Source File
	error.go

Belonging Package
	os

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

package os

import (
	"internal/poll"
	"io/fs"
)

// Portable analogs of some common system call errors.
//
// Errors returned from this package may be tested against these errors
// with [errors.Is].
var (
	// ErrInvalid indicates an invalid argument.
	// Methods on File will return this error when the receiver is nil.
	ErrInvalid = fs.ErrInvalid // "invalid argument"

	ErrPermission = fs.ErrPermission // "permission denied"
	ErrExist      = fs.ErrExist      // "file already exists"
	ErrNotExist   = fs.ErrNotExist   // "file does not exist"
	ErrClosed     = fs.ErrClosed     // "file already closed"

	ErrNoDeadline       = errNoDeadline()       // "file type does not support deadline"
	ErrDeadlineExceeded = errDeadlineExceeded() // "i/o timeout"
)

func errNoDeadline() error { return poll.ErrNoDeadline }

// errDeadlineExceeded returns the value for os.ErrDeadlineExceeded.
// This error comes from the internal/poll package, which is also
// used by package net. Doing it this way ensures that the net
// package will return os.ErrDeadlineExceeded for an exceeded deadline,
// as documented by net.Conn.SetDeadline, without requiring any extra
// work in the net package and without requiring the internal/poll
// package to import os (which it can't, because that would be circular).
func errDeadlineExceeded() error { return poll.ErrDeadlineExceeded }

type timeout interface {
	Timeout() bool
}

// PathError records an error and the operation and file path that caused it.
type PathError = fs.PathError

// SyscallError records an error from a specific system call.
type SyscallError struct {
	Syscall string
	Err     error
}

func (e *SyscallError) Error() string { return e.Syscall + ": " + e.Err.Error() }

func (e *SyscallError) Unwrap() error { return e.Err }

// Timeout reports whether this error represents a timeout.
func (e *SyscallError) Timeout() bool {
	t, ok := e.Err.(timeout)
	return ok && t.Timeout()
}

// NewSyscallError returns, as an error, a new [SyscallError]
// with the given system call name and error details.
// As a convenience, if err is nil, NewSyscallError returns nil.
func NewSyscallError(syscall string, err error) error {
	if err == nil {
		return nil
	}
	return &SyscallError{syscall, err}
}

// IsExist returns a boolean indicating whether its argument is known to report
// that a file or directory already exists. It is satisfied by [ErrExist] as
// well as some syscall errors.
//
// This function predates [errors.Is]. It only supports errors returned by
// the os package. New code should use errors.Is(err, fs.ErrExist).
func IsExist(err error) bool {
	return underlyingErrorIs(err, ErrExist)
}

// IsNotExist returns a boolean indicating whether its argument is known to
// report that a file or directory does not exist. It is satisfied by
// [ErrNotExist] as well as some syscall errors.
//
// This function predates [errors.Is]. It only supports errors returned by
// the os package. New code should use errors.Is(err, fs.ErrNotExist).
func IsNotExist(err error) bool {
	return underlyingErrorIs(err, ErrNotExist)
}

// IsPermission returns a boolean indicating whether its argument is known to
// report that permission is denied. It is satisfied by [ErrPermission] as well
// as some syscall errors.
//
// This function predates [errors.Is]. It only supports errors returned by
// the os package. New code should use errors.Is(err, fs.ErrPermission).
func IsPermission(err error) bool {
	return underlyingErrorIs(err, ErrPermission)
}

// IsTimeout returns a boolean indicating whether its argument is known
// to report that a timeout occurred.
//
// This function predates [errors.Is], and the notion of whether an
// error indicates a timeout can be ambiguous. For example, the Unix
// error EWOULDBLOCK sometimes indicates a timeout and sometimes does not.
// New code should use errors.Is with a value appropriate to the call
// returning the error, such as [os.ErrDeadlineExceeded].
func IsTimeout(err error) bool {
	terr, ok := underlyingError(err).(timeout)
	return ok && terr.Timeout()
}

func underlyingErrorIs(err, target error) bool {
	// Note that this function is not errors.Is:
	// underlyingError only unwraps the specific error-wrapping types
	// that it historically did, not all errors implementing Unwrap().
	err = underlyingError(err)
	if err == target {
		return true
	}
	// To preserve prior behavior, only examine syscall errors.
	e, ok := err.(syscallErrorType)
	return ok && e.Is(target)
}

// underlyingError returns the underlying error for known os error types.
func underlyingError(err error) error {
	switch err := err.(type) {
	case *PathError:
		return err.Err
	case *LinkError:
		return err.Err
	case *SyscallError:
		return err.Err
	}
	return err
}

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