConstError",
+}, {
+	arg:    (*error)(nil),
+	target: errors.ConstError("foo"),
+	msg:    "wrong argument type *error for errors.ConstError",
+}}
+
+func (s *ErrorSuite) TestErrorIs(c *gc.C) {
+	for i, test := range errorIsTests {
+		c.Logf("test %d. %T %T", i, test.arg, test.target)
+		result, msg := jc.ErrorIs.Check([]interface{}{test.arg, test.target}, nil)
+		c.Check(result, gc.Equals, test.result)
+		c.Check(msg, gc.Equals, test.msg)
+	}
+}