Fork github.com/google/renameio

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This is a fork of the github.com/google/renameio Go package at commit 353f8196982447d8b12c64f69530e657331e3dbc.
The inital commit of this package will carry the original package contents.
The Original License is the Apache License in Version 2.0 and the copyright of the forked package is held by Google Inc.
Any changes are recorded in the git history, which is part of this project.
---
The `renameio` Go package provides a way to atomically create or replace a file or
symbolic link.
## Atomicity vs durability
`renameio` concerns itself *only* with atomicity, i.e. making sure applications
never see unexpected file content (a half-written file, or a 0-byte file).
As a practical example, consider https://manpages.debian.org/: if there is a
power outage while the site is updating, we are okay with losing the manpages
which were being rendered at the time of the power outage. They will be added in
a later run of the software. We are not okay with having a manpage replaced by a
0-byte file under any circumstances, though.
## Advantages of this package
There are other packages for atomically replacing files, and sometimes ad-hoc
implementations can be found in programs.
A naive approach to the problem is to create a temporary file followed by a call
to `os.Rename()`. However, there are a number of subtleties which make the
correct sequence of operations hard to identify:
* The temporary file should be removed when an error occurs, but a remove must
not be attempted if the rename succeeded, as a new file might have been
created with the same name. This renders a throwaway `defer
os.Remove(t.Name())` insufficient; state must be kept.
* The temporary file must be created on the same file system (same mount point)
for the rename to work, but the TMPDIR environment variable should still be
respected, e.g. to direct temporary files into a separate directory outside of
the webservers document root but on the same file system.
* On POSIX operating systems, the
[`fsync`](https://manpages.debian.org/stretch/manpages-dev/fsync.2) system
call must be used to ensure that the `os.Rename()` call will not result in a
0-length file.
This package attempts to get all of these details right, provides an intuitive,
yet flexible API and caters to use-cases where high performance is required.
## Disclaimer
This is not an official Google product (experimental or otherwise), it
is just code that happens to be owned by Google.
This project is not affiliated with the Go project.

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// Copyright 2018 Google Inc.
//
// 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 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
package renameio

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// Copyright 2018 Google Inc.
//
// 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 renameio_test
import (
"fmt"
"log"
"github.com/safing/portbase/utils/renameio"
)
func ExampleTempFile_justone() {
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)
}
}
func ExampleTempFile_many() {
// 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)
}
}
}

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// Copyright 2018 Google Inc.
//
// 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.
// +build darwin dragonfly freebsd linux nacl netbsd openbsd solaris windows
package renameio
import (
"bytes"
"io/ioutil"
"os"
"path/filepath"
"testing"
)
func TestSymlink(t *testing.T) {
d, err := ioutil.TempDir("", "tempdirtest")
if err != nil {
t.Fatal(err)
}
defer os.RemoveAll(d)
want := []byte("Hello World")
if err := ioutil.WriteFile(filepath.Join(d, "hello.txt"), want, 0644); err != nil {
t.Fatal(err)
}
for i := 0; i < 2; i++ {
if err := Symlink("hello.txt", filepath.Join(d, "hi.txt")); err != nil {
t.Fatal(err)
}
got, err := ioutil.ReadFile(filepath.Join(d, "hi.txt"))
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(got, want) {
t.Fatalf("unexpected content: got %q, want %q", string(got), string(want))
}
}
}

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// Copyright 2018 Google Inc.
//
// 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 renameio
import (
"io/ioutil"
"os"
"path/filepath"
)
// 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 TempDir(dest string) string {
return tempDir("", filepath.Join(dest, "renameio-TempDir"))
}
func tempDir(dir, dest string) string {
if dir != "" {
return dir // caller-specified directory always wins
}
// Chose the destination directory as temporary directory so that we
// definitely can rename the file, for which both temporary and destination
// file need to point to the same mount point.
fallback := filepath.Dir(dest)
// The user might have overridden the os.TempDir() return value by setting
// the TMPDIR environment variable.
tmpdir := os.TempDir()
testsrc, err := ioutil.TempFile(tmpdir, "."+filepath.Base(dest))
if err != nil {
return fallback
}
cleanup := true
defer func() {
if cleanup {
os.Remove(testsrc.Name())
}
}()
testsrc.Close()
testdest, err := ioutil.TempFile(filepath.Dir(dest), "."+filepath.Base(dest))
if err != nil {
return fallback
}
defer os.Remove(testdest.Name())
testdest.Close()
if err := os.Rename(testsrc.Name(), testdest.Name()); err != nil {
return fallback
}
cleanup = false // testsrc no longer exists
return tmpdir
}
// PendingFile is a pending temporary file, waiting to replace the destination
// path in a call to CloseAtomicallyReplace.
type PendingFile struct {
*os.File
path string
done bool
closed bool
}
// Cleanup is a no-op if CloseAtomicallyReplace succeeded, and otherwise closes
// and removes the temporary file.
func (t *PendingFile) Cleanup() error {
if t.done {
return nil
}
// An error occurred. Close and remove the tempfile. Errors are returned for
// reporting, there is nothing the caller can recover here.
var closeErr error
if !t.closed {
closeErr = t.Close()
}
if err := os.Remove(t.Name()); err != nil {
return err
}
return closeErr
}
// 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.
func (t *PendingFile) CloseAtomicallyReplace() error {
// Even on an ordered file system (e.g. ext4 with data=ordered) or file
// systems with write barriers, we cannot skip the fsync(2) call as per
// Theodore Ts'o (ext2/3/4 lead developer):
//
// > data=ordered only guarantees the avoidance of stale data (e.g., the previous
// > contents of a data block showing up after a crash, where the previous data
// > could be someone's love letters, medical records, etc.). Without the fsync(2)
// > a zero-length file is a valid and possible outcome after the rename.
if err := t.Sync(); err != nil {
return err
}
t.closed = true
if err := t.Close(); err != nil {
return err
}
if err := os.Rename(t.Name(), t.path); err != nil {
return err
}
t.done = true
return nil
}
// TempFile wraps ioutil.TempFile for the use case of 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 by default. You can change these by
// explicitly calling Chmod on the returned PendingFile.
func TempFile(dir, path string) (*PendingFile, error) {
f, err := ioutil.TempFile(tempDir(dir, path), "."+filepath.Base(path))
if err != nil {
return nil, err
}
return &PendingFile{File: f, path: path}, nil
}
// 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 Symlink(oldname, newname string) error {
// Fast path: if newname does not exist yet, we can skip the whole dance
// below.
if err := os.Symlink(oldname, newname); err == nil || !os.IsExist(err) {
return err
}
// We need to use ioutil.TempDir, as we cannot overwrite a ioutil.TempFile,
// and removing+symlinking creates a TOCTOU race.
d, err := ioutil.TempDir(filepath.Dir(newname), "."+filepath.Base(newname))
if err != nil {
return err
}
cleanup := true
defer func() {
if cleanup {
os.RemoveAll(d)
}
}()
symlink := filepath.Join(d, "tmp.symlink")
if err := os.Symlink(oldname, symlink); err != nil {
return err
}
if err := os.Rename(symlink, newname); err != nil {
return err
}
cleanup = false
return os.RemoveAll(d)
}

