Code examples to go with https://incident.io/blog/golang-errors

README.md

pkg/errors

Our incident.io pkg/errors, as explained in:

• https://incident.io/blog/golang-errors

errors.go

package errors

import (
	"fmt"
	"reflect"
	"runtime"
	"unsafe"

	"github.com/pkg/errors"
)

// Export a number of functions or variables from pkg/errors. We want people to be able to
// use them, if only via the entrypoints we've vetted in this file.
var (
	As     = errors.As
	Is     = errors.Is
	Cause  = errors.Cause
	Unwrap = errors.Unwrap
)

// StackTrace should be aliases rather than newtype'd, so it can work with any of the
// functions we export from pkg/errors.
type StackTrace = errors.StackTrace

type StackTracer interface {
	StackTrace() errors.StackTrace
}

// Sentinel is used to create compile-time errors that are intended to be value only, with
// no associated stack trace.
func Sentinel(msg string, args ...interface{}) error {
	return fmt.Errorf(msg, args...)
}

// New acts as pkg/errors.New does, producing a stack traced error, but supports
// interpolating of message parameters. Use this when you want the stack trace to start at
// the place you create the error.
func New(msg string, args ...interface{}) error {
	return PopStack(errors.New(fmt.Sprintf(msg, args...)))
}

// Wrap creates a new error from a cause, decorating the original error message with a
// prefix.
//
// It differs from the pkg/errors Wrap/Wrapf by idempotently creating a stack trace,
// meaning we won't create another stack trace when there is already a stack trace present
// that matches our current program position.
func Wrap(cause error, msg string, args ...interface{}) error {
	causeStackTracer := new(StackTracer)
	if errors.As(cause, causeStackTracer) {
		// If our cause has set a stack trace, and that trace is a child of our own function
		// as inferred by prefix matching our current program counter stack, then we only want
		// to decorate the error message rather than add a redundant stack trace.
		if ancestorOfCause(callers(1), (*causeStackTracer).StackTrace()) {
			return errors.WithMessagef(cause, msg, args...) // no stack added, no pop required
		}
	}

	// Otherwise we can't see a stack trace that represents ourselves, so let's add one.
	return PopStack(errors.Wrapf(cause, msg, args...))
}

// ancestorOfCause returns true if the caller looks to be an ancestor of the given stack
// trace. We check this by seeing whether our stack prefix-matches the cause stack, which
// should imply the error was generated directly from our goroutine.
func ancestorOfCause(ourStack []uintptr, causeStack errors.StackTrace) bool {
	// Stack traces are ordered such that the deepest frame is first. We'll want to check
	// for prefix matching in reverse.
	//
	// As an example, imagine we have a prefix-matching stack for ourselves:
	// [
	//   "github.com/onsi/ginkgo/internal/leafnodes.(*runner).runSync",
	//   "github.com/incident-io/core/server/pkg/errors_test.TestSuite",
	//   "testing.tRunner",
	//   "runtime.goexit"
	// ]
	//
	// We'll want to compare this against an error cause that will have happened further
	// down the stack. An example stack trace from such an error might be:
	// [
	//   "github.com/incident-io/core/server/pkg/errors.New",
	//   "github.com/incident-io/core/server/pkg/errors_test.glob..func1.2.2.2.1",,
	//   "github.com/onsi/ginkgo/internal/leafnodes.(*runner).runSync",
	//   "github.com/incident-io/core/server/pkg/errors_test.TestSuite",
	//   "testing.tRunner",
	//   "runtime.goexit"
	// ]
	//
	// They prefix match, but we'll have to handle the match carefully as we need to match
	// from back to forward.

