// Package vterrors provides simple error handling primitives for Vitess
//
// In all Vitess code, errors should be propagated using vterrors.Wrapf()
// and not fmt.Errorf(). This makes sure that stacktraces are kept and
// propagated correctly.
//
// New errors should be created using vterrors.New or vterrors.Errorf
//
// Vitess uses canonical error codes for error reporting. This is based
// on years of industry experience with error reporting. This idea is
// that errors should be classified into a small set of errors (10 or so)
// with very specific meaning. Each error has a code, and a message. When
// errors are passed around (even through RPCs), the code is
// propagated. To handle errors, only the code should be looked at (and
// not string-matching on the error message).
//
// Error codes are defined in /proto/vtrpc.proto. Along with an
// RPCError message that can be used to transmit errors through RPCs, in
// the message payloads. These codes match the names and numbers defined
// by gRPC.
//
// A standardized error implementation that allows you to build an error
// with an associated canonical code is also defined.
// While sending an error through gRPC, these codes are transmitted
// using gRPC's error propagation mechanism and decoded back to
// the original code on the other end.
//
// Retrieving the cause of an error
//
// Using vterrors.Wrap constructs a stack of errors, adding context to the
// preceding error, instead of simply building up a string.
// Depending on the nature of the error it may be necessary to reverse the
// operation of errors.Wrap to retrieve the original error for inspection.
// Any error value which implements this interface
//
//     type causer interface {
//             Cause() error
//     }
//
// can be inspected by vterrors.Cause and vterrors.RootCause.
//
// * vterrors.Cause will find the immediate cause if one is available, or nil
//   if the error is not a `causer` or if no cause is available.
// * vterrors.RootCause will recursively retrieve
//   the topmost error which does not implement causer, which is assumed to be
//   the original cause. For example:
//
//     switch err := errors.RootCause(err).(type) {
//     case *MyError:
//             // handle specifically
//     default:
//             // unknown error
//     }
//
// causer interface is not exported by this package, but is considered a part
// of stable public API.
//
// Formatted printing of errors
//
// All error values returned from this package implement fmt.Formatter and can
// be formatted by the fmt package. The following verbs are supported
//
//     %s    print the error. If the error has a Cause it will be
//           printed recursively
//     %v    extended format. Each Frame of the error's StackTrace will
//           be printed in detail.
//
// Most but not all of the code in this file was originally copied from
// https://github.com/pkg/errors/blob/v0.8.0/errors.go
package vterrors

import (
	"flag"
	"fmt"
	"io"

	"golang.org/x/net/context"
	vtrpcpb "vitess.io/vitess/go/vt/proto/vtrpc"
)

// LogErrStacks controls whether or not printing errors includes the
// embedded stack trace in the output.
var LogErrStacks bool

func init() {
	flag.BoolVar(&LogErrStacks, "log_err_stacks", false, "log stack traces for errors")
}

// New returns an error with the supplied message.
// New also records the stack trace at the point it was called.
func New(code vtrpcpb.Code, message string) error {
	return &fundamental{
		msg:   message,
		code:  code,
		stack: callers(),
	}
}

// NewWithoutCode returns an error when no applicable error code is available
// It will record the stack trace when creating the error
func NewWithoutCode(message string) error {
	return &fundamental{
		msg:   message,
		code:  vtrpcpb.Code_UNKNOWN,
		stack: callers(),
	}
}

// Errorf formats according to a format specifier and returns the string
// as a value that satisfies error.
// Errorf also records the stack trace at the point it was called.
func Errorf(code vtrpcpb.Code, format string, args ...interface{}) error {
	return &fundamental{
		msg:   fmt.Sprintf(format, args...),
		code:  code,
		stack: callers(),
	}
}