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// Copyright 2018 Google Inc.
//
// 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.
// +build linux
package renameio
import (
"io/ioutil"
"os"
"path/filepath"
"syscall"
"testing"
)
func TestTempDir(t *testing.T) {
if tmpdir, ok := os.LookupEnv("TMPDIR"); ok {
defer os.Setenv("TMPDIR", tmpdir) // restore
} else {
defer os.Unsetenv("TMPDIR") // restore
}
mount1, err := ioutil.TempDir("", "tempdirtest")
if err != nil {
t.Fatal(err)
}
defer os.RemoveAll(mount1)
mount2, err := ioutil.TempDir("", "tempdirtest")
if err != nil {
t.Fatal(err)
}
defer os.RemoveAll(mount2)
if err := syscall.Mount("tmpfs", mount1, "tmpfs", 0, ""); err != nil {
t.Skipf("cannot mount tmpfs on %s: %v", mount1, err)
}
defer syscall.Unmount(mount1, 0)
if err := syscall.Mount("tmpfs", mount2, "tmpfs", 0, ""); err != nil {
t.Skipf("cannot mount tmpfs on %s: %v", mount2, err)
}
defer syscall.Unmount(mount2, 0)
tests := []struct {
name string
dir string
path string
TMPDIR string
want string
}{
{
name: "implicit TMPDIR",
path: filepath.Join(os.TempDir(), "foo.txt"),
want: os.TempDir(),
},
{
name: "explicit TMPDIR",
path: filepath.Join(mount1, "foo.txt"),
TMPDIR: mount1,
want: mount1,
},
{
name: "explicit unsuitable TMPDIR",
path: filepath.Join(mount1, "foo.txt"),
TMPDIR: mount2,
want: mount1,
},
{
name: "nonexistant TMPDIR",
path: filepath.Join(mount1, "foo.txt"),
TMPDIR: "/nonexistant",
want: mount1,
},
{
name: "caller-specified",
dir: "/overridden",
path: filepath.Join(mount1, "foo.txt"),
TMPDIR: "/nonexistant",
want: "/overridden",
},
}
for _, tt := range tests {
t.Run(tt.name, func(t *testing.T) {
if tt.TMPDIR == "" {
os.Unsetenv("TMPDIR")
} else {
os.Setenv("TMPDIR", tt.TMPDIR)
}
if got := tempDir(tt.dir, tt.path); got != tt.want {
t.Fatalf("tempDir(%q, %q): got %q, want %q", tt.dir, tt.path, got, tt.want)
}
})
}
}

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// Copyright 2018 Google Inc.
//
// 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 renameio
import "os"
// WriteFile mirrors ioutil.WriteFile, replacing an existing file with the same
// name atomically.
func WriteFile(filename string, data []byte, perm os.FileMode) error {
t, err := TempFile("", filename)
if err != nil {
return err
}
defer t.Cleanup()
// Set permissions before writing data, in case the data is sensitive.
if err := t.Chmod(perm); err != nil {
return err
}
if _, err := t.Write(data); err != nil {
return err
}
return t.CloseAtomicallyReplace()
}

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// Copyright 2018 Google Inc.
//
// 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.
// +build darwin dragonfly freebsd linux nacl netbsd openbsd solaris windows
package renameio
import (
"bytes"
"io/ioutil"
"os"
"path/filepath"
"testing"
)
func TestWriteFile(t *testing.T) {
d, err := ioutil.TempDir("", "tempdirtest")
if err != nil {
t.Fatal(err)
}
defer os.RemoveAll(d)
filename := filepath.Join(d, "hello.sh")
wantData := []byte("#!/bin/sh\necho \"Hello World\"\n")
wantPerm := os.FileMode(0755)
if err := WriteFile(filename, wantData, wantPerm); err != nil {
t.Fatal(err)
}
gotData, err := ioutil.ReadFile(filename)
if err != nil {
t.Fatal(err)
}
if !bytes.Equal(gotData, wantData) {
t.Errorf("got data %v, want data %v", gotData, wantData)
}
fi, err := os.Stat(filename)
if err != nil {
t.Fatal(err)
}
if gotPerm := fi.Mode() & os.ModePerm; gotPerm != wantPerm {
t.Errorf("got permissions 0%o, want permissions 0%o", gotPerm, wantPerm)
}
}