	// We can't possibly prefix match if our stack is larger than the cause stack.
	if len(ourStack) > len(causeStack) {
		return false
	}

	// We know the sizes are compatible, so compare program counters from back to front.
	for idx := 0; idx < len(ourStack); idx++ {
		if ourStack[len(ourStack)-1] != (uintptr)(causeStack[len(causeStack)-1]) {
			return false
		}
	}

	// All comparisons checked out, these stacks match
	return true
}

func callers(skip int) []uintptr {
	pc := make([]uintptr, 32)        // assume we'll have at most 32 frames
	n := runtime.Callers(skip+3, pc) // capture those frames, skipping runtime.Callers, ourself and the calling function

	return pc[:n] // return everything that we captured
}

// RecoverPanic turns a panic into an error, adjusting the stacktrace so it originates at
// the line that caused it.
//
// Example:
//
// func Do() (err error) {
//   defer func() {
//     errors.RecoverPanic(recover(), &err)
//   }()
// }
func RecoverPanic(r interface{}, errPtr *error) {
	var err error
	if r != nil {
		if panicErr, ok := r.(error); ok {
			err = errors.Wrap(panicErr, "caught panic")
		} else {
			err = errors.New(fmt.Sprintf("caught panic: %v", r))
		}
	}

	if err != nil {
		// Pop twice: once for the errors package, then again for the defer function we must
		// run this under. We want the stacktrace to originate at the source of the panic, not
		// in the infrastructure that catches it.
		err = PopStack(err) // errors.go
		err = PopStack(err) // defer

		*errPtr = err
	}
}

// PopStack removes the top of the stack from an errors stack trace.
func PopStack(err error) error {
	if err == nil {
		return err
	}

	// We want to remove us, the internal/errors.New function, from the error stack we just
	// produced. There's no official way of reaching into the error and adjusting this, as
	// the stack is stored as a private field on an unexported struct.
	//
	// This does some unsafe badness to adjust that field, which should not be repeated
	// anywhere else.
	stackField := reflect.ValueOf(err).Elem().FieldByName("stack")
	if stackField.IsZero() {
		return err
	}
	stackFieldPtr := (**[]uintptr)(unsafe.Pointer(stackField.UnsafeAddr()))

	// Remove the first of the frames, dropping 'us' from the error stack trace.
	frames := (**stackFieldPtr)[1:]

	// Assign to the internal stack field
	*stackFieldPtr = &frames

	return err
}

errors_test.go

package errors_test

import (
	"fmt"

	"github.com/incident-io/core/server/pkg/errors"

	. "github.com/onsi/ginkgo"
	. "github.com/onsi/gomega"
)

func getStackTraces(err error) []errors.StackTrace {
	traces := []errors.StackTrace{}
	if err, ok := err.(errors.StackTracer); ok {
		traces = append(traces, err.StackTrace())
	}

	if err := errors.Unwrap(err); err != nil {
		traces = append(traces, getStackTraces(err)...)
	}

	return traces
}

var _ = Describe("errors", func() {
	Describe("New", func() {
		It("generates an error with a stack trace", func() {
			err := errors.New("oops")
			Expect(getStackTraces(err)).To(HaveLen(1))
		})
	})

	Describe("Wrap", func() {
		Context("when cause has no stack trace", func() {
			It("wraps the error and takes stack trace", func() {
				err := errors.Wrap(fmt.Errorf("cause"), "description")
				Expect(err.Error()).To(Equal("description: cause"))

				cause := errors.Cause(err)
				Expect(cause).To(MatchError("cause"))

				Expect(getStackTraces(err)).To(HaveLen(1))
			})
		})

		Context("when cause has stack trace", func() {
			Context("which is not an ancestor of our own", func() {
				It("creates a new stack trace", func() {
					errChan := make(chan error)
					go func() {
						errChan <- errors.New("unrelated") // created with a stack trace
					}()

					err := errors.Wrap(<-errChan, "helpful description")
					Expect(err.Error()).To(Equal("helpful description: unrelated"))

					Expect(getStackTraces(err)).To(HaveLen(2))
				})
			})

			Context("with a frame from our current method", func() {
				It("does not create new stack trace", func() {
					err := errors.Wrap(errors.New("related"), "helpful description")
					Expect(err.Error()).To(Equal("helpful description: related"))

					Expect(getStackTraces(err)).To(HaveLen(1))
				})
			})
		})
	})
})

suite_test.go

package errors_test

import (
	"context"
	"testing"

	"github.com/incident-io/core/server/pkg/pkgdb"
	"github.com/incident-io/core/server/pkg/spec"
)

var (
	ctx context.Context
	pg  *pkgdb.Postgres
	tx  *pkgdb.Postgres
)

func TestSuite(t *testing.T) {
	spec.NewSuite(t, &ctx, &pg, &tx, spec.WithEnforceSafeDB(false))
}