// fundamental is an error that has a message and a stack, but no caller.
type fundamental struct {
	msg  string
	code vtrpcpb.Code
	*stack
}

func (f *fundamental) Error() string { return f.msg }

func (f *fundamental) Format(s fmt.State, verb rune) {
	switch verb {
	case 'v':
		panicIfError(io.WriteString(s, "Code: "+f.code.String()+"\n"))
		panicIfError(io.WriteString(s, f.msg+"\n"))
		if LogErrStacks {
			f.stack.Format(s, verb)
		}
		return
	case 's':
		panicIfError(io.WriteString(s, f.msg))
	case 'q':
		panicIfError(fmt.Fprintf(s, "%q", f.msg))
	}
}

// Code returns the error code if it's a vtError.
// If err is nil, it returns ok.
func Code(err error) vtrpcpb.Code {
	if err == nil {
		return vtrpcpb.Code_OK
	}
	if err, ok := err.(*fundamental); ok {
		return err.code
	}

	cause := Cause(err)
	if cause != err && cause != nil {
		// If we did not find an error code at the outer level, let's find the cause and check it's code
		return Code(cause)
	}

	// Handle some special cases.
	switch err {
	case context.Canceled:
		return vtrpcpb.Code_CANCELED
	case context.DeadlineExceeded:
		return vtrpcpb.Code_DEADLINE_EXCEEDED
	}
	return vtrpcpb.Code_UNKNOWN
}

// Wrap returns an error annotating err with a stack trace
// at the point Wrap is called, and the supplied message.
// If err is nil, Wrap returns nil.
func Wrap(err error, message string) error {
	if err == nil {
		return nil
	}
	return &wrapping{
		cause: err,
		msg:   message,
		stack: callers(),
	}
}

// Wrapf returns an error annotating err with a stack trace
// at the point Wrapf is call, and the format specifier.
// If err is nil, Wrapf returns nil.
func Wrapf(err error, format string, args ...interface{}) error {
	if err == nil {
		return nil
	}
	return &wrapping{
		cause: err,
		msg:   fmt.Sprintf(format, args...),
		stack: callers(),
	}
}

type wrapping struct {
	cause error
	msg   string
	stack *stack
}

func (w *wrapping) Error() string { return w.msg + ": " + w.cause.Error() }
func (w *wrapping) Cause() error  { return w.cause }

func (w *wrapping) Format(s fmt.State, verb rune) {
	if rune('v') == verb {
		panicIfError(fmt.Fprintf(s, "%v\n", w.Cause()))
		panicIfError(io.WriteString(s, w.msg))
		if LogErrStacks {
			w.stack.Format(s, verb)
		}
		return
	}

	if rune('s') == verb || rune('q') == verb {
		panicIfError(io.WriteString(s, w.Error()))
	}
}

// since we can't return an error, let's panic if something goes wrong here
func panicIfError(_ int, err error) {
	if err != nil {
		panic(err)
	}
}

// RootCause returns the underlying cause of the error, if possible.
// An error value has a cause if it implements the following
// interface:
//
//     type causer interface {
//            Cause() error
//     }
//
// If the error does not implement Cause, the original error will
// be returned. If the error is nil, nil will be returned without further
// investigation.
func RootCause(err error) error {
	for {
		cause := Cause(err)
		if cause == nil {
			return err
		}
		err = cause
	}
}

//
// Cause will return the immediate cause, if possible.
// An error value has a cause if it implements the following
// interface:
//
//     type causer interface {
//            Cause() error
//     }
// If the error does not implement Cause, nil will be returned
func Cause(err error) error {
	type causer interface {
		Cause() error
	}

	causerObj, ok := err.(causer)
	if !ok {
		return nil
	}

	return causerObj.Cause()
}

// Equals returns true iff the error message and the code returned by Code()
// are equal.
func Equals(a, b error) bool {
	if a == nil && b == nil {
		// Both are nil.
		return true
	}

	if a == nil || b == nil {
		// One of the two is nil, since we know both are not nil.
		return false
	}

	return a.Error() == b.Error() && Code(a) == Code(b)
}

// Print is meant to print the vtError object in test failures.
// For comparing two vterrors, use Equals() instead.
func Print(err error) string {
	return fmt.Sprintf("%v: %v\n", Code(err), err.Error())
}