package renameio

import "github.com/google/renameio/v2"

Package renameio provides a way to atomically create or replace a file or symbolic link.

Caveat: this package requires the file system rename(2) implementation to be atomic. Notably, this is not the case when using NFS with multiple clients: https://stackoverflow.com/a/41396801

Index ¶

• func Symlink(oldname, newname string) error
• func TempDir(dest string) string
• func WriteFile(filename string, data []byte, perm os.FileMode, opts ...Option) error
• type Option
• func IgnoreUmask() Option
• func WithExistingPermissions() Option
• func WithPermissions(perm os.FileMode) Option
• func WithStaticPermissions(perm os.FileMode) Option
• func WithTempDir(dir string) Option
• type PendingFile
• func NewPendingFile(path string, opts ...Option) (*PendingFile, error)
• func TempFile(dir, path string) (*PendingFile, error)
• func (t *PendingFile) Cleanup() error
• func (t *PendingFile) CloseAtomicallyReplace() error

Examples ¶

• TempFile (Justone)
• TempFile (Many)

Functions ¶

func Symlink ¶

func Symlink(oldname, newname string) error

Symlink wraps os.Symlink, replacing an existing symlink with the same name atomically (os.Symlink fails when newname already exists, at least on Linux).

func TempDir ¶

func TempDir(dest string) string

TempDir checks whether os.TempDir() can be used as a temporary directory for later atomically replacing files within dest. If no (os.TempDir() resides on a different mount point), dest is returned.

Note that the returned value ceases to be valid once either os.TempDir() changes (e.g. on Linux, once the TMPDIR environment variable changes) or the file system is unmounted.

func WriteFile ¶

func WriteFile(filename string, data []byte, perm os.FileMode, opts ...Option) error

WriteFile mirrors ioutil.WriteFile, replacing an existing file with the same name atomically.

Types ¶

type Option ¶

type Option interface {
	// contains filtered or unexported methods
}

Option is the interface implemented by all configuration function return values.

func IgnoreUmask ¶

func IgnoreUmask() Option

IgnoreUmask causes the permissions configured using WithPermissions to be applied directly without applying the umask.

func WithExistingPermissions ¶

func WithExistingPermissions() Option

WithExistingPermissions configures the file creation to try to use the permissions from an already existing target file. If the target file doesn't exist yet or is not a regular file the default permissions are used unless overridden using WithPermissions or WithStaticPermissions.

func WithPermissions ¶

func WithPermissions(perm os.FileMode) Option

WithPermissions sets the permissions for the target file while respecting the umask(2). Bits set in the umask are removed from the permissions given unless IgnoreUmask is used.

func WithStaticPermissions ¶

func WithStaticPermissions(perm os.FileMode) Option

WithStaticPermissions sets the permissions for the target file ignoring the umask(2). This is equivalent to calling Chmod() on the file handle or using WithPermissions in combination with IgnoreUmask.

func WithTempDir ¶

func WithTempDir(dir string) Option

WithTempDir configures the directory to use for temporary, uncommitted files. Suitable for using a cached directory from TempDir(filepath.Base(path)).

type PendingFile ¶

type PendingFile struct {
	*os.File
	// contains filtered or unexported fields
}

PendingFile is a pending temporary file, waiting to replace the destination path in a call to CloseAtomicallyReplace.

func NewPendingFile ¶

func NewPendingFile(path string, opts ...Option) (*PendingFile, error)

NewPendingFile creates a temporary file destined to atomically creating or replacing the destination file at path.

TempDir(filepath.Base(path)) is used to store the temporary file. If you are going to write a large number of files to the same file system, use the result of TempDir(filepath.Base(path)) with the WithTempDir option.

The file's permissions will be (0600 & ^umask). Use WithPermissions, IgnoreUmask, WithStaticPermissions and WithExistingPermissions to control them.

func TempFile ¶

func TempFile(dir, path string) (*PendingFile, error)

TempFile creates a temporary file destined to atomically creating or replacing the destination file at path.

If dir is the empty string, TempDir(filepath.Base(path)) is used. If you are going to write a large number of files to the same file system, store the result of TempDir(filepath.Base(path)) and pass it instead of the empty string.

The file's permissions will be 0600. You can change these by explicitly calling Chmod on the returned PendingFile.

{
	persist := func(temperature float64) error {
		t, err := renameio.TempFile("", "/srv/www/metrics.txt")
		if err != nil {
			return err
		}
		defer t.Cleanup()
		if _, err := fmt.Fprintf(t, "temperature_degc %f\n", temperature); err != nil {
			return err
		}
		return t.CloseAtomicallyReplace()
	}
	// Thanks to the write package, a webserver exposing /srv/www never
	// serves an incomplete or missing file.
	if err := persist(31.2); err != nil {
		log.Fatal(err)
	}
}

{
	// Prepare for writing files to /srv/www, effectively caching calls to
	// TempDir which TempFile would otherwise need to make.
	dir := renameio.TempDir("/srv/www")
	persist := func(temperature float64) error {
		t, err := renameio.TempFile(dir, "/srv/www/metrics.txt")
		if err != nil {
			return err
		}
		defer t.Cleanup()
		if _, err := fmt.Fprintf(t, "temperature_degc %f\n", temperature); err != nil {
			return err
		}
		return t.CloseAtomicallyReplace()
	}

	// Imagine this was an endless loop, reading temperature sensor values.
	// Thanks to the write package, a webserver exposing /srv/www never
	// serves an incomplete or missing file.
	for {
		if err := persist(31.2); err != nil {
			log.Fatal(err)
		}
	}
}

func (*PendingFile) Cleanup ¶

func (t *PendingFile) Cleanup() error

Cleanup is a no-op if CloseAtomicallyReplace succeeded, and otherwise closes and removes the temporary file.

This method is not safe for concurrent use by multiple goroutines.

func (*PendingFile) CloseAtomicallyReplace ¶

func (t *PendingFile) CloseAtomicallyReplace() error

CloseAtomicallyReplace closes the temporary file and atomically replaces the destination file with it, i.e., a concurrent open(2) call will either open the file previously located at the destination path (if any), or the just written file, but the file will always be present.

This method is not safe for concurrent use by multiple goroutines.

Source Files ¶

doc.go option.go tempfile.go writefile.go

Directories ¶