Add an errors package with a similar API to github.com/pkg/errors

Go introduced a 'native' way to wrap errors back in v1.13. At that point we were
already using github.com/pkg/errors to 'wrap' errors with context, and we never
got around to migrating. In addition to pure inertia, I've personally avoided
making the switch because I prefer the github.com/pkg/errors API. Specifically I
like that errors.Wrap handles the "outer context: inner context" error format
that Go uses by convention, and that errors.Wrap will return nil when passed a
nil error.

Given that github.com/pkg/errors has long been in maintenance mode, and is (per
pkg/errors#245) no longer used by its original author
now seems as good a time as any to migrate. This commit attempts to ease that
migration for the Crossplane project - and to retain the nice API - by adding a
package that acts as a small github.com/pkg/errors style shim layer around the
stdlib pkg/errors (and friends, like fmt.Errorf).

Signed-off-by: Nic Cope <[email protected]>

pkg/errors/errors.go

@@ -0,0 +1,120 @@
+/*
+Copyright 2021 The Crossplane Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+// Package errors is a github.com/pkg/errors compatible API for native errors.
+// It includes only the subset of the github.com/pkg/errors API that is used by
+// the Crossplane project.
+package errors
+
+import (
+	"errors"
+	"fmt"
+)
+
+// New returns an error that formats as the given text. Each call to New returns
+// a distinct error value even if the text is identical.
+func New(text string) error { return errors.New(text) }
+
+// Is reports whether any error in err's chain matches target.
+//
+// The chain consists of err itself followed by the sequence of errors obtained
+// by repeatedly calling Unwrap.
+//
+// An error is considered to match a target if it is equal to that target or if
+// it implements a method Is(error) bool such that Is(target) returns true.
+//
+// An error type might provide an Is method so it can be treated as equivalent
+// to an existing error. For example, if MyError defines
+//
+//	func (m MyError) Is(target error) bool { return target == fs.ErrExist }
+//
+// then Is(MyError{}, fs.ErrExist) returns true. See syscall.Errno.Is for
+// an example in the standard library.
+func Is(err, target error) bool { return errors.Is(err, target) }
+
+// As finds the first error in err's chain that matches target, and if so, sets
+// target to that error value and returns true. Otherwise, it returns false.
+//
+// The chain consists of err itself followed by the sequence of errors obtained
+// by repeatedly calling Unwrap.
+//
+// An error matches target if the error's concrete value is assignable to the
+// value pointed to by target, or if the error has a method As(interface{}) bool
+// such that As(target) returns true. In the latter case, the As method is
+// responsible for setting target.
+//
+// An error type might provide an As method so it can be treated as if it were a
+// different error type.
+//
+// As panics if target is not a non-nil pointer to either a type that implements
+// error, or to any interface type.
+func As(err error, target interface{}) bool { return errors.As(err, target) }
+
+// Unwrap returns the result of calling the Unwrap method on err, if err's type
+// contains an Unwrap method returning error. Otherwise, Unwrap returns nil.
+func Unwrap(err error) error { return errors.Unwrap(err) }
+
+// Errorf formats according to a format specifier and returns the string as a
+// value that satisfies error.
+//
+// If the format specifier includes a %w verb with an error operand, the
+// returned error will implement an Unwrap method returning the operand. It is
+// invalid to include more than one %w verb or to supply it with an operand that
+// does not implement the error interface. The %w verb is otherwise a synonym
+// for %v.
+func Errorf(format string, a ...interface{}) error { return fmt.Errorf(format, a...) }
+
+// Wrap the supplied error with the supplied context message. Returns nil if the
+// supplied error is nil. If the supplied error is not nil the returned error
+// will have an Unwrap method.
+func Wrap(err error, message string) error {
+	if err == nil {
+		return nil
+	}
+	return fmt.Errorf("%s: %w", message, err)
+}
+
+// Wrapf wraps the supplied error with the supplied context message. Returns nil
+// if the supplied error is nil. If the supplied error is not nil the returned
+// error will have an Unwrap method.
+func Wrapf(err error, message string, args ...interface{}) error {
+	if err == nil {
+		return nil
+	}
+	return fmt.Errorf("%s: %w", fmt.Sprintf(message, args...), err)
+}
+
+// Cause calls Unwrap on each error it finds. It returns the first error it
+// finds that does not have an Unwrap method - i.e. the first error that was not
+// the result of a Wrap call, a Wrapf call, or an Errorf call with %w wrapping.
+func Cause(err error) error {
+	type wrapped interface {
+		Unwrap() error
+	}
+
+	for err != nil {
+		// We're ignoring errorlint telling us to use errors.As because
+		// we actually do want to check the outermost error.
+		//nolint:errorlint
+		w, ok := err.(wrapped)
+		if !ok {
+			return err
+		}
+		err = w.Unwrap()
+	}
+
+	return err
+}

pkg/errors/errors_test.go

@@ -0,0 +1,126 @@
+/*
+Copyright 2021 The Crossplane Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package errors
+
+import (
+	"testing"
+
+	"github.com/google/go-cmp/cmp"
+
+	"github.com/crossplane/crossplane-runtime/pkg/test"
+)
+
+func TestWrap(t *testing.T) {
+	type args struct {
+		err     error
+		message string
+	}
+	cases := map[string]struct {
+		args args
+		want error
+	}{
+		"NilError": {
+			args: args{
+				err:     nil,
+				message: "very useful context",
+			},
+			want: nil,
+		},
+		"NonNilError": {
+			args: args{
+				err:     New("boom"),
+				message: "very useful context",
+			},
+			want: Errorf("very useful context: %w", New("boom")),
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			got := Wrap(tc.args.err, tc.args.message)
+			if diff := cmp.Diff(tc.want, got, test.EquateErrors()); diff != "" {
+				t.Errorf("Wrap(...): -want, +got:\n%s", diff)
+			}
+		})
+	}
+}
+
+func TestWrapf(t *testing.T) {
+	type args struct {
+		err     error
+		message string
+		args    []interface{}
+	}
+	cases := map[string]struct {
+		args args
+		want error
+	}{
+		"NilError": {
+			args: args{
+				err:     nil,
+				message: "very useful context",
+			},
+			want: nil,
+		},
+		"NonNilError": {
+			args: args{
+				err:     New("boom"),
+				message: "very useful context about %s",
+				args:    []interface{}{"ducks"},
+			},
+			want: Errorf("very useful context about %s: %w", "ducks", New("boom")),
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			got := Wrapf(tc.args.err, tc.args.message, tc.args.args...)
+			if diff := cmp.Diff(tc.want, got, test.EquateErrors()); diff != "" {
+				t.Errorf("Wrapf(...): -want, +got:\n%s", diff)
+			}
+		})
+	}
+}
+
+func TestCause(t *testing.T) {
+	cases := map[string]struct {
+		err  error
+		want error
+	}{
+		"NilError": {
+			err:  nil,
+			want: nil,
+		},
+		"BareError": {
+			err:  New("boom"),
+			want: New("boom"),
+		},
+		"WrappedError": {
+			err:  Wrap(Wrap(New("boom"), "interstitial context"), "very important context"),
+			want: New("boom"),
+		},
+	}
+
+	for name, tc := range cases {
+		t.Run(name, func(t *testing.T) {
+			got := Cause(tc.err)
+			if diff := cmp.Diff(tc.want, got, test.EquateErrors()); diff != "" {
+				t.Errorf("Cause(...): -want, +got:\n%s", diff)
+			}
+		})
+	}
